In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?

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In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?



I just dipped into a book, The Drunkard's Walk - How Randomness Rules Our Lives, by Leonard Mlodinow, Vintage Books, 2008. On p.107 Mlodinow says the chances are 1 in 3.



It seems obvious to me that the chances are 1 in 2. Am I correct? Is this not exactly analogous to having a bowl with an infinite number of marbles, half black and half red? Without looking I draw out a black marble. The probability of the second marble I draw being black is 1/2.







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  • 3




    In your "obvoius" case, you don't count the case where the first child is a boy but the second child is a girl.
    – Rawling
    Dec 21 '10 at 12:22






  • 2




    @Rawling Say I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no? Or let's say my wife and I have our first child, a girl. The probability that the next child we have will also be a girl is 1/2, no?
    – NotSuper
    Dec 21 '10 at 12:45






  • 3




    In the second case, yes; in the first case, I think the answer is also yes (given that one of the two children walk with equal probability). Take a look at en.wikipedia.org/wiki/Boy_or_Girl_paradox
    – Rawling
    Dec 21 '10 at 13:58










  • See also: math.stackexchange.com/questions/4400/…
    – BlueRaja - Danny Pflughoeft
    Dec 21 '10 at 18:08










  • So re-order the question - "in a family with two children, at least one of whom is a girl, what are the chances that the other is also a girl?" and the 1 in 3 answer might make a little more sense.
    – Bill Michell
    Sep 27 '11 at 16:01














up vote
73
down vote

favorite
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In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?



I just dipped into a book, The Drunkard's Walk - How Randomness Rules Our Lives, by Leonard Mlodinow, Vintage Books, 2008. On p.107 Mlodinow says the chances are 1 in 3.



It seems obvious to me that the chances are 1 in 2. Am I correct? Is this not exactly analogous to having a bowl with an infinite number of marbles, half black and half red? Without looking I draw out a black marble. The probability of the second marble I draw being black is 1/2.







share|cite|improve this question


















  • 3




    In your "obvoius" case, you don't count the case where the first child is a boy but the second child is a girl.
    – Rawling
    Dec 21 '10 at 12:22






  • 2




    @Rawling Say I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no? Or let's say my wife and I have our first child, a girl. The probability that the next child we have will also be a girl is 1/2, no?
    – NotSuper
    Dec 21 '10 at 12:45






  • 3




    In the second case, yes; in the first case, I think the answer is also yes (given that one of the two children walk with equal probability). Take a look at en.wikipedia.org/wiki/Boy_or_Girl_paradox
    – Rawling
    Dec 21 '10 at 13:58










  • See also: math.stackexchange.com/questions/4400/…
    – BlueRaja - Danny Pflughoeft
    Dec 21 '10 at 18:08










  • So re-order the question - "in a family with two children, at least one of whom is a girl, what are the chances that the other is also a girl?" and the 1 in 3 answer might make a little more sense.
    – Bill Michell
    Sep 27 '11 at 16:01












up vote
73
down vote

favorite
37









up vote
73
down vote

favorite
37






37





In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?



I just dipped into a book, The Drunkard's Walk - How Randomness Rules Our Lives, by Leonard Mlodinow, Vintage Books, 2008. On p.107 Mlodinow says the chances are 1 in 3.



It seems obvious to me that the chances are 1 in 2. Am I correct? Is this not exactly analogous to having a bowl with an infinite number of marbles, half black and half red? Without looking I draw out a black marble. The probability of the second marble I draw being black is 1/2.







share|cite|improve this question














In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?



I just dipped into a book, The Drunkard's Walk - How Randomness Rules Our Lives, by Leonard Mlodinow, Vintage Books, 2008. On p.107 Mlodinow says the chances are 1 in 3.



It seems obvious to me that the chances are 1 in 2. Am I correct? Is this not exactly analogous to having a bowl with an infinite number of marbles, half black and half red? Without looking I draw out a black marble. The probability of the second marble I draw being black is 1/2.









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edited Apr 18 '11 at 12:52









t.b.

61.1k7202280




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asked Dec 21 '10 at 12:17









NotSuper

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6533814







  • 3




    In your "obvoius" case, you don't count the case where the first child is a boy but the second child is a girl.
    – Rawling
    Dec 21 '10 at 12:22






  • 2




    @Rawling Say I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no? Or let's say my wife and I have our first child, a girl. The probability that the next child we have will also be a girl is 1/2, no?
    – NotSuper
    Dec 21 '10 at 12:45






  • 3




    In the second case, yes; in the first case, I think the answer is also yes (given that one of the two children walk with equal probability). Take a look at en.wikipedia.org/wiki/Boy_or_Girl_paradox
    – Rawling
    Dec 21 '10 at 13:58










  • See also: math.stackexchange.com/questions/4400/…
    – BlueRaja - Danny Pflughoeft
    Dec 21 '10 at 18:08










  • So re-order the question - "in a family with two children, at least one of whom is a girl, what are the chances that the other is also a girl?" and the 1 in 3 answer might make a little more sense.
    – Bill Michell
    Sep 27 '11 at 16:01












  • 3




    In your "obvoius" case, you don't count the case where the first child is a boy but the second child is a girl.
    – Rawling
    Dec 21 '10 at 12:22






  • 2




    @Rawling Say I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no? Or let's say my wife and I have our first child, a girl. The probability that the next child we have will also be a girl is 1/2, no?
    – NotSuper
    Dec 21 '10 at 12:45






  • 3




    In the second case, yes; in the first case, I think the answer is also yes (given that one of the two children walk with equal probability). Take a look at en.wikipedia.org/wiki/Boy_or_Girl_paradox
    – Rawling
    Dec 21 '10 at 13:58










  • See also: math.stackexchange.com/questions/4400/…
    – BlueRaja - Danny Pflughoeft
    Dec 21 '10 at 18:08










  • So re-order the question - "in a family with two children, at least one of whom is a girl, what are the chances that the other is also a girl?" and the 1 in 3 answer might make a little more sense.
    – Bill Michell
    Sep 27 '11 at 16:01







3




3




In your "obvoius" case, you don't count the case where the first child is a boy but the second child is a girl.
– Rawling
Dec 21 '10 at 12:22




In your "obvoius" case, you don't count the case where the first child is a boy but the second child is a girl.
– Rawling
Dec 21 '10 at 12:22




2




2




@Rawling Say I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no? Or let's say my wife and I have our first child, a girl. The probability that the next child we have will also be a girl is 1/2, no?
– NotSuper
Dec 21 '10 at 12:45




@Rawling Say I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no? Or let's say my wife and I have our first child, a girl. The probability that the next child we have will also be a girl is 1/2, no?
– NotSuper
Dec 21 '10 at 12:45




3




3




In the second case, yes; in the first case, I think the answer is also yes (given that one of the two children walk with equal probability). Take a look at en.wikipedia.org/wiki/Boy_or_Girl_paradox
– Rawling
Dec 21 '10 at 13:58




In the second case, yes; in the first case, I think the answer is also yes (given that one of the two children walk with equal probability). Take a look at en.wikipedia.org/wiki/Boy_or_Girl_paradox
– Rawling
Dec 21 '10 at 13:58












See also: math.stackexchange.com/questions/4400/…
– BlueRaja - Danny Pflughoeft
Dec 21 '10 at 18:08




See also: math.stackexchange.com/questions/4400/…
– BlueRaja - Danny Pflughoeft
Dec 21 '10 at 18:08












So re-order the question - "in a family with two children, at least one of whom is a girl, what are the chances that the other is also a girl?" and the 1 in 3 answer might make a little more sense.
– Bill Michell
Sep 27 '11 at 16:01




So re-order the question - "in a family with two children, at least one of whom is a girl, what are the chances that the other is also a girl?" and the 1 in 3 answer might make a little more sense.
– Bill Michell
Sep 27 '11 at 16:01










18 Answers
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I think this question confuses a lot of people because there's a lack of intuitive context -- I'll try to supply that.



Suppose there is a birthday party to which all of the girls (and none of the boys) in a small town are invited. If you run into a mother who has dropped off a kid at this birthday party and who has two children, the chance that she has two girls is $1/3$. Why? $3/4$ of the mothers with two children will have a daughter at the birthday party, the ones with two girls ($1/4$ of the total mothers with two children) and the ones with one girl and one boy ($1/2$ of the total mothers with two children). Out of these $3/4$ of the mothers, $1/3$ have two girls.



On the other hand, if the birthday party is only for fifth-grade girls, you get a different answer. Assuming there are no siblings who are both in the fifth grade, the answer in this case is $1/2$. The child in fifth grade is a girl, but the other child has probability $1/2$ of being a girl. Situations of this kind arise in real life much more commonly than situations of the other kind, so the answer of $1/3$ is quite nonintuitive.






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    In a family with 2 children there are four possibilities:



    1) the first child is a boy and the second child is a boy (bb)



    2) the first child is a boy and the second child is a girl (bg)



    3) the first child is a girl and the second child is a boy (gb)



    4) the first child is a girl and the second child is a girl (gg)



    Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg. Out of those three possibilities the only one with two girls is gg. Hence the probability is $frac13$.






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    • 12




      Your answer is correct for the OP's question, but not how YOU pose the question. Your text says "the first child is a...". That being said, if we know the first child is a girl, then out of bb, bg, gb, and gg, only gb, and gg are left available. Hence the probability is 1/2. Had you phrased it like the OP ("if we have a family with one girl (not necessarily the first child), what is the probability that both are girls?"), then your answer would be correct.
      – Nemi
      Dec 21 '10 at 18:06







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      Yes, @Nemi is correct - I think the OP has stated the question such that gb and bg are the same thing.
      – Maxim Zaslavsky
      Dec 21 '10 at 20:12






    • 6




      @Nemi You are correct that there is an important distinction between "one of the two children is a girl" and "the first child is a girl". My text says: "Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg". I did not say that we are given that the first child is a girl. The three possibilities arise from considering whether the first/second born was a girl/boy.
      – Travis Service
      Dec 21 '10 at 20:29










    • sorry, re-reading your answer now I see that you have stated it correctly. Not sure if I read it wrong earlier or you have edited it. It is most certainly correct as stated.
      – Nemi
      Dec 22 '10 at 3:54






    • 3




      While true, this doesn't explain the apparent paradox.
      – BlueRaja - Danny Pflughoeft
      Dec 23 '10 at 21:30

















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    I think that the reason that these puzzles are so often confusing is that they rely on the limitations of the English language rather than on any mathematical difficulties. Of course this is not unique to English, and I think you should be able to find similar similar puzzles in pretty much any natural language.



    Here is an example that brings out the difficulty more clearly. First, consider the following similar puzzle:



    • A family has two children, Robin and Lindsay. Lindsay is a girl. What is the probability that both children are girls?

    In elementary probability class, they teach you to answer this by making a table of the four options




    Robin Lindsay
    B B
    * B G
    G B
    * G G


    The starred rows are the ones where Lindsay is a girl, and we compute from them that the chance that both children are girls is 1/2.



    Now consider this puzzle



    • A family has two children, Robin and Lindsay. At least one of them is a girl. What is the probability that both children are girls?

    The elementary probability method gives the following table, and a probability of 1/3. The difference is that we gain one more row, compared to the previous puzzle.




    Robin Lindsay
    B B
    * B G
    * G B
    * G G


    After looking at these, you can see that the difficulty of the original puzzle comes because, in English, "one of the children" can mean several different things:



    • "one" can mean "a particular one". If you read the original puzzle like this, it becomes analogous to the first puzzle I wrote, and the answer will be 1/2.


    • "one" can mean "at least one". If you read the original puzzle like this, it become analogous to the second puzzle I wrote, and the answer is 1/3.


    • "one" can mean "exactly one". If you read the original puzzle like this, the answer is 0.


    There is a common convention in mathematics that "one" usually means "at least one". For example, this is the sense intended in the following sentence, which is a typical example of mathematical English: "if a natural number $n$ is a multiple of a prime number $p$, and $n = ab$, then one of $a$ and $b$ is divisible by $p$." We would not read this as saying that exactly one of $a$ and $b$ is divisible by $p$.



    I don't believe this convention is very common in non-mathematical English. If I say, "one of my children is a girl", in normal English this means that the other is a boy. Similar discrepancies between mathematical and non-mathematical English come up with our use of the word "or" and our use of the phrase "if/then". When we teach mathematics, we have to spend time explaining this mathematical argot to students, so they can use the same English conventions that we do.



    Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks.






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    • I now, at last, agree that the correct answer is 1/3. (See my question "2 slightly different situations in which 2 coins are tossed. Does the knowledge of an observer effect the probabilities of the outcomes?" at goo.gl/yyOlK and my comment beginning with "@all: It suddenly hit me that I didn't need that truthful observer.", about half way down the page, under Willie Wong's answer.)
      – NotSuper
      Dec 27 '10 at 16:25











    • (continued) I'd award you the 100 bounty points for the clarity of your explanation of the mathematics, if you hadn't added your idea that "Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks." In my case I wasn't tricked -- I immediately assumed that "one of my children is a girl" meant that "at least one of my children is a girl".
      – NotSuper
      Dec 27 '10 at 16:30










    • (continued) I assumed this because to interpret "one of my children is a girl" as meaning "exactly one of my children is a girl" would destroy the problem/puzzle in that the answer would be too obviously zero. Also, because I had first seen the question in a book about randomness and probability, I was already in math English mode.
      – NotSuper
      Dec 27 '10 at 16:30










    • @NotSuper: Thanks for your kind comments.
      – Carl Mummert
      Dec 28 '10 at 3:07






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      Should be the accepted answer.
      – 355durch113
      Oct 17 '15 at 3:58

















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    Please ignore the old comments - I have radically changed my answer. It's long, but bear with me - after reading this answer, you should feel intuitively-comfortable with almost all other probabilistic fallacies, not just this one.



    The first and most important thing is to define what we mean by "probability." Let's define it to mean "the expected percentage of positive outcomes when repeating an observation over a large sample" (Go ahead, read that again, more slowly). Also, let's call the method we use to chose this large sample the "model."



    This may sound trivial, but it has some important implications. For example, what is the probability you will die before age 40? According to our definition, this question has no meaning: we can't observe you multiple times before age forty and record how many times you die. Instead, we observe other people below age 40, and record how many of them die.



    So let's say we observe all other people on earth below age 40 (our model), and find that 1/2 of them die before hitting the big four-oh. Does this mean the probability of you dying before 40 is 50%? Well, according to this model, it does! However, this is hardly a fair model. Perhaps you live in a first-world country - now when we revise our model to include only people below 40 in first-world countries, your chances become a much-less-grim 1/10†. But you are also not a smoker, and don't live in the city, and ride your bike on Sundays with your wife and crazy mother-in-law, which makes your chances 123/4567! That's much better... however, our model still doesn't take into account that you are also an avid skydiver ;)
    † I am pulling these numbers out of nowhere, they are not real statistics.



    So the point is, asking for a "probability" only makes sense in the context of a certain model - a way of repeating our observation many times. Without that, asking for a probability is meaningless.




    Now, back to the original question. Before we can assign a probability, we must choose a model; how are we choosing the families to sample from? I see two obvious choices, which will lead to different answers:



    1. Consider only families which have two children, one of whom is a girl, and choose one randomly.

    2. Consider only girls who have exactly one sibling, and choose one randomly.

    Do you see the difference? In the first case, every family has the same probability of being chosen. However, in the second case, the families with two girls are more likely to be chosen than families with only one girl, because every girl has an equal chance of being chosen: the two-girl families have doubled their odds by having two girls. If children were raffle tickets, they would have bought two tickets while the one-girl families bought only one.



    Thus, we should expect the probabilities in these two cases to be different. Let's calculate them more rigorously (writing BG to mean "boy was born, then girl):



    1. There are three equally probable family-types: BG, GB, and GG (BB was removed from consideration, because they have no girls). Since only one of the three has two girls, our chances of having two girls are 1/3.

    2. We have the same possibilities as above, but now GG is twice as likely as BG or GB. Thus, the probabilities are GG: 2/4, GB: 1/4, and BG: 1/4, meaning the probability of a girl-sibling is 2/4 = 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

    Here lies the fallacy: the model our intuition assumes is the second one, but the way the problem is worded strongly implies the first one. When we think in terms of "randomly choosing a family (over a large number of families)," our intuition meshes perfectly with the result.




    Let's take a look at another similar problem




    In a family of two children, where the oldest child is a girl, what is the probability they are both girls?




    Once again, I can see two different, plausible models for observing our random sample:



    1. Consider only families with two children, the oldest of whom is a girl, and choose one randomly.

    2. Consider only girls who have exactly one (younger) sibling, and choose one at random.

    So once again, the question strongly implies the first model, though arguments for either could be made. However, when we actually calculate the probability...



    1. Same as before, but we've also eliminated BG, where the first child was a boy. This leaves only two equally-probable possibilities, GB and GG. Thus, the chances are 1/2.

    2. We've also eliminated BG from this case, leaving GB and GG. However, unlike the original question, GG is no longer twice as likely, since the younger child can no longer be the one who was randomly chosen. Thus, GB and GG are equally likely, and we again have a probability of 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

    ...we find that the choice between these two models doesn't matter, because in this case both have the same probability! Among two-child families with an older daughter, it doesn't matter if we randomly choose the family or the older-daughter, because in both cases there is only one of each per family.




    Hopefully that all made sense. For bonus points, try applying this reasoning to the Monty Hall problem. What is our model - how are we making repeated observations? Why does it clash with our intuition?



    For even more bonus points, try to figure out the following question; the math isn't too difficult, but it took me a long while to figure out why, intuitively, the answer should be correct:




    In a family of two children, one of whom is a girl named Florida, what are the chances of two girls?




    (If you have troubles, post a question and leave a link to it in the comments, and I'll try to answer it there as best I can :) )






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    • 4




      I don't completely agree with this analysis. Mathematically, there is no problem at all. Once we list the possible events and the favorable events, the probability is just the corresponding fraction. So the difficulty in these puzzles is just that English hasn't evolved a clear way to describe a probability space. There are two reasonable probability spaces here, and usual English is not clear about which of them is intended. But regardless what anyone knows about the children, I can still compute the probabilities in both these spaces, to get 1/3 and 1/2, respectively.
      – Carl Mummert
      Dec 23 '10 at 15:06











    • @Carl: Then you must believe that the Monty Hall problem is ambiguous in exactly the same way, no? I'm afraid I have to disagree with that.
      – BlueRaja - Danny Pflughoeft
      Dec 23 '10 at 17:15







    • 2




      I do feel that the difficulty in the Monty Hall problem is also not "mathematical", in the sense that as soon as any probability model has been rigorously stated then the probabilities can be calculated using basic methods. The arguments about the Monty Hall problem stem from a lack of rigour in the statement, which I think is often intentional in order to make an otherwise easy probability exercise into a "puzzle".
      – Carl Mummert
      Dec 23 '10 at 17:44










    • @Carl: In both cases, it's not that it's ambiguous or not mathematically-defined, it's just counter-intuitive.
      – BlueRaja - Danny Pflughoeft
      Dec 23 '10 at 21:29






    • 2




      The two puzzles are similar, and you've made a nice comparison. They each have two simple probability models, and it's clear which gives the clever, counter-intuitive solution. But if you admit any more complexity, there's a lot of ambiguity in both problems. For example, given a room full of people convinced that switching doors is the right strategy, an adversarial Monty Hall can make sure no one wins by offering the switch only to those who first choose the car. In real life, I would assume ratings and TV gimmicks affect what choices I'm given, and I have no probability model for that!
      – Jonas Kibelbek
      Dec 23 '10 at 23:04

















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    Here's some idea.



    Suppose you sample $4n$ families with two children, where $n$ is a very large integer. Then, with a very high probability, about $n$ of them have two boys, about $n$ have two girls, and about $2n$ have one girl and one boy.
    Since you ignore the families with two boys, the desired probability is given by $n/(3n) = 1/3$.



    EDIT: The above solution corresponds to the following situation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you check if one of the kids is a girl. If no, you ignore this family. If yes, you check if both kids are girls.



    EDIT: Among all the OP's examples, only one is relevant here. Namely, "I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no?". Well, in this situation the probability is indeed $1/2$, assuming the following interpretation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you wait until one of the kids comes into the room. If it is a boy, you ignore this family; otherwise you check if both kids are girls. It is readily understood that this leads to a probability of $1/2$.






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    • 4




      Of course, a lot depends on how we interpret the question.
      – Shai Covo
      Dec 21 '10 at 12:52






    • 1




      @NotSuper: It all depends a lot on the precise formulation, as Shai already pointed out. Let's say you visit a friend who flips a coin twice. Given that at least one of the outcomes is Heads, the probability of two Heads is indeed 1/3.
      – Hendrik Vogt
      Dec 21 '10 at 13:35






    • 1




      @NotSuper: Shai is right indeed: You can see only one of them, say the one closer to you, and it's a head. So you know more than in the experiment I formulated. In that experiment, I assumed that you know that at least one of the two outcomes is Heads, but if it's only one head, then you don't know which coin it is! In your experiment you know that that coin lying closer to you is a head, so it's a different experiment.
      – Hendrik Vogt
      Dec 21 '10 at 15:38







    • 1




      @NotSuper: I understand that this is a bit difficult. Let me put my experiment in different words: When I write "Given that at least one of the outcomes is Heads", then this means: someone looked at both coins and provided you with the information that one of the outcomes is Heads. In your experiment, you look only at one of the coins.
      – Hendrik Vogt
      Dec 21 '10 at 17:46






    • 1




      @NotSuper: No. Looking at only one coin and looking a two coins gives different knowledge. Sorry, I can't explain it better.
      – Hendrik Vogt
      Dec 21 '10 at 18:14

















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    One possible solution for this problem might be though using conditional probability.



    Let $B_i$ and $G_i$ represent $i$th boy and $i$th girl respectively. Here the sample space could be expressed as $S =$ $(B_1,B_2),(B_1,G_2),(G_1,B_2),(G_1,G_2)$



    Let $A$ = An event of having both girl chil.
    $B$ = An event of having at-least one girl child.



    $A = $$(G_1,G_2)$ and $B =$$(B_1,G_2),(G_1,B_2),(G_1,G_2)$



    So what you are asking is $$P(A / B) = fracP(A cap B)P(B) = fracfrac14frac34 = frac13 $$






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      It really depends on what knowledge you have to restrict the probability space. (The original question is a little ambiguous, but I agree that 1/3 is the best answer.)



      The marble example and the example of the firstborn daughter are the same situation-- the second trial is independent, and the odds are 1/2. In these cases you know the first trial is independent, and you can ignore it.



      The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter. For example, suppose there is a social science study on 2 child families with at least 1 daughter-- in this situation, about 1/3 of the families will be daughter-daughter, 1/3 will be daughter-son, and 1/3 will be son-daughter. You have to consider the full probability space of two trials (d-d,d-s,s-d,s-s) and eliminate the s-s possibility. This leaves a 1/3 chance of 2 daughters.



      For other variations on this question, you have to decide the appropriate probability space before answering. (One common, confusing variation is to suppose the family has two children, one of which is a son born on a Tuesday-- what are the odds of two sons?)



      In your example where the first child in the room is a daughter, if we suppose that the order the children enter has nothing to do with sex, then the probability the other child is a daughter is 1/2. However, maybe the family encourages their daughters to play inside and their sons outside-- then you could assume the other child is a son, since he hasn't come in yet. Maybe the culture has a tradition that the oldest daughter in the household (if there is a daughter) offers guests drinks-- then you know the family has at least one daughter, and the odds that they have another daughter is 1/3. Once you've chosen your assumptions, you can define the probability space and compute the answer.






      share|cite|improve this answer
















      • 1




        @Jonas It isn't at all clear that "The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter." Instead I imagined I hadn't seen my nephew in a decade, and we hadn't kept in touch. But I had heard that he had 2 kids and one was a girl. I was going to visit him soon and was wondering about the other child. Girl? or boy? I figured the chances of a girl was 1 in 2, and I still do. I found the Wikipedia article linked to by Rawling enlightening.
        – NotSuper
        Dec 21 '10 at 15:15











      • @NotSuper: Yes, I agree it's ambiguous. It is clear that I gave Mlodinow's intended interpretation only because he gave the answer 1/3. But your nephew scenario is ambiguous as well-- how did you hear that one child is a girl? You could learn that among their children is at least one daughter (maybe they send an old dollhouse to your grandchildren), which gives 1/3. Or, you could specify one of their children (oldest, first to answer the phone, etc., as long as it's independent of sex) and learn she is a girl, which gives 1/2.
        – Jonas Kibelbek
        Dec 21 '10 at 15:42










      • These correspond to the two probability spaces described in my answer and in the Wikipedia article. When you hear that one child is a girl, can you cross off son-son and son-daughter, or can you just cross off son-son? (With other information and messier probability models, we could get other answers than 1/2 and 1/3 as well. But these are the two most sensible models to apply.)
        – Jonas Kibelbek
        Dec 21 '10 at 15:46











      • @Jonas "How did you hear that one child is a girl?" I wrote "I had heard that he had 2 kids and one was a girl." I think that implies that someone told me that (or possibly wrote that to me in a letter or email), and that's all I know about my nephew's children. Isn't that the standard interpretation?
        – NotSuper
        Dec 24 '10 at 13:34











      • @NotSuper: To quote from Carl's answer, "one child" can be interpreted several ways in English: "one particular child," "at least one child," or "exactly one child." If the letter or e-mail gives you enough context to pick one interpretation, then it's easy to compute the probability. Your intuitive answer of 1/2 is a correct answer, provided you recognize you are using the "one particular child" interpretation. But, since this is a puzzle or riddle, the smallest assumption ("at least one child"), which gives the counter-intuitive answer, is the riddler's preferred answer. Do you agree?
        – Jonas Kibelbek
        Dec 24 '10 at 16:34

















      up vote
      2
      down vote













      This is really a continuation of BlueRaja's answer, where I'm changing the question slightly to give another illustration of the underlying point. Consider the following two procedures:



      Procedure A: I flip two coins, a nickel and a penny. After the flip, I
      pick one at random, and report its value to you.



      Procedure B: I flip two coins, a nickel and a penny. If exactly one lands
      heads, I report that one's value to you, i.e. I say either "the nickel
      landed heads" or "the penny landed heads" as appropriate. If both land
      heads, I report on one of them chosen at random. If neither lands
      heads, I say "both landed tails."



      Now, if I follow Procedure A and say "the nickel landed
      heads", then the probability that the penny landed heads is 50/50. But
      if I say the very same thing after following Procedure B, the
      probability that the penny landed heads is 1 in 3.



      Now take the following problem: I flip two coins and say to you "the
      nickel landed heads". What is the probability that the penny landed
      heads? Clearly there's no way to answer without knowing or assuming something about the procedure I'm following.



      (All this having been said, I agree that the best answer to the original question is 1/3.)






      share|cite|improve this answer



























        up vote
        2
        down vote













        In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?



        0%. You already said that ONE is a girl. The other must be a boy.






        share|cite|improve this answer
















        • 5




          If a family has two children and both are girls, it is not incorrect to say "one of the children is a girl." "One" doesn't mean "exactly one."
          – manthanomen
          May 6 '13 at 23:11







        • 4




          Does one mean at least one, exactly one, at most one, a specific one? I think Judie's point was that the parameters of the problem are not clearly stated -- you have to make an assumption about the intent. The given answers generally assume that in this case one was intended to mean one or two, but maybe we don't know that from the question by itself unless we recognize it due to prior exposure to this type of problem.
          – nobar
          May 11 '13 at 22:34

















        up vote
        1
        down vote













        If you are given that the first child is a girl or that the second child is a girl, then the obvious answer of 50% is correct.



        In this case we are given that one or both of the children is a girl, but not given any information about which one(s) -- so a simple evaluation of the 4 possible scenarios (for a two-child family) reveals the answer of 33%.



        The confusion stems from the slightly ambiguous language used. The phrase one of the children was used to mean one or both of the children. Based on that language, our brains trick us into thinking that a specific one of the children is given and therefore we can evaluate the other child independently.






        share|cite|improve this answer




















        • A link to this Wikipedia article was given in the main comments, but I wanted to point out that the article addresses the psychology of this problem, in an attempt to expose the seeming "paradox" of common intuition.
          – nobar
          May 13 '13 at 2:20











        • youtu.be/be2wuOaglFY?t=37m0s "This problem is famous for being ill-posed."
          – nobar
          Mar 3 '17 at 1:43










        • There is some ambiguity about whether I should have said "4 possible scenarios" or "3 possible scenarios".
          – nobar
          Mar 3 '17 at 1:46

















        up vote
        1
        down vote













        We can solve this from first principles (maybe they're technically "second" principles, who knows).



        Define:



        1. $p_G$ be the probability that a child is a girl

        2. $p_B$ be the probability that a child is a boy

        3. $N_G$ be the number of girls in a family

        4. $N_B$ be the number of boys in a family

        5. $N_T = N_G + N_B$

        We want to find: $mathbbP(N_G=2 | N_G geq 1 , N_T=2)$



        $$mathbbP(N_G=2 | N_G geq 1 , N_T=2) = fracmathbbP(N_G=2 , N_G geq 1 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



        $$=fracmathbbP(N_G=2 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



        $$=fracp_G^2p_G^2 + 2p_Gp_B$$



        If we take $p_G = p_B = frac12$ then we get the book's answer of $frac1over4frac14+frac12$



        $$=frac1over43over4 = frac13$$



        NOTE: In the above, the second factor in the denominator is derived since the order in which the children are born does matter.






        share|cite|improve this answer






















        • The question does not say "$N_G geq 1$", though, it just says "one of the children is a girl". Once we have interpreted this to mean $N_g geq 1$, we have already solved the problem, as sketched here. The issue is not whether $P(N_G = 2| N_G geq 1, N_T = 2) = 1/3$ - the issue is what probability the English is asking for in the first place.
          – Carl Mummert
          Aug 21 at 20:51

















        up vote
        1
        down vote













        I feel like people are always left with a feeling of "sure, I see how you can obtain that result but why isn't my interpretation correct?" Here's my latest attempt to address this issue.



        https://math.stackexchange.com/a/1801261/342573.






        share|cite|improve this answer





























          up vote
          1
          down vote













          There are at least three interpretations of the statement that "one child is a
          girl". Two of them lead to the 1/2 and 1/3 answers which have been discussed
          at length, but I found only one brief
          answer covering the third
          interpretation (though I did only skim).



          The meaning of the statement, and consequently the correct answer, can be
          altered by considering it to answer different questions.



          1. "How many of your children are girls?" "One of my children is a girl."

          2. "Is one of your children a girl?" "Yes, one of my children is a girl."

          3. "Is the child in the stroller a boy or a girl?" "This one is a girl."

          Suppose you've just had the first discussion; what do you think the other child
          will be in that case? There is no chance the other is a girl.



          The second question would prompt most people to search their set of children
          for a girl and answer "yes" if they can find one or "no" otherwise.
          Consequently, out of the 3/4 of two-child parents who can answer yes, about 1/3
          of those will have another girl.



          However it's conceivable that some kind of contrarian may resolve "one of your
          children" even before they have considered the rest of the question, choosing
          one at random regardless of its gender, and answering the rest of the question
          with respect to that random choice. This renders the situation equivalent to
          the third case.



          In the third case, obviously, the gender of the other child is independent of
          all the information we have. So it's a 1/2 chance.






          share|cite|improve this answer






















          • This is entirely incorrect. You are misunderstanding what "one of" means.
            – The Count
            Feb 22 '17 at 19:00










          • @TheCount, who's the authority on the meaning of the words, here?
            – sh1
            Feb 22 '17 at 19:07










          • Fair point, but I would argue that the term "one of them" does not imply the exclusion of "both of them" as in the famous and idiotic puzzle "I have two coins that add to 30 cents and one is not a nickel". The question was clearly not intended to be taken that way.
            – The Count
            Feb 22 '17 at 19:08






          • 1




            @TheCount, the point I'm trying to make is that the confusion between 1/2 and 1/3 comes about because we don't exactly know what we've been told. There's a third interpretation; not necessarily more correct, but also not invalid. It's a more "it couldn't possibly mean that!" case which is meant to highlight that the question is not well framed.
            – sh1
            Feb 22 '17 at 19:13










          • I see. Well, you make a fair point, but I suppose my confusion comes about from the fact that your answer seemingly declares your interpretation to be the correct one, instead of explaining it to me as you just did. I think including that info in the answer itself would be helpful.
            – The Count
            Feb 22 '17 at 19:14

















          up vote
          0
          down vote













          Let $X$ be the event "one of the children is a girl" Let $BG$ refer to the event the family has one boy and one girl (I'm not distinguishing $BG$ and $GB$), and so forth. Everything is conditioned on the family having two children.



          $$ P(GG | X) = fracP(GG wedge X)P(X)
          = fracP(XGG) P(GG) + P(X
          = frac1 cdot frac14
          1 cdot frac14 + P(X
          = frac1BG)
          $$



          (note that I am assuming this isn't taken as a "trick" question, so that $P(X|GG) = 1$ is meant, rather than $P(X|GG)=0$)



          So everything boils down to the probability of "one of the children is a girl" given that the family has a boy and a girl.



          Now, what does the phrase "one of the children is a girl" mean? There are two typical options



          • This phrase simply means that this is not an all-boy family. In this case, $P(X|BG) = 1$, and $P(GG|X) = 1/3$.

          • The phrase means that a particular child was chosen and it could just has easily have been the boy, so that $P(X|BG) = 1/2$, and $P(GG|X) = 1/2$.

          In my opinion, an English speaker would almost always interpret the phrase the first way rather than the second way.



          However, there is a nasty linguistic trap: even meaning it the first way, we may often pick a girl to be the referent of "one" (e.g. so that we can speak of the "other" child). And since we've picked a girl, we get stuck in the second interpretation. This is exacerbated by the fact the second interpretation has a very appealing shortcut for reasoning out the solution.






          share|cite|improve this answer





























            up vote
            0
            down vote













            You have to read carefully. Here, the probability of a girl means the probability of at least one girl. You have
            $$P(hboxone child is a girl) = 1 - P(hboxno child is a girl)
            = 1 - P(hboxtwo boys) = frac14. $$
            Now you are calculating the conditional probabilty
            $$P(hboxtwo girls| hboxat least 1 girl = P(hboxtwo girls)over P(hboxat least one girl) = 1over 3.$$






            share|cite|improve this answer



























              up vote
              0
              down vote













              You reach into a bag and pull two marbles. You look at the first and see that it is black. There is a 1/2 chance that the other is red.



              You reach into a bag and pull two marbles. Your friend tells you whether or not you have 2 red marbles. He says you don't have 2 red marbles. The chance you have 2 black marbles isn't 1/2 or 1/4, it's 1/3.



              The assertion that at least one child is a girl is equivalent to saying that you don't have 2 boys. It is analogous to the second case above, not the first.






              share|cite|improve this answer



























                up vote
                0
                down vote













                There are many ways to explain this, but let me share with you how I do it in my class. I set $q=P(mboxYou say that he has at least one daughter mid mboxHe has $B,G$)$.



                If you sample at family level, then you ask the father if he has at least a daughter or you check official records. In that case $q=1$.



                If you sample at child level, by for instance meeting the father in the company of one child or otherwise happening to find out about one child, then $q=1/2$. In this case you will half the time miss this family because you happen to see the son first.



                Using Bayes' formula, we get
                $$
                P(B,G) mid mboxYou say that he has at least one daughter)
                = fracP(mboxYou say that he has at least one daughter mid B,G) P(B,G)
                P(mboxYou say that he has at least one daughter).
                $$



                Since
                $$P(mboxYou say that he has at least one daughter) = P(mboxYou say that he has at least one daughter mid B,G) P(B,G) + P(mboxYou say that he has at least one daughter mid G, G) P(G, G),
                $$



                we get
                $$
                P(B,G mid mboxYou say that he has at least one daughter) = (q times 1/2)/(q times 1/2 + 1 times 1/4) = 2q/(1 + 2q),
                $$



                and it follows that
                $$
                P(G, G mid mboxYou say that he has at least one daughter)=1/(1 + 2q).
                $$



                If $q=1/2$, we get $P=1/2$, and $q=1$ gives $P=1/3$.






                share|cite|improve this answer




















                • The issue with the question is that it does not say "at least one daughter", and instead says "one of the children is a girl". Once we re-interpret the problem to say 'at least one is a girl' we have already solved it.
                  – Carl Mummert
                  Aug 21 at 20:48

















                up vote
                -1
                down vote













                I think that the answer is going to be 1/3



                As the ways in which your neighbor can have children are as follow:



                (where B= Boy; G= Girl)



                BB, GG, BG, GB where BB is not possible as one of them is a girl. So the probability of the other child being a G is GG out of the sample space= GG, BG, GB



                I know some of you might argue that BG and GB are similar things but we don’t know whether the Sister is the elder one or the smaller one. So, it’s gonna work like that.






                share|cite|improve this answer



















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                  up vote
                  56
                  down vote



                  accepted










                  I think this question confuses a lot of people because there's a lack of intuitive context -- I'll try to supply that.



                  Suppose there is a birthday party to which all of the girls (and none of the boys) in a small town are invited. If you run into a mother who has dropped off a kid at this birthday party and who has two children, the chance that she has two girls is $1/3$. Why? $3/4$ of the mothers with two children will have a daughter at the birthday party, the ones with two girls ($1/4$ of the total mothers with two children) and the ones with one girl and one boy ($1/2$ of the total mothers with two children). Out of these $3/4$ of the mothers, $1/3$ have two girls.



                  On the other hand, if the birthday party is only for fifth-grade girls, you get a different answer. Assuming there are no siblings who are both in the fifth grade, the answer in this case is $1/2$. The child in fifth grade is a girl, but the other child has probability $1/2$ of being a girl. Situations of this kind arise in real life much more commonly than situations of the other kind, so the answer of $1/3$ is quite nonintuitive.






                  share|cite|improve this answer


























                    up vote
                    56
                    down vote



                    accepted










                    I think this question confuses a lot of people because there's a lack of intuitive context -- I'll try to supply that.



                    Suppose there is a birthday party to which all of the girls (and none of the boys) in a small town are invited. If you run into a mother who has dropped off a kid at this birthday party and who has two children, the chance that she has two girls is $1/3$. Why? $3/4$ of the mothers with two children will have a daughter at the birthday party, the ones with two girls ($1/4$ of the total mothers with two children) and the ones with one girl and one boy ($1/2$ of the total mothers with two children). Out of these $3/4$ of the mothers, $1/3$ have two girls.



                    On the other hand, if the birthday party is only for fifth-grade girls, you get a different answer. Assuming there are no siblings who are both in the fifth grade, the answer in this case is $1/2$. The child in fifth grade is a girl, but the other child has probability $1/2$ of being a girl. Situations of this kind arise in real life much more commonly than situations of the other kind, so the answer of $1/3$ is quite nonintuitive.






                    share|cite|improve this answer
























                      up vote
                      56
                      down vote



                      accepted







                      up vote
                      56
                      down vote



                      accepted






                      I think this question confuses a lot of people because there's a lack of intuitive context -- I'll try to supply that.



                      Suppose there is a birthday party to which all of the girls (and none of the boys) in a small town are invited. If you run into a mother who has dropped off a kid at this birthday party and who has two children, the chance that she has two girls is $1/3$. Why? $3/4$ of the mothers with two children will have a daughter at the birthday party, the ones with two girls ($1/4$ of the total mothers with two children) and the ones with one girl and one boy ($1/2$ of the total mothers with two children). Out of these $3/4$ of the mothers, $1/3$ have two girls.



                      On the other hand, if the birthday party is only for fifth-grade girls, you get a different answer. Assuming there are no siblings who are both in the fifth grade, the answer in this case is $1/2$. The child in fifth grade is a girl, but the other child has probability $1/2$ of being a girl. Situations of this kind arise in real life much more commonly than situations of the other kind, so the answer of $1/3$ is quite nonintuitive.






                      share|cite|improve this answer














                      I think this question confuses a lot of people because there's a lack of intuitive context -- I'll try to supply that.



                      Suppose there is a birthday party to which all of the girls (and none of the boys) in a small town are invited. If you run into a mother who has dropped off a kid at this birthday party and who has two children, the chance that she has two girls is $1/3$. Why? $3/4$ of the mothers with two children will have a daughter at the birthday party, the ones with two girls ($1/4$ of the total mothers with two children) and the ones with one girl and one boy ($1/2$ of the total mothers with two children). Out of these $3/4$ of the mothers, $1/3$ have two girls.



                      On the other hand, if the birthday party is only for fifth-grade girls, you get a different answer. Assuming there are no siblings who are both in the fifth grade, the answer in this case is $1/2$. The child in fifth grade is a girl, but the other child has probability $1/2$ of being a girl. Situations of this kind arise in real life much more commonly than situations of the other kind, so the answer of $1/3$ is quite nonintuitive.







                      share|cite|improve this answer














                      share|cite|improve this answer



                      share|cite|improve this answer








                      edited Dec 30 '10 at 1:54

























                      answered Dec 23 '10 at 1:05









                      Peter Shor

                      2,5731522




                      2,5731522




















                          up vote
                          63
                          down vote













                          In a family with 2 children there are four possibilities:



                          1) the first child is a boy and the second child is a boy (bb)



                          2) the first child is a boy and the second child is a girl (bg)



                          3) the first child is a girl and the second child is a boy (gb)



                          4) the first child is a girl and the second child is a girl (gg)



                          Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg. Out of those three possibilities the only one with two girls is gg. Hence the probability is $frac13$.






                          share|cite|improve this answer
















                          • 12




                            Your answer is correct for the OP's question, but not how YOU pose the question. Your text says "the first child is a...". That being said, if we know the first child is a girl, then out of bb, bg, gb, and gg, only gb, and gg are left available. Hence the probability is 1/2. Had you phrased it like the OP ("if we have a family with one girl (not necessarily the first child), what is the probability that both are girls?"), then your answer would be correct.
                            – Nemi
                            Dec 21 '10 at 18:06







                          • 3




                            Yes, @Nemi is correct - I think the OP has stated the question such that gb and bg are the same thing.
                            – Maxim Zaslavsky
                            Dec 21 '10 at 20:12






                          • 6




                            @Nemi You are correct that there is an important distinction between "one of the two children is a girl" and "the first child is a girl". My text says: "Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg". I did not say that we are given that the first child is a girl. The three possibilities arise from considering whether the first/second born was a girl/boy.
                            – Travis Service
                            Dec 21 '10 at 20:29










                          • sorry, re-reading your answer now I see that you have stated it correctly. Not sure if I read it wrong earlier or you have edited it. It is most certainly correct as stated.
                            – Nemi
                            Dec 22 '10 at 3:54






                          • 3




                            While true, this doesn't explain the apparent paradox.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 21:30














                          up vote
                          63
                          down vote













                          In a family with 2 children there are four possibilities:



                          1) the first child is a boy and the second child is a boy (bb)



                          2) the first child is a boy and the second child is a girl (bg)



                          3) the first child is a girl and the second child is a boy (gb)



                          4) the first child is a girl and the second child is a girl (gg)



                          Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg. Out of those three possibilities the only one with two girls is gg. Hence the probability is $frac13$.






                          share|cite|improve this answer
















                          • 12




                            Your answer is correct for the OP's question, but not how YOU pose the question. Your text says "the first child is a...". That being said, if we know the first child is a girl, then out of bb, bg, gb, and gg, only gb, and gg are left available. Hence the probability is 1/2. Had you phrased it like the OP ("if we have a family with one girl (not necessarily the first child), what is the probability that both are girls?"), then your answer would be correct.
                            – Nemi
                            Dec 21 '10 at 18:06







                          • 3




                            Yes, @Nemi is correct - I think the OP has stated the question such that gb and bg are the same thing.
                            – Maxim Zaslavsky
                            Dec 21 '10 at 20:12






                          • 6




                            @Nemi You are correct that there is an important distinction between "one of the two children is a girl" and "the first child is a girl". My text says: "Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg". I did not say that we are given that the first child is a girl. The three possibilities arise from considering whether the first/second born was a girl/boy.
                            – Travis Service
                            Dec 21 '10 at 20:29










                          • sorry, re-reading your answer now I see that you have stated it correctly. Not sure if I read it wrong earlier or you have edited it. It is most certainly correct as stated.
                            – Nemi
                            Dec 22 '10 at 3:54






                          • 3




                            While true, this doesn't explain the apparent paradox.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 21:30












                          up vote
                          63
                          down vote










                          up vote
                          63
                          down vote









                          In a family with 2 children there are four possibilities:



                          1) the first child is a boy and the second child is a boy (bb)



                          2) the first child is a boy and the second child is a girl (bg)



                          3) the first child is a girl and the second child is a boy (gb)



                          4) the first child is a girl and the second child is a girl (gg)



                          Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg. Out of those three possibilities the only one with two girls is gg. Hence the probability is $frac13$.






                          share|cite|improve this answer












                          In a family with 2 children there are four possibilities:



                          1) the first child is a boy and the second child is a boy (bb)



                          2) the first child is a boy and the second child is a girl (bg)



                          3) the first child is a girl and the second child is a boy (gb)



                          4) the first child is a girl and the second child is a girl (gg)



                          Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg. Out of those three possibilities the only one with two girls is gg. Hence the probability is $frac13$.







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered Dec 21 '10 at 12:42









                          Travis Service

                          80457




                          80457







                          • 12




                            Your answer is correct for the OP's question, but not how YOU pose the question. Your text says "the first child is a...". That being said, if we know the first child is a girl, then out of bb, bg, gb, and gg, only gb, and gg are left available. Hence the probability is 1/2. Had you phrased it like the OP ("if we have a family with one girl (not necessarily the first child), what is the probability that both are girls?"), then your answer would be correct.
                            – Nemi
                            Dec 21 '10 at 18:06







                          • 3




                            Yes, @Nemi is correct - I think the OP has stated the question such that gb and bg are the same thing.
                            – Maxim Zaslavsky
                            Dec 21 '10 at 20:12






                          • 6




                            @Nemi You are correct that there is an important distinction between "one of the two children is a girl" and "the first child is a girl". My text says: "Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg". I did not say that we are given that the first child is a girl. The three possibilities arise from considering whether the first/second born was a girl/boy.
                            – Travis Service
                            Dec 21 '10 at 20:29










                          • sorry, re-reading your answer now I see that you have stated it correctly. Not sure if I read it wrong earlier or you have edited it. It is most certainly correct as stated.
                            – Nemi
                            Dec 22 '10 at 3:54






                          • 3




                            While true, this doesn't explain the apparent paradox.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 21:30












                          • 12




                            Your answer is correct for the OP's question, but not how YOU pose the question. Your text says "the first child is a...". That being said, if we know the first child is a girl, then out of bb, bg, gb, and gg, only gb, and gg are left available. Hence the probability is 1/2. Had you phrased it like the OP ("if we have a family with one girl (not necessarily the first child), what is the probability that both are girls?"), then your answer would be correct.
                            – Nemi
                            Dec 21 '10 at 18:06







                          • 3




                            Yes, @Nemi is correct - I think the OP has stated the question such that gb and bg are the same thing.
                            – Maxim Zaslavsky
                            Dec 21 '10 at 20:12






                          • 6




                            @Nemi You are correct that there is an important distinction between "one of the two children is a girl" and "the first child is a girl". My text says: "Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg". I did not say that we are given that the first child is a girl. The three possibilities arise from considering whether the first/second born was a girl/boy.
                            – Travis Service
                            Dec 21 '10 at 20:29










                          • sorry, re-reading your answer now I see that you have stated it correctly. Not sure if I read it wrong earlier or you have edited it. It is most certainly correct as stated.
                            – Nemi
                            Dec 22 '10 at 3:54






                          • 3




                            While true, this doesn't explain the apparent paradox.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 21:30







                          12




                          12




                          Your answer is correct for the OP's question, but not how YOU pose the question. Your text says "the first child is a...". That being said, if we know the first child is a girl, then out of bb, bg, gb, and gg, only gb, and gg are left available. Hence the probability is 1/2. Had you phrased it like the OP ("if we have a family with one girl (not necessarily the first child), what is the probability that both are girls?"), then your answer would be correct.
                          – Nemi
                          Dec 21 '10 at 18:06





                          Your answer is correct for the OP's question, but not how YOU pose the question. Your text says "the first child is a...". That being said, if we know the first child is a girl, then out of bb, bg, gb, and gg, only gb, and gg are left available. Hence the probability is 1/2. Had you phrased it like the OP ("if we have a family with one girl (not necessarily the first child), what is the probability that both are girls?"), then your answer would be correct.
                          – Nemi
                          Dec 21 '10 at 18:06





                          3




                          3




                          Yes, @Nemi is correct - I think the OP has stated the question such that gb and bg are the same thing.
                          – Maxim Zaslavsky
                          Dec 21 '10 at 20:12




                          Yes, @Nemi is correct - I think the OP has stated the question such that gb and bg are the same thing.
                          – Maxim Zaslavsky
                          Dec 21 '10 at 20:12




                          6




                          6




                          @Nemi You are correct that there is an important distinction between "one of the two children is a girl" and "the first child is a girl". My text says: "Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg". I did not say that we are given that the first child is a girl. The three possibilities arise from considering whether the first/second born was a girl/boy.
                          – Travis Service
                          Dec 21 '10 at 20:29




                          @Nemi You are correct that there is an important distinction between "one of the two children is a girl" and "the first child is a girl". My text says: "Since we are given that at least one child is a girl there are three possibilities: bg, gb, or gg". I did not say that we are given that the first child is a girl. The three possibilities arise from considering whether the first/second born was a girl/boy.
                          – Travis Service
                          Dec 21 '10 at 20:29












                          sorry, re-reading your answer now I see that you have stated it correctly. Not sure if I read it wrong earlier or you have edited it. It is most certainly correct as stated.
                          – Nemi
                          Dec 22 '10 at 3:54




                          sorry, re-reading your answer now I see that you have stated it correctly. Not sure if I read it wrong earlier or you have edited it. It is most certainly correct as stated.
                          – Nemi
                          Dec 22 '10 at 3:54




                          3




                          3




                          While true, this doesn't explain the apparent paradox.
                          – BlueRaja - Danny Pflughoeft
                          Dec 23 '10 at 21:30




                          While true, this doesn't explain the apparent paradox.
                          – BlueRaja - Danny Pflughoeft
                          Dec 23 '10 at 21:30










                          up vote
                          27
                          down vote













                          I think that the reason that these puzzles are so often confusing is that they rely on the limitations of the English language rather than on any mathematical difficulties. Of course this is not unique to English, and I think you should be able to find similar similar puzzles in pretty much any natural language.



                          Here is an example that brings out the difficulty more clearly. First, consider the following similar puzzle:



                          • A family has two children, Robin and Lindsay. Lindsay is a girl. What is the probability that both children are girls?

                          In elementary probability class, they teach you to answer this by making a table of the four options




                          Robin Lindsay
                          B B
                          * B G
                          G B
                          * G G


                          The starred rows are the ones where Lindsay is a girl, and we compute from them that the chance that both children are girls is 1/2.



                          Now consider this puzzle



                          • A family has two children, Robin and Lindsay. At least one of them is a girl. What is the probability that both children are girls?

                          The elementary probability method gives the following table, and a probability of 1/3. The difference is that we gain one more row, compared to the previous puzzle.




                          Robin Lindsay
                          B B
                          * B G
                          * G B
                          * G G


                          After looking at these, you can see that the difficulty of the original puzzle comes because, in English, "one of the children" can mean several different things:



                          • "one" can mean "a particular one". If you read the original puzzle like this, it becomes analogous to the first puzzle I wrote, and the answer will be 1/2.


                          • "one" can mean "at least one". If you read the original puzzle like this, it become analogous to the second puzzle I wrote, and the answer is 1/3.


                          • "one" can mean "exactly one". If you read the original puzzle like this, the answer is 0.


                          There is a common convention in mathematics that "one" usually means "at least one". For example, this is the sense intended in the following sentence, which is a typical example of mathematical English: "if a natural number $n$ is a multiple of a prime number $p$, and $n = ab$, then one of $a$ and $b$ is divisible by $p$." We would not read this as saying that exactly one of $a$ and $b$ is divisible by $p$.



                          I don't believe this convention is very common in non-mathematical English. If I say, "one of my children is a girl", in normal English this means that the other is a boy. Similar discrepancies between mathematical and non-mathematical English come up with our use of the word "or" and our use of the phrase "if/then". When we teach mathematics, we have to spend time explaining this mathematical argot to students, so they can use the same English conventions that we do.



                          Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks.






                          share|cite|improve this answer




















                          • I now, at last, agree that the correct answer is 1/3. (See my question "2 slightly different situations in which 2 coins are tossed. Does the knowledge of an observer effect the probabilities of the outcomes?" at goo.gl/yyOlK and my comment beginning with "@all: It suddenly hit me that I didn't need that truthful observer.", about half way down the page, under Willie Wong's answer.)
                            – NotSuper
                            Dec 27 '10 at 16:25











                          • (continued) I'd award you the 100 bounty points for the clarity of your explanation of the mathematics, if you hadn't added your idea that "Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks." In my case I wasn't tricked -- I immediately assumed that "one of my children is a girl" meant that "at least one of my children is a girl".
                            – NotSuper
                            Dec 27 '10 at 16:30










                          • (continued) I assumed this because to interpret "one of my children is a girl" as meaning "exactly one of my children is a girl" would destroy the problem/puzzle in that the answer would be too obviously zero. Also, because I had first seen the question in a book about randomness and probability, I was already in math English mode.
                            – NotSuper
                            Dec 27 '10 at 16:30










                          • @NotSuper: Thanks for your kind comments.
                            – Carl Mummert
                            Dec 28 '10 at 3:07






                          • 2




                            Should be the accepted answer.
                            – 355durch113
                            Oct 17 '15 at 3:58














                          up vote
                          27
                          down vote













                          I think that the reason that these puzzles are so often confusing is that they rely on the limitations of the English language rather than on any mathematical difficulties. Of course this is not unique to English, and I think you should be able to find similar similar puzzles in pretty much any natural language.



                          Here is an example that brings out the difficulty more clearly. First, consider the following similar puzzle:



                          • A family has two children, Robin and Lindsay. Lindsay is a girl. What is the probability that both children are girls?

                          In elementary probability class, they teach you to answer this by making a table of the four options




                          Robin Lindsay
                          B B
                          * B G
                          G B
                          * G G


                          The starred rows are the ones where Lindsay is a girl, and we compute from them that the chance that both children are girls is 1/2.



                          Now consider this puzzle



                          • A family has two children, Robin and Lindsay. At least one of them is a girl. What is the probability that both children are girls?

                          The elementary probability method gives the following table, and a probability of 1/3. The difference is that we gain one more row, compared to the previous puzzle.




                          Robin Lindsay
                          B B
                          * B G
                          * G B
                          * G G


                          After looking at these, you can see that the difficulty of the original puzzle comes because, in English, "one of the children" can mean several different things:



                          • "one" can mean "a particular one". If you read the original puzzle like this, it becomes analogous to the first puzzle I wrote, and the answer will be 1/2.


                          • "one" can mean "at least one". If you read the original puzzle like this, it become analogous to the second puzzle I wrote, and the answer is 1/3.


                          • "one" can mean "exactly one". If you read the original puzzle like this, the answer is 0.


                          There is a common convention in mathematics that "one" usually means "at least one". For example, this is the sense intended in the following sentence, which is a typical example of mathematical English: "if a natural number $n$ is a multiple of a prime number $p$, and $n = ab$, then one of $a$ and $b$ is divisible by $p$." We would not read this as saying that exactly one of $a$ and $b$ is divisible by $p$.



                          I don't believe this convention is very common in non-mathematical English. If I say, "one of my children is a girl", in normal English this means that the other is a boy. Similar discrepancies between mathematical and non-mathematical English come up with our use of the word "or" and our use of the phrase "if/then". When we teach mathematics, we have to spend time explaining this mathematical argot to students, so they can use the same English conventions that we do.



                          Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks.






                          share|cite|improve this answer




















                          • I now, at last, agree that the correct answer is 1/3. (See my question "2 slightly different situations in which 2 coins are tossed. Does the knowledge of an observer effect the probabilities of the outcomes?" at goo.gl/yyOlK and my comment beginning with "@all: It suddenly hit me that I didn't need that truthful observer.", about half way down the page, under Willie Wong's answer.)
                            – NotSuper
                            Dec 27 '10 at 16:25











                          • (continued) I'd award you the 100 bounty points for the clarity of your explanation of the mathematics, if you hadn't added your idea that "Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks." In my case I wasn't tricked -- I immediately assumed that "one of my children is a girl" meant that "at least one of my children is a girl".
                            – NotSuper
                            Dec 27 '10 at 16:30










                          • (continued) I assumed this because to interpret "one of my children is a girl" as meaning "exactly one of my children is a girl" would destroy the problem/puzzle in that the answer would be too obviously zero. Also, because I had first seen the question in a book about randomness and probability, I was already in math English mode.
                            – NotSuper
                            Dec 27 '10 at 16:30










                          • @NotSuper: Thanks for your kind comments.
                            – Carl Mummert
                            Dec 28 '10 at 3:07






                          • 2




                            Should be the accepted answer.
                            – 355durch113
                            Oct 17 '15 at 3:58












                          up vote
                          27
                          down vote










                          up vote
                          27
                          down vote









                          I think that the reason that these puzzles are so often confusing is that they rely on the limitations of the English language rather than on any mathematical difficulties. Of course this is not unique to English, and I think you should be able to find similar similar puzzles in pretty much any natural language.



                          Here is an example that brings out the difficulty more clearly. First, consider the following similar puzzle:



                          • A family has two children, Robin and Lindsay. Lindsay is a girl. What is the probability that both children are girls?

                          In elementary probability class, they teach you to answer this by making a table of the four options




                          Robin Lindsay
                          B B
                          * B G
                          G B
                          * G G


                          The starred rows are the ones where Lindsay is a girl, and we compute from them that the chance that both children are girls is 1/2.



                          Now consider this puzzle



                          • A family has two children, Robin and Lindsay. At least one of them is a girl. What is the probability that both children are girls?

                          The elementary probability method gives the following table, and a probability of 1/3. The difference is that we gain one more row, compared to the previous puzzle.




                          Robin Lindsay
                          B B
                          * B G
                          * G B
                          * G G


                          After looking at these, you can see that the difficulty of the original puzzle comes because, in English, "one of the children" can mean several different things:



                          • "one" can mean "a particular one". If you read the original puzzle like this, it becomes analogous to the first puzzle I wrote, and the answer will be 1/2.


                          • "one" can mean "at least one". If you read the original puzzle like this, it become analogous to the second puzzle I wrote, and the answer is 1/3.


                          • "one" can mean "exactly one". If you read the original puzzle like this, the answer is 0.


                          There is a common convention in mathematics that "one" usually means "at least one". For example, this is the sense intended in the following sentence, which is a typical example of mathematical English: "if a natural number $n$ is a multiple of a prime number $p$, and $n = ab$, then one of $a$ and $b$ is divisible by $p$." We would not read this as saying that exactly one of $a$ and $b$ is divisible by $p$.



                          I don't believe this convention is very common in non-mathematical English. If I say, "one of my children is a girl", in normal English this means that the other is a boy. Similar discrepancies between mathematical and non-mathematical English come up with our use of the word "or" and our use of the phrase "if/then". When we teach mathematics, we have to spend time explaining this mathematical argot to students, so they can use the same English conventions that we do.



                          Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks.






                          share|cite|improve this answer












                          I think that the reason that these puzzles are so often confusing is that they rely on the limitations of the English language rather than on any mathematical difficulties. Of course this is not unique to English, and I think you should be able to find similar similar puzzles in pretty much any natural language.



                          Here is an example that brings out the difficulty more clearly. First, consider the following similar puzzle:



                          • A family has two children, Robin and Lindsay. Lindsay is a girl. What is the probability that both children are girls?

                          In elementary probability class, they teach you to answer this by making a table of the four options




                          Robin Lindsay
                          B B
                          * B G
                          G B
                          * G G


                          The starred rows are the ones where Lindsay is a girl, and we compute from them that the chance that both children are girls is 1/2.



                          Now consider this puzzle



                          • A family has two children, Robin and Lindsay. At least one of them is a girl. What is the probability that both children are girls?

                          The elementary probability method gives the following table, and a probability of 1/3. The difference is that we gain one more row, compared to the previous puzzle.




                          Robin Lindsay
                          B B
                          * B G
                          * G B
                          * G G


                          After looking at these, you can see that the difficulty of the original puzzle comes because, in English, "one of the children" can mean several different things:



                          • "one" can mean "a particular one". If you read the original puzzle like this, it becomes analogous to the first puzzle I wrote, and the answer will be 1/2.


                          • "one" can mean "at least one". If you read the original puzzle like this, it become analogous to the second puzzle I wrote, and the answer is 1/3.


                          • "one" can mean "exactly one". If you read the original puzzle like this, the answer is 0.


                          There is a common convention in mathematics that "one" usually means "at least one". For example, this is the sense intended in the following sentence, which is a typical example of mathematical English: "if a natural number $n$ is a multiple of a prime number $p$, and $n = ab$, then one of $a$ and $b$ is divisible by $p$." We would not read this as saying that exactly one of $a$ and $b$ is divisible by $p$.



                          I don't believe this convention is very common in non-mathematical English. If I say, "one of my children is a girl", in normal English this means that the other is a boy. Similar discrepancies between mathematical and non-mathematical English come up with our use of the word "or" and our use of the phrase "if/then". When we teach mathematics, we have to spend time explaining this mathematical argot to students, so they can use the same English conventions that we do.



                          Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks.







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered Dec 23 '10 at 15:47









                          Carl Mummert

                          64k7128238




                          64k7128238











                          • I now, at last, agree that the correct answer is 1/3. (See my question "2 slightly different situations in which 2 coins are tossed. Does the knowledge of an observer effect the probabilities of the outcomes?" at goo.gl/yyOlK and my comment beginning with "@all: It suddenly hit me that I didn't need that truthful observer.", about half way down the page, under Willie Wong's answer.)
                            – NotSuper
                            Dec 27 '10 at 16:25











                          • (continued) I'd award you the 100 bounty points for the clarity of your explanation of the mathematics, if you hadn't added your idea that "Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks." In my case I wasn't tricked -- I immediately assumed that "one of my children is a girl" meant that "at least one of my children is a girl".
                            – NotSuper
                            Dec 27 '10 at 16:30










                          • (continued) I assumed this because to interpret "one of my children is a girl" as meaning "exactly one of my children is a girl" would destroy the problem/puzzle in that the answer would be too obviously zero. Also, because I had first seen the question in a book about randomness and probability, I was already in math English mode.
                            – NotSuper
                            Dec 27 '10 at 16:30










                          • @NotSuper: Thanks for your kind comments.
                            – Carl Mummert
                            Dec 28 '10 at 3:07






                          • 2




                            Should be the accepted answer.
                            – 355durch113
                            Oct 17 '15 at 3:58
















                          • I now, at last, agree that the correct answer is 1/3. (See my question "2 slightly different situations in which 2 coins are tossed. Does the knowledge of an observer effect the probabilities of the outcomes?" at goo.gl/yyOlK and my comment beginning with "@all: It suddenly hit me that I didn't need that truthful observer.", about half way down the page, under Willie Wong's answer.)
                            – NotSuper
                            Dec 27 '10 at 16:25











                          • (continued) I'd award you the 100 bounty points for the clarity of your explanation of the mathematics, if you hadn't added your idea that "Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks." In my case I wasn't tricked -- I immediately assumed that "one of my children is a girl" meant that "at least one of my children is a girl".
                            – NotSuper
                            Dec 27 '10 at 16:30










                          • (continued) I assumed this because to interpret "one of my children is a girl" as meaning "exactly one of my children is a girl" would destroy the problem/puzzle in that the answer would be too obviously zero. Also, because I had first seen the question in a book about randomness and probability, I was already in math English mode.
                            – NotSuper
                            Dec 27 '10 at 16:30










                          • @NotSuper: Thanks for your kind comments.
                            – Carl Mummert
                            Dec 28 '10 at 3:07






                          • 2




                            Should be the accepted answer.
                            – 355durch113
                            Oct 17 '15 at 3:58















                          I now, at last, agree that the correct answer is 1/3. (See my question "2 slightly different situations in which 2 coins are tossed. Does the knowledge of an observer effect the probabilities of the outcomes?" at goo.gl/yyOlK and my comment beginning with "@all: It suddenly hit me that I didn't need that truthful observer.", about half way down the page, under Willie Wong's answer.)
                          – NotSuper
                          Dec 27 '10 at 16:25





                          I now, at last, agree that the correct answer is 1/3. (See my question "2 slightly different situations in which 2 coins are tossed. Does the knowledge of an observer effect the probabilities of the outcomes?" at goo.gl/yyOlK and my comment beginning with "@all: It suddenly hit me that I didn't need that truthful observer.", about half way down the page, under Willie Wong's answer.)
                          – NotSuper
                          Dec 27 '10 at 16:25













                          (continued) I'd award you the 100 bounty points for the clarity of your explanation of the mathematics, if you hadn't added your idea that "Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks." In my case I wasn't tricked -- I immediately assumed that "one of my children is a girl" meant that "at least one of my children is a girl".
                          – NotSuper
                          Dec 27 '10 at 16:30




                          (continued) I'd award you the 100 bounty points for the clarity of your explanation of the mathematics, if you hadn't added your idea that "Probability puzzles like the one you're asking about rely on these differences of English meaning, rather than on any logical or mathematical problem. In that sense, they aren't really puzzles, they're just tricks." In my case I wasn't tricked -- I immediately assumed that "one of my children is a girl" meant that "at least one of my children is a girl".
                          – NotSuper
                          Dec 27 '10 at 16:30












                          (continued) I assumed this because to interpret "one of my children is a girl" as meaning "exactly one of my children is a girl" would destroy the problem/puzzle in that the answer would be too obviously zero. Also, because I had first seen the question in a book about randomness and probability, I was already in math English mode.
                          – NotSuper
                          Dec 27 '10 at 16:30




                          (continued) I assumed this because to interpret "one of my children is a girl" as meaning "exactly one of my children is a girl" would destroy the problem/puzzle in that the answer would be too obviously zero. Also, because I had first seen the question in a book about randomness and probability, I was already in math English mode.
                          – NotSuper
                          Dec 27 '10 at 16:30












                          @NotSuper: Thanks for your kind comments.
                          – Carl Mummert
                          Dec 28 '10 at 3:07




                          @NotSuper: Thanks for your kind comments.
                          – Carl Mummert
                          Dec 28 '10 at 3:07




                          2




                          2




                          Should be the accepted answer.
                          – 355durch113
                          Oct 17 '15 at 3:58




                          Should be the accepted answer.
                          – 355durch113
                          Oct 17 '15 at 3:58










                          up vote
                          21
                          down vote













                          Please ignore the old comments - I have radically changed my answer. It's long, but bear with me - after reading this answer, you should feel intuitively-comfortable with almost all other probabilistic fallacies, not just this one.



                          The first and most important thing is to define what we mean by "probability." Let's define it to mean "the expected percentage of positive outcomes when repeating an observation over a large sample" (Go ahead, read that again, more slowly). Also, let's call the method we use to chose this large sample the "model."



                          This may sound trivial, but it has some important implications. For example, what is the probability you will die before age 40? According to our definition, this question has no meaning: we can't observe you multiple times before age forty and record how many times you die. Instead, we observe other people below age 40, and record how many of them die.



                          So let's say we observe all other people on earth below age 40 (our model), and find that 1/2 of them die before hitting the big four-oh. Does this mean the probability of you dying before 40 is 50%? Well, according to this model, it does! However, this is hardly a fair model. Perhaps you live in a first-world country - now when we revise our model to include only people below 40 in first-world countries, your chances become a much-less-grim 1/10†. But you are also not a smoker, and don't live in the city, and ride your bike on Sundays with your wife and crazy mother-in-law, which makes your chances 123/4567! That's much better... however, our model still doesn't take into account that you are also an avid skydiver ;)
                          † I am pulling these numbers out of nowhere, they are not real statistics.



                          So the point is, asking for a "probability" only makes sense in the context of a certain model - a way of repeating our observation many times. Without that, asking for a probability is meaningless.




                          Now, back to the original question. Before we can assign a probability, we must choose a model; how are we choosing the families to sample from? I see two obvious choices, which will lead to different answers:



                          1. Consider only families which have two children, one of whom is a girl, and choose one randomly.

                          2. Consider only girls who have exactly one sibling, and choose one randomly.

                          Do you see the difference? In the first case, every family has the same probability of being chosen. However, in the second case, the families with two girls are more likely to be chosen than families with only one girl, because every girl has an equal chance of being chosen: the two-girl families have doubled their odds by having two girls. If children were raffle tickets, they would have bought two tickets while the one-girl families bought only one.



                          Thus, we should expect the probabilities in these two cases to be different. Let's calculate them more rigorously (writing BG to mean "boy was born, then girl):



                          1. There are three equally probable family-types: BG, GB, and GG (BB was removed from consideration, because they have no girls). Since only one of the three has two girls, our chances of having two girls are 1/3.

                          2. We have the same possibilities as above, but now GG is twice as likely as BG or GB. Thus, the probabilities are GG: 2/4, GB: 1/4, and BG: 1/4, meaning the probability of a girl-sibling is 2/4 = 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

                          Here lies the fallacy: the model our intuition assumes is the second one, but the way the problem is worded strongly implies the first one. When we think in terms of "randomly choosing a family (over a large number of families)," our intuition meshes perfectly with the result.




                          Let's take a look at another similar problem




                          In a family of two children, where the oldest child is a girl, what is the probability they are both girls?




                          Once again, I can see two different, plausible models for observing our random sample:



                          1. Consider only families with two children, the oldest of whom is a girl, and choose one randomly.

                          2. Consider only girls who have exactly one (younger) sibling, and choose one at random.

                          So once again, the question strongly implies the first model, though arguments for either could be made. However, when we actually calculate the probability...



                          1. Same as before, but we've also eliminated BG, where the first child was a boy. This leaves only two equally-probable possibilities, GB and GG. Thus, the chances are 1/2.

                          2. We've also eliminated BG from this case, leaving GB and GG. However, unlike the original question, GG is no longer twice as likely, since the younger child can no longer be the one who was randomly chosen. Thus, GB and GG are equally likely, and we again have a probability of 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

                          ...we find that the choice between these two models doesn't matter, because in this case both have the same probability! Among two-child families with an older daughter, it doesn't matter if we randomly choose the family or the older-daughter, because in both cases there is only one of each per family.




                          Hopefully that all made sense. For bonus points, try applying this reasoning to the Monty Hall problem. What is our model - how are we making repeated observations? Why does it clash with our intuition?



                          For even more bonus points, try to figure out the following question; the math isn't too difficult, but it took me a long while to figure out why, intuitively, the answer should be correct:




                          In a family of two children, one of whom is a girl named Florida, what are the chances of two girls?




                          (If you have troubles, post a question and leave a link to it in the comments, and I'll try to answer it there as best I can :) )






                          share|cite|improve this answer


















                          • 4




                            I don't completely agree with this analysis. Mathematically, there is no problem at all. Once we list the possible events and the favorable events, the probability is just the corresponding fraction. So the difficulty in these puzzles is just that English hasn't evolved a clear way to describe a probability space. There are two reasonable probability spaces here, and usual English is not clear about which of them is intended. But regardless what anyone knows about the children, I can still compute the probabilities in both these spaces, to get 1/3 and 1/2, respectively.
                            – Carl Mummert
                            Dec 23 '10 at 15:06











                          • @Carl: Then you must believe that the Monty Hall problem is ambiguous in exactly the same way, no? I'm afraid I have to disagree with that.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 17:15







                          • 2




                            I do feel that the difficulty in the Monty Hall problem is also not "mathematical", in the sense that as soon as any probability model has been rigorously stated then the probabilities can be calculated using basic methods. The arguments about the Monty Hall problem stem from a lack of rigour in the statement, which I think is often intentional in order to make an otherwise easy probability exercise into a "puzzle".
                            – Carl Mummert
                            Dec 23 '10 at 17:44










                          • @Carl: In both cases, it's not that it's ambiguous or not mathematically-defined, it's just counter-intuitive.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 21:29






                          • 2




                            The two puzzles are similar, and you've made a nice comparison. They each have two simple probability models, and it's clear which gives the clever, counter-intuitive solution. But if you admit any more complexity, there's a lot of ambiguity in both problems. For example, given a room full of people convinced that switching doors is the right strategy, an adversarial Monty Hall can make sure no one wins by offering the switch only to those who first choose the car. In real life, I would assume ratings and TV gimmicks affect what choices I'm given, and I have no probability model for that!
                            – Jonas Kibelbek
                            Dec 23 '10 at 23:04














                          up vote
                          21
                          down vote













                          Please ignore the old comments - I have radically changed my answer. It's long, but bear with me - after reading this answer, you should feel intuitively-comfortable with almost all other probabilistic fallacies, not just this one.



                          The first and most important thing is to define what we mean by "probability." Let's define it to mean "the expected percentage of positive outcomes when repeating an observation over a large sample" (Go ahead, read that again, more slowly). Also, let's call the method we use to chose this large sample the "model."



                          This may sound trivial, but it has some important implications. For example, what is the probability you will die before age 40? According to our definition, this question has no meaning: we can't observe you multiple times before age forty and record how many times you die. Instead, we observe other people below age 40, and record how many of them die.



                          So let's say we observe all other people on earth below age 40 (our model), and find that 1/2 of them die before hitting the big four-oh. Does this mean the probability of you dying before 40 is 50%? Well, according to this model, it does! However, this is hardly a fair model. Perhaps you live in a first-world country - now when we revise our model to include only people below 40 in first-world countries, your chances become a much-less-grim 1/10†. But you are also not a smoker, and don't live in the city, and ride your bike on Sundays with your wife and crazy mother-in-law, which makes your chances 123/4567! That's much better... however, our model still doesn't take into account that you are also an avid skydiver ;)
                          † I am pulling these numbers out of nowhere, they are not real statistics.



                          So the point is, asking for a "probability" only makes sense in the context of a certain model - a way of repeating our observation many times. Without that, asking for a probability is meaningless.




                          Now, back to the original question. Before we can assign a probability, we must choose a model; how are we choosing the families to sample from? I see two obvious choices, which will lead to different answers:



                          1. Consider only families which have two children, one of whom is a girl, and choose one randomly.

                          2. Consider only girls who have exactly one sibling, and choose one randomly.

                          Do you see the difference? In the first case, every family has the same probability of being chosen. However, in the second case, the families with two girls are more likely to be chosen than families with only one girl, because every girl has an equal chance of being chosen: the two-girl families have doubled their odds by having two girls. If children were raffle tickets, they would have bought two tickets while the one-girl families bought only one.



                          Thus, we should expect the probabilities in these two cases to be different. Let's calculate them more rigorously (writing BG to mean "boy was born, then girl):



                          1. There are three equally probable family-types: BG, GB, and GG (BB was removed from consideration, because they have no girls). Since only one of the three has two girls, our chances of having two girls are 1/3.

                          2. We have the same possibilities as above, but now GG is twice as likely as BG or GB. Thus, the probabilities are GG: 2/4, GB: 1/4, and BG: 1/4, meaning the probability of a girl-sibling is 2/4 = 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

                          Here lies the fallacy: the model our intuition assumes is the second one, but the way the problem is worded strongly implies the first one. When we think in terms of "randomly choosing a family (over a large number of families)," our intuition meshes perfectly with the result.




                          Let's take a look at another similar problem




                          In a family of two children, where the oldest child is a girl, what is the probability they are both girls?




                          Once again, I can see two different, plausible models for observing our random sample:



                          1. Consider only families with two children, the oldest of whom is a girl, and choose one randomly.

                          2. Consider only girls who have exactly one (younger) sibling, and choose one at random.

                          So once again, the question strongly implies the first model, though arguments for either could be made. However, when we actually calculate the probability...



                          1. Same as before, but we've also eliminated BG, where the first child was a boy. This leaves only two equally-probable possibilities, GB and GG. Thus, the chances are 1/2.

                          2. We've also eliminated BG from this case, leaving GB and GG. However, unlike the original question, GG is no longer twice as likely, since the younger child can no longer be the one who was randomly chosen. Thus, GB and GG are equally likely, and we again have a probability of 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

                          ...we find that the choice between these two models doesn't matter, because in this case both have the same probability! Among two-child families with an older daughter, it doesn't matter if we randomly choose the family or the older-daughter, because in both cases there is only one of each per family.




                          Hopefully that all made sense. For bonus points, try applying this reasoning to the Monty Hall problem. What is our model - how are we making repeated observations? Why does it clash with our intuition?



                          For even more bonus points, try to figure out the following question; the math isn't too difficult, but it took me a long while to figure out why, intuitively, the answer should be correct:




                          In a family of two children, one of whom is a girl named Florida, what are the chances of two girls?




                          (If you have troubles, post a question and leave a link to it in the comments, and I'll try to answer it there as best I can :) )






                          share|cite|improve this answer


















                          • 4




                            I don't completely agree with this analysis. Mathematically, there is no problem at all. Once we list the possible events and the favorable events, the probability is just the corresponding fraction. So the difficulty in these puzzles is just that English hasn't evolved a clear way to describe a probability space. There are two reasonable probability spaces here, and usual English is not clear about which of them is intended. But regardless what anyone knows about the children, I can still compute the probabilities in both these spaces, to get 1/3 and 1/2, respectively.
                            – Carl Mummert
                            Dec 23 '10 at 15:06











                          • @Carl: Then you must believe that the Monty Hall problem is ambiguous in exactly the same way, no? I'm afraid I have to disagree with that.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 17:15







                          • 2




                            I do feel that the difficulty in the Monty Hall problem is also not "mathematical", in the sense that as soon as any probability model has been rigorously stated then the probabilities can be calculated using basic methods. The arguments about the Monty Hall problem stem from a lack of rigour in the statement, which I think is often intentional in order to make an otherwise easy probability exercise into a "puzzle".
                            – Carl Mummert
                            Dec 23 '10 at 17:44










                          • @Carl: In both cases, it's not that it's ambiguous or not mathematically-defined, it's just counter-intuitive.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 21:29






                          • 2




                            The two puzzles are similar, and you've made a nice comparison. They each have two simple probability models, and it's clear which gives the clever, counter-intuitive solution. But if you admit any more complexity, there's a lot of ambiguity in both problems. For example, given a room full of people convinced that switching doors is the right strategy, an adversarial Monty Hall can make sure no one wins by offering the switch only to those who first choose the car. In real life, I would assume ratings and TV gimmicks affect what choices I'm given, and I have no probability model for that!
                            – Jonas Kibelbek
                            Dec 23 '10 at 23:04












                          up vote
                          21
                          down vote










                          up vote
                          21
                          down vote









                          Please ignore the old comments - I have radically changed my answer. It's long, but bear with me - after reading this answer, you should feel intuitively-comfortable with almost all other probabilistic fallacies, not just this one.



                          The first and most important thing is to define what we mean by "probability." Let's define it to mean "the expected percentage of positive outcomes when repeating an observation over a large sample" (Go ahead, read that again, more slowly). Also, let's call the method we use to chose this large sample the "model."



                          This may sound trivial, but it has some important implications. For example, what is the probability you will die before age 40? According to our definition, this question has no meaning: we can't observe you multiple times before age forty and record how many times you die. Instead, we observe other people below age 40, and record how many of them die.



                          So let's say we observe all other people on earth below age 40 (our model), and find that 1/2 of them die before hitting the big four-oh. Does this mean the probability of you dying before 40 is 50%? Well, according to this model, it does! However, this is hardly a fair model. Perhaps you live in a first-world country - now when we revise our model to include only people below 40 in first-world countries, your chances become a much-less-grim 1/10†. But you are also not a smoker, and don't live in the city, and ride your bike on Sundays with your wife and crazy mother-in-law, which makes your chances 123/4567! That's much better... however, our model still doesn't take into account that you are also an avid skydiver ;)
                          † I am pulling these numbers out of nowhere, they are not real statistics.



                          So the point is, asking for a "probability" only makes sense in the context of a certain model - a way of repeating our observation many times. Without that, asking for a probability is meaningless.




                          Now, back to the original question. Before we can assign a probability, we must choose a model; how are we choosing the families to sample from? I see two obvious choices, which will lead to different answers:



                          1. Consider only families which have two children, one of whom is a girl, and choose one randomly.

                          2. Consider only girls who have exactly one sibling, and choose one randomly.

                          Do you see the difference? In the first case, every family has the same probability of being chosen. However, in the second case, the families with two girls are more likely to be chosen than families with only one girl, because every girl has an equal chance of being chosen: the two-girl families have doubled their odds by having two girls. If children were raffle tickets, they would have bought two tickets while the one-girl families bought only one.



                          Thus, we should expect the probabilities in these two cases to be different. Let's calculate them more rigorously (writing BG to mean "boy was born, then girl):



                          1. There are three equally probable family-types: BG, GB, and GG (BB was removed from consideration, because they have no girls). Since only one of the three has two girls, our chances of having two girls are 1/3.

                          2. We have the same possibilities as above, but now GG is twice as likely as BG or GB. Thus, the probabilities are GG: 2/4, GB: 1/4, and BG: 1/4, meaning the probability of a girl-sibling is 2/4 = 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

                          Here lies the fallacy: the model our intuition assumes is the second one, but the way the problem is worded strongly implies the first one. When we think in terms of "randomly choosing a family (over a large number of families)," our intuition meshes perfectly with the result.




                          Let's take a look at another similar problem




                          In a family of two children, where the oldest child is a girl, what is the probability they are both girls?




                          Once again, I can see two different, plausible models for observing our random sample:



                          1. Consider only families with two children, the oldest of whom is a girl, and choose one randomly.

                          2. Consider only girls who have exactly one (younger) sibling, and choose one at random.

                          So once again, the question strongly implies the first model, though arguments for either could be made. However, when we actually calculate the probability...



                          1. Same as before, but we've also eliminated BG, where the first child was a boy. This leaves only two equally-probable possibilities, GB and GG. Thus, the chances are 1/2.

                          2. We've also eliminated BG from this case, leaving GB and GG. However, unlike the original question, GG is no longer twice as likely, since the younger child can no longer be the one who was randomly chosen. Thus, GB and GG are equally likely, and we again have a probability of 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

                          ...we find that the choice between these two models doesn't matter, because in this case both have the same probability! Among two-child families with an older daughter, it doesn't matter if we randomly choose the family or the older-daughter, because in both cases there is only one of each per family.




                          Hopefully that all made sense. For bonus points, try applying this reasoning to the Monty Hall problem. What is our model - how are we making repeated observations? Why does it clash with our intuition?



                          For even more bonus points, try to figure out the following question; the math isn't too difficult, but it took me a long while to figure out why, intuitively, the answer should be correct:




                          In a family of two children, one of whom is a girl named Florida, what are the chances of two girls?




                          (If you have troubles, post a question and leave a link to it in the comments, and I'll try to answer it there as best I can :) )






                          share|cite|improve this answer














                          Please ignore the old comments - I have radically changed my answer. It's long, but bear with me - after reading this answer, you should feel intuitively-comfortable with almost all other probabilistic fallacies, not just this one.



                          The first and most important thing is to define what we mean by "probability." Let's define it to mean "the expected percentage of positive outcomes when repeating an observation over a large sample" (Go ahead, read that again, more slowly). Also, let's call the method we use to chose this large sample the "model."



                          This may sound trivial, but it has some important implications. For example, what is the probability you will die before age 40? According to our definition, this question has no meaning: we can't observe you multiple times before age forty and record how many times you die. Instead, we observe other people below age 40, and record how many of them die.



                          So let's say we observe all other people on earth below age 40 (our model), and find that 1/2 of them die before hitting the big four-oh. Does this mean the probability of you dying before 40 is 50%? Well, according to this model, it does! However, this is hardly a fair model. Perhaps you live in a first-world country - now when we revise our model to include only people below 40 in first-world countries, your chances become a much-less-grim 1/10†. But you are also not a smoker, and don't live in the city, and ride your bike on Sundays with your wife and crazy mother-in-law, which makes your chances 123/4567! That's much better... however, our model still doesn't take into account that you are also an avid skydiver ;)
                          † I am pulling these numbers out of nowhere, they are not real statistics.



                          So the point is, asking for a "probability" only makes sense in the context of a certain model - a way of repeating our observation many times. Without that, asking for a probability is meaningless.




                          Now, back to the original question. Before we can assign a probability, we must choose a model; how are we choosing the families to sample from? I see two obvious choices, which will lead to different answers:



                          1. Consider only families which have two children, one of whom is a girl, and choose one randomly.

                          2. Consider only girls who have exactly one sibling, and choose one randomly.

                          Do you see the difference? In the first case, every family has the same probability of being chosen. However, in the second case, the families with two girls are more likely to be chosen than families with only one girl, because every girl has an equal chance of being chosen: the two-girl families have doubled their odds by having two girls. If children were raffle tickets, they would have bought two tickets while the one-girl families bought only one.



                          Thus, we should expect the probabilities in these two cases to be different. Let's calculate them more rigorously (writing BG to mean "boy was born, then girl):



                          1. There are three equally probable family-types: BG, GB, and GG (BB was removed from consideration, because they have no girls). Since only one of the three has two girls, our chances of having two girls are 1/3.

                          2. We have the same possibilities as above, but now GG is twice as likely as BG or GB. Thus, the probabilities are GG: 2/4, GB: 1/4, and BG: 1/4, meaning the probability of a girl-sibling is 2/4 = 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

                          Here lies the fallacy: the model our intuition assumes is the second one, but the way the problem is worded strongly implies the first one. When we think in terms of "randomly choosing a family (over a large number of families)," our intuition meshes perfectly with the result.




                          Let's take a look at another similar problem




                          In a family of two children, where the oldest child is a girl, what is the probability they are both girls?




                          Once again, I can see two different, plausible models for observing our random sample:



                          1. Consider only families with two children, the oldest of whom is a girl, and choose one randomly.

                          2. Consider only girls who have exactly one (younger) sibling, and choose one at random.

                          So once again, the question strongly implies the first model, though arguments for either could be made. However, when we actually calculate the probability...



                          1. Same as before, but we've also eliminated BG, where the first child was a boy. This leaves only two equally-probable possibilities, GB and GG. Thus, the chances are 1/2.

                          2. We've also eliminated BG from this case, leaving GB and GG. However, unlike the original question, GG is no longer twice as likely, since the younger child can no longer be the one who was randomly chosen. Thus, GB and GG are equally likely, and we again have a probability of 1/2 (alternatively, we could have noted that there are only two equally-probable possibilities for the sibling: boy or girl).

                          ...we find that the choice between these two models doesn't matter, because in this case both have the same probability! Among two-child families with an older daughter, it doesn't matter if we randomly choose the family or the older-daughter, because in both cases there is only one of each per family.




                          Hopefully that all made sense. For bonus points, try applying this reasoning to the Monty Hall problem. What is our model - how are we making repeated observations? Why does it clash with our intuition?



                          For even more bonus points, try to figure out the following question; the math isn't too difficult, but it took me a long while to figure out why, intuitively, the answer should be correct:




                          In a family of two children, one of whom is a girl named Florida, what are the chances of two girls?




                          (If you have troubles, post a question and leave a link to it in the comments, and I'll try to answer it there as best I can :) )







                          share|cite|improve this answer














                          share|cite|improve this answer



                          share|cite|improve this answer








                          edited Jan 31 '14 at 16:57

























                          answered Dec 21 '10 at 18:36









                          BlueRaja - Danny Pflughoeft

                          5,34432842




                          5,34432842







                          • 4




                            I don't completely agree with this analysis. Mathematically, there is no problem at all. Once we list the possible events and the favorable events, the probability is just the corresponding fraction. So the difficulty in these puzzles is just that English hasn't evolved a clear way to describe a probability space. There are two reasonable probability spaces here, and usual English is not clear about which of them is intended. But regardless what anyone knows about the children, I can still compute the probabilities in both these spaces, to get 1/3 and 1/2, respectively.
                            – Carl Mummert
                            Dec 23 '10 at 15:06











                          • @Carl: Then you must believe that the Monty Hall problem is ambiguous in exactly the same way, no? I'm afraid I have to disagree with that.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 17:15







                          • 2




                            I do feel that the difficulty in the Monty Hall problem is also not "mathematical", in the sense that as soon as any probability model has been rigorously stated then the probabilities can be calculated using basic methods. The arguments about the Monty Hall problem stem from a lack of rigour in the statement, which I think is often intentional in order to make an otherwise easy probability exercise into a "puzzle".
                            – Carl Mummert
                            Dec 23 '10 at 17:44










                          • @Carl: In both cases, it's not that it's ambiguous or not mathematically-defined, it's just counter-intuitive.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 21:29






                          • 2




                            The two puzzles are similar, and you've made a nice comparison. They each have two simple probability models, and it's clear which gives the clever, counter-intuitive solution. But if you admit any more complexity, there's a lot of ambiguity in both problems. For example, given a room full of people convinced that switching doors is the right strategy, an adversarial Monty Hall can make sure no one wins by offering the switch only to those who first choose the car. In real life, I would assume ratings and TV gimmicks affect what choices I'm given, and I have no probability model for that!
                            – Jonas Kibelbek
                            Dec 23 '10 at 23:04












                          • 4




                            I don't completely agree with this analysis. Mathematically, there is no problem at all. Once we list the possible events and the favorable events, the probability is just the corresponding fraction. So the difficulty in these puzzles is just that English hasn't evolved a clear way to describe a probability space. There are two reasonable probability spaces here, and usual English is not clear about which of them is intended. But regardless what anyone knows about the children, I can still compute the probabilities in both these spaces, to get 1/3 and 1/2, respectively.
                            – Carl Mummert
                            Dec 23 '10 at 15:06











                          • @Carl: Then you must believe that the Monty Hall problem is ambiguous in exactly the same way, no? I'm afraid I have to disagree with that.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 17:15







                          • 2




                            I do feel that the difficulty in the Monty Hall problem is also not "mathematical", in the sense that as soon as any probability model has been rigorously stated then the probabilities can be calculated using basic methods. The arguments about the Monty Hall problem stem from a lack of rigour in the statement, which I think is often intentional in order to make an otherwise easy probability exercise into a "puzzle".
                            – Carl Mummert
                            Dec 23 '10 at 17:44










                          • @Carl: In both cases, it's not that it's ambiguous or not mathematically-defined, it's just counter-intuitive.
                            – BlueRaja - Danny Pflughoeft
                            Dec 23 '10 at 21:29






                          • 2




                            The two puzzles are similar, and you've made a nice comparison. They each have two simple probability models, and it's clear which gives the clever, counter-intuitive solution. But if you admit any more complexity, there's a lot of ambiguity in both problems. For example, given a room full of people convinced that switching doors is the right strategy, an adversarial Monty Hall can make sure no one wins by offering the switch only to those who first choose the car. In real life, I would assume ratings and TV gimmicks affect what choices I'm given, and I have no probability model for that!
                            – Jonas Kibelbek
                            Dec 23 '10 at 23:04







                          4




                          4




                          I don't completely agree with this analysis. Mathematically, there is no problem at all. Once we list the possible events and the favorable events, the probability is just the corresponding fraction. So the difficulty in these puzzles is just that English hasn't evolved a clear way to describe a probability space. There are two reasonable probability spaces here, and usual English is not clear about which of them is intended. But regardless what anyone knows about the children, I can still compute the probabilities in both these spaces, to get 1/3 and 1/2, respectively.
                          – Carl Mummert
                          Dec 23 '10 at 15:06





                          I don't completely agree with this analysis. Mathematically, there is no problem at all. Once we list the possible events and the favorable events, the probability is just the corresponding fraction. So the difficulty in these puzzles is just that English hasn't evolved a clear way to describe a probability space. There are two reasonable probability spaces here, and usual English is not clear about which of them is intended. But regardless what anyone knows about the children, I can still compute the probabilities in both these spaces, to get 1/3 and 1/2, respectively.
                          – Carl Mummert
                          Dec 23 '10 at 15:06













                          @Carl: Then you must believe that the Monty Hall problem is ambiguous in exactly the same way, no? I'm afraid I have to disagree with that.
                          – BlueRaja - Danny Pflughoeft
                          Dec 23 '10 at 17:15





                          @Carl: Then you must believe that the Monty Hall problem is ambiguous in exactly the same way, no? I'm afraid I have to disagree with that.
                          – BlueRaja - Danny Pflughoeft
                          Dec 23 '10 at 17:15





                          2




                          2




                          I do feel that the difficulty in the Monty Hall problem is also not "mathematical", in the sense that as soon as any probability model has been rigorously stated then the probabilities can be calculated using basic methods. The arguments about the Monty Hall problem stem from a lack of rigour in the statement, which I think is often intentional in order to make an otherwise easy probability exercise into a "puzzle".
                          – Carl Mummert
                          Dec 23 '10 at 17:44




                          I do feel that the difficulty in the Monty Hall problem is also not "mathematical", in the sense that as soon as any probability model has been rigorously stated then the probabilities can be calculated using basic methods. The arguments about the Monty Hall problem stem from a lack of rigour in the statement, which I think is often intentional in order to make an otherwise easy probability exercise into a "puzzle".
                          – Carl Mummert
                          Dec 23 '10 at 17:44












                          @Carl: In both cases, it's not that it's ambiguous or not mathematically-defined, it's just counter-intuitive.
                          – BlueRaja - Danny Pflughoeft
                          Dec 23 '10 at 21:29




                          @Carl: In both cases, it's not that it's ambiguous or not mathematically-defined, it's just counter-intuitive.
                          – BlueRaja - Danny Pflughoeft
                          Dec 23 '10 at 21:29




                          2




                          2




                          The two puzzles are similar, and you've made a nice comparison. They each have two simple probability models, and it's clear which gives the clever, counter-intuitive solution. But if you admit any more complexity, there's a lot of ambiguity in both problems. For example, given a room full of people convinced that switching doors is the right strategy, an adversarial Monty Hall can make sure no one wins by offering the switch only to those who first choose the car. In real life, I would assume ratings and TV gimmicks affect what choices I'm given, and I have no probability model for that!
                          – Jonas Kibelbek
                          Dec 23 '10 at 23:04




                          The two puzzles are similar, and you've made a nice comparison. They each have two simple probability models, and it's clear which gives the clever, counter-intuitive solution. But if you admit any more complexity, there's a lot of ambiguity in both problems. For example, given a room full of people convinced that switching doors is the right strategy, an adversarial Monty Hall can make sure no one wins by offering the switch only to those who first choose the car. In real life, I would assume ratings and TV gimmicks affect what choices I'm given, and I have no probability model for that!
                          – Jonas Kibelbek
                          Dec 23 '10 at 23:04










                          up vote
                          10
                          down vote













                          Here's some idea.



                          Suppose you sample $4n$ families with two children, where $n$ is a very large integer. Then, with a very high probability, about $n$ of them have two boys, about $n$ have two girls, and about $2n$ have one girl and one boy.
                          Since you ignore the families with two boys, the desired probability is given by $n/(3n) = 1/3$.



                          EDIT: The above solution corresponds to the following situation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you check if one of the kids is a girl. If no, you ignore this family. If yes, you check if both kids are girls.



                          EDIT: Among all the OP's examples, only one is relevant here. Namely, "I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no?". Well, in this situation the probability is indeed $1/2$, assuming the following interpretation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you wait until one of the kids comes into the room. If it is a boy, you ignore this family; otherwise you check if both kids are girls. It is readily understood that this leads to a probability of $1/2$.






                          share|cite|improve this answer


















                          • 4




                            Of course, a lot depends on how we interpret the question.
                            – Shai Covo
                            Dec 21 '10 at 12:52






                          • 1




                            @NotSuper: It all depends a lot on the precise formulation, as Shai already pointed out. Let's say you visit a friend who flips a coin twice. Given that at least one of the outcomes is Heads, the probability of two Heads is indeed 1/3.
                            – Hendrik Vogt
                            Dec 21 '10 at 13:35






                          • 1




                            @NotSuper: Shai is right indeed: You can see only one of them, say the one closer to you, and it's a head. So you know more than in the experiment I formulated. In that experiment, I assumed that you know that at least one of the two outcomes is Heads, but if it's only one head, then you don't know which coin it is! In your experiment you know that that coin lying closer to you is a head, so it's a different experiment.
                            – Hendrik Vogt
                            Dec 21 '10 at 15:38







                          • 1




                            @NotSuper: I understand that this is a bit difficult. Let me put my experiment in different words: When I write "Given that at least one of the outcomes is Heads", then this means: someone looked at both coins and provided you with the information that one of the outcomes is Heads. In your experiment, you look only at one of the coins.
                            – Hendrik Vogt
                            Dec 21 '10 at 17:46






                          • 1




                            @NotSuper: No. Looking at only one coin and looking a two coins gives different knowledge. Sorry, I can't explain it better.
                            – Hendrik Vogt
                            Dec 21 '10 at 18:14














                          up vote
                          10
                          down vote













                          Here's some idea.



                          Suppose you sample $4n$ families with two children, where $n$ is a very large integer. Then, with a very high probability, about $n$ of them have two boys, about $n$ have two girls, and about $2n$ have one girl and one boy.
                          Since you ignore the families with two boys, the desired probability is given by $n/(3n) = 1/3$.



                          EDIT: The above solution corresponds to the following situation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you check if one of the kids is a girl. If no, you ignore this family. If yes, you check if both kids are girls.



                          EDIT: Among all the OP's examples, only one is relevant here. Namely, "I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no?". Well, in this situation the probability is indeed $1/2$, assuming the following interpretation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you wait until one of the kids comes into the room. If it is a boy, you ignore this family; otherwise you check if both kids are girls. It is readily understood that this leads to a probability of $1/2$.






                          share|cite|improve this answer


















                          • 4




                            Of course, a lot depends on how we interpret the question.
                            – Shai Covo
                            Dec 21 '10 at 12:52






                          • 1




                            @NotSuper: It all depends a lot on the precise formulation, as Shai already pointed out. Let's say you visit a friend who flips a coin twice. Given that at least one of the outcomes is Heads, the probability of two Heads is indeed 1/3.
                            – Hendrik Vogt
                            Dec 21 '10 at 13:35






                          • 1




                            @NotSuper: Shai is right indeed: You can see only one of them, say the one closer to you, and it's a head. So you know more than in the experiment I formulated. In that experiment, I assumed that you know that at least one of the two outcomes is Heads, but if it's only one head, then you don't know which coin it is! In your experiment you know that that coin lying closer to you is a head, so it's a different experiment.
                            – Hendrik Vogt
                            Dec 21 '10 at 15:38







                          • 1




                            @NotSuper: I understand that this is a bit difficult. Let me put my experiment in different words: When I write "Given that at least one of the outcomes is Heads", then this means: someone looked at both coins and provided you with the information that one of the outcomes is Heads. In your experiment, you look only at one of the coins.
                            – Hendrik Vogt
                            Dec 21 '10 at 17:46






                          • 1




                            @NotSuper: No. Looking at only one coin and looking a two coins gives different knowledge. Sorry, I can't explain it better.
                            – Hendrik Vogt
                            Dec 21 '10 at 18:14












                          up vote
                          10
                          down vote










                          up vote
                          10
                          down vote









                          Here's some idea.



                          Suppose you sample $4n$ families with two children, where $n$ is a very large integer. Then, with a very high probability, about $n$ of them have two boys, about $n$ have two girls, and about $2n$ have one girl and one boy.
                          Since you ignore the families with two boys, the desired probability is given by $n/(3n) = 1/3$.



                          EDIT: The above solution corresponds to the following situation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you check if one of the kids is a girl. If no, you ignore this family. If yes, you check if both kids are girls.



                          EDIT: Among all the OP's examples, only one is relevant here. Namely, "I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no?". Well, in this situation the probability is indeed $1/2$, assuming the following interpretation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you wait until one of the kids comes into the room. If it is a boy, you ignore this family; otherwise you check if both kids are girls. It is readily understood that this leads to a probability of $1/2$.






                          share|cite|improve this answer














                          Here's some idea.



                          Suppose you sample $4n$ families with two children, where $n$ is a very large integer. Then, with a very high probability, about $n$ of them have two boys, about $n$ have two girls, and about $2n$ have one girl and one boy.
                          Since you ignore the families with two boys, the desired probability is given by $n/(3n) = 1/3$.



                          EDIT: The above solution corresponds to the following situation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you check if one of the kids is a girl. If no, you ignore this family. If yes, you check if both kids are girls.



                          EDIT: Among all the OP's examples, only one is relevant here. Namely, "I visit this family. I know they have 2 kids. One of them, a girl, comes into the room. The probability that the 2nd kid is also a girl is 1/2, no?". Well, in this situation the probability is indeed $1/2$, assuming the following interpretation. You visit a large number of families. In each family, you check if there are two kids. If no, you ignore this family. If yes, you wait until one of the kids comes into the room. If it is a boy, you ignore this family; otherwise you check if both kids are girls. It is readily understood that this leads to a probability of $1/2$.







                          share|cite|improve this answer














                          share|cite|improve this answer



                          share|cite|improve this answer








                          edited Dec 21 '10 at 13:52

























                          answered Dec 21 '10 at 12:47









                          Shai Covo

                          21k22853




                          21k22853







                          • 4




                            Of course, a lot depends on how we interpret the question.
                            – Shai Covo
                            Dec 21 '10 at 12:52






                          • 1




                            @NotSuper: It all depends a lot on the precise formulation, as Shai already pointed out. Let's say you visit a friend who flips a coin twice. Given that at least one of the outcomes is Heads, the probability of two Heads is indeed 1/3.
                            – Hendrik Vogt
                            Dec 21 '10 at 13:35






                          • 1




                            @NotSuper: Shai is right indeed: You can see only one of them, say the one closer to you, and it's a head. So you know more than in the experiment I formulated. In that experiment, I assumed that you know that at least one of the two outcomes is Heads, but if it's only one head, then you don't know which coin it is! In your experiment you know that that coin lying closer to you is a head, so it's a different experiment.
                            – Hendrik Vogt
                            Dec 21 '10 at 15:38







                          • 1




                            @NotSuper: I understand that this is a bit difficult. Let me put my experiment in different words: When I write "Given that at least one of the outcomes is Heads", then this means: someone looked at both coins and provided you with the information that one of the outcomes is Heads. In your experiment, you look only at one of the coins.
                            – Hendrik Vogt
                            Dec 21 '10 at 17:46






                          • 1




                            @NotSuper: No. Looking at only one coin and looking a two coins gives different knowledge. Sorry, I can't explain it better.
                            – Hendrik Vogt
                            Dec 21 '10 at 18:14












                          • 4




                            Of course, a lot depends on how we interpret the question.
                            – Shai Covo
                            Dec 21 '10 at 12:52






                          • 1




                            @NotSuper: It all depends a lot on the precise formulation, as Shai already pointed out. Let's say you visit a friend who flips a coin twice. Given that at least one of the outcomes is Heads, the probability of two Heads is indeed 1/3.
                            – Hendrik Vogt
                            Dec 21 '10 at 13:35






                          • 1




                            @NotSuper: Shai is right indeed: You can see only one of them, say the one closer to you, and it's a head. So you know more than in the experiment I formulated. In that experiment, I assumed that you know that at least one of the two outcomes is Heads, but if it's only one head, then you don't know which coin it is! In your experiment you know that that coin lying closer to you is a head, so it's a different experiment.
                            – Hendrik Vogt
                            Dec 21 '10 at 15:38







                          • 1




                            @NotSuper: I understand that this is a bit difficult. Let me put my experiment in different words: When I write "Given that at least one of the outcomes is Heads", then this means: someone looked at both coins and provided you with the information that one of the outcomes is Heads. In your experiment, you look only at one of the coins.
                            – Hendrik Vogt
                            Dec 21 '10 at 17:46






                          • 1




                            @NotSuper: No. Looking at only one coin and looking a two coins gives different knowledge. Sorry, I can't explain it better.
                            – Hendrik Vogt
                            Dec 21 '10 at 18:14







                          4




                          4




                          Of course, a lot depends on how we interpret the question.
                          – Shai Covo
                          Dec 21 '10 at 12:52




                          Of course, a lot depends on how we interpret the question.
                          – Shai Covo
                          Dec 21 '10 at 12:52




                          1




                          1




                          @NotSuper: It all depends a lot on the precise formulation, as Shai already pointed out. Let's say you visit a friend who flips a coin twice. Given that at least one of the outcomes is Heads, the probability of two Heads is indeed 1/3.
                          – Hendrik Vogt
                          Dec 21 '10 at 13:35




                          @NotSuper: It all depends a lot on the precise formulation, as Shai already pointed out. Let's say you visit a friend who flips a coin twice. Given that at least one of the outcomes is Heads, the probability of two Heads is indeed 1/3.
                          – Hendrik Vogt
                          Dec 21 '10 at 13:35




                          1




                          1




                          @NotSuper: Shai is right indeed: You can see only one of them, say the one closer to you, and it's a head. So you know more than in the experiment I formulated. In that experiment, I assumed that you know that at least one of the two outcomes is Heads, but if it's only one head, then you don't know which coin it is! In your experiment you know that that coin lying closer to you is a head, so it's a different experiment.
                          – Hendrik Vogt
                          Dec 21 '10 at 15:38





                          @NotSuper: Shai is right indeed: You can see only one of them, say the one closer to you, and it's a head. So you know more than in the experiment I formulated. In that experiment, I assumed that you know that at least one of the two outcomes is Heads, but if it's only one head, then you don't know which coin it is! In your experiment you know that that coin lying closer to you is a head, so it's a different experiment.
                          – Hendrik Vogt
                          Dec 21 '10 at 15:38





                          1




                          1




                          @NotSuper: I understand that this is a bit difficult. Let me put my experiment in different words: When I write "Given that at least one of the outcomes is Heads", then this means: someone looked at both coins and provided you with the information that one of the outcomes is Heads. In your experiment, you look only at one of the coins.
                          – Hendrik Vogt
                          Dec 21 '10 at 17:46




                          @NotSuper: I understand that this is a bit difficult. Let me put my experiment in different words: When I write "Given that at least one of the outcomes is Heads", then this means: someone looked at both coins and provided you with the information that one of the outcomes is Heads. In your experiment, you look only at one of the coins.
                          – Hendrik Vogt
                          Dec 21 '10 at 17:46




                          1




                          1




                          @NotSuper: No. Looking at only one coin and looking a two coins gives different knowledge. Sorry, I can't explain it better.
                          – Hendrik Vogt
                          Dec 21 '10 at 18:14




                          @NotSuper: No. Looking at only one coin and looking a two coins gives different knowledge. Sorry, I can't explain it better.
                          – Hendrik Vogt
                          Dec 21 '10 at 18:14










                          up vote
                          6
                          down vote













                          One possible solution for this problem might be though using conditional probability.



                          Let $B_i$ and $G_i$ represent $i$th boy and $i$th girl respectively. Here the sample space could be expressed as $S =$ $(B_1,B_2),(B_1,G_2),(G_1,B_2),(G_1,G_2)$



                          Let $A$ = An event of having both girl chil.
                          $B$ = An event of having at-least one girl child.



                          $A = $$(G_1,G_2)$ and $B =$$(B_1,G_2),(G_1,B_2),(G_1,G_2)$



                          So what you are asking is $$P(A / B) = fracP(A cap B)P(B) = fracfrac14frac34 = frac13 $$






                          share|cite|improve this answer
























                            up vote
                            6
                            down vote













                            One possible solution for this problem might be though using conditional probability.



                            Let $B_i$ and $G_i$ represent $i$th boy and $i$th girl respectively. Here the sample space could be expressed as $S =$ $(B_1,B_2),(B_1,G_2),(G_1,B_2),(G_1,G_2)$



                            Let $A$ = An event of having both girl chil.
                            $B$ = An event of having at-least one girl child.



                            $A = $$(G_1,G_2)$ and $B =$$(B_1,G_2),(G_1,B_2),(G_1,G_2)$



                            So what you are asking is $$P(A / B) = fracP(A cap B)P(B) = fracfrac14frac34 = frac13 $$






                            share|cite|improve this answer






















                              up vote
                              6
                              down vote










                              up vote
                              6
                              down vote









                              One possible solution for this problem might be though using conditional probability.



                              Let $B_i$ and $G_i$ represent $i$th boy and $i$th girl respectively. Here the sample space could be expressed as $S =$ $(B_1,B_2),(B_1,G_2),(G_1,B_2),(G_1,G_2)$



                              Let $A$ = An event of having both girl chil.
                              $B$ = An event of having at-least one girl child.



                              $A = $$(G_1,G_2)$ and $B =$$(B_1,G_2),(G_1,B_2),(G_1,G_2)$



                              So what you are asking is $$P(A / B) = fracP(A cap B)P(B) = fracfrac14frac34 = frac13 $$






                              share|cite|improve this answer












                              One possible solution for this problem might be though using conditional probability.



                              Let $B_i$ and $G_i$ represent $i$th boy and $i$th girl respectively. Here the sample space could be expressed as $S =$ $(B_1,B_2),(B_1,G_2),(G_1,B_2),(G_1,G_2)$



                              Let $A$ = An event of having both girl chil.
                              $B$ = An event of having at-least one girl child.



                              $A = $$(G_1,G_2)$ and $B =$$(B_1,G_2),(G_1,B_2),(G_1,G_2)$



                              So what you are asking is $$P(A / B) = fracP(A cap B)P(B) = fracfrac14frac34 = frac13 $$







                              share|cite|improve this answer












                              share|cite|improve this answer



                              share|cite|improve this answer










                              answered Dec 21 '10 at 14:36









                              Quixotic

                              11.5k2178169




                              11.5k2178169




















                                  up vote
                                  4
                                  down vote













                                  It really depends on what knowledge you have to restrict the probability space. (The original question is a little ambiguous, but I agree that 1/3 is the best answer.)



                                  The marble example and the example of the firstborn daughter are the same situation-- the second trial is independent, and the odds are 1/2. In these cases you know the first trial is independent, and you can ignore it.



                                  The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter. For example, suppose there is a social science study on 2 child families with at least 1 daughter-- in this situation, about 1/3 of the families will be daughter-daughter, 1/3 will be daughter-son, and 1/3 will be son-daughter. You have to consider the full probability space of two trials (d-d,d-s,s-d,s-s) and eliminate the s-s possibility. This leaves a 1/3 chance of 2 daughters.



                                  For other variations on this question, you have to decide the appropriate probability space before answering. (One common, confusing variation is to suppose the family has two children, one of which is a son born on a Tuesday-- what are the odds of two sons?)



                                  In your example where the first child in the room is a daughter, if we suppose that the order the children enter has nothing to do with sex, then the probability the other child is a daughter is 1/2. However, maybe the family encourages their daughters to play inside and their sons outside-- then you could assume the other child is a son, since he hasn't come in yet. Maybe the culture has a tradition that the oldest daughter in the household (if there is a daughter) offers guests drinks-- then you know the family has at least one daughter, and the odds that they have another daughter is 1/3. Once you've chosen your assumptions, you can define the probability space and compute the answer.






                                  share|cite|improve this answer
















                                  • 1




                                    @Jonas It isn't at all clear that "The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter." Instead I imagined I hadn't seen my nephew in a decade, and we hadn't kept in touch. But I had heard that he had 2 kids and one was a girl. I was going to visit him soon and was wondering about the other child. Girl? or boy? I figured the chances of a girl was 1 in 2, and I still do. I found the Wikipedia article linked to by Rawling enlightening.
                                    – NotSuper
                                    Dec 21 '10 at 15:15











                                  • @NotSuper: Yes, I agree it's ambiguous. It is clear that I gave Mlodinow's intended interpretation only because he gave the answer 1/3. But your nephew scenario is ambiguous as well-- how did you hear that one child is a girl? You could learn that among their children is at least one daughter (maybe they send an old dollhouse to your grandchildren), which gives 1/3. Or, you could specify one of their children (oldest, first to answer the phone, etc., as long as it's independent of sex) and learn she is a girl, which gives 1/2.
                                    – Jonas Kibelbek
                                    Dec 21 '10 at 15:42










                                  • These correspond to the two probability spaces described in my answer and in the Wikipedia article. When you hear that one child is a girl, can you cross off son-son and son-daughter, or can you just cross off son-son? (With other information and messier probability models, we could get other answers than 1/2 and 1/3 as well. But these are the two most sensible models to apply.)
                                    – Jonas Kibelbek
                                    Dec 21 '10 at 15:46











                                  • @Jonas "How did you hear that one child is a girl?" I wrote "I had heard that he had 2 kids and one was a girl." I think that implies that someone told me that (or possibly wrote that to me in a letter or email), and that's all I know about my nephew's children. Isn't that the standard interpretation?
                                    – NotSuper
                                    Dec 24 '10 at 13:34











                                  • @NotSuper: To quote from Carl's answer, "one child" can be interpreted several ways in English: "one particular child," "at least one child," or "exactly one child." If the letter or e-mail gives you enough context to pick one interpretation, then it's easy to compute the probability. Your intuitive answer of 1/2 is a correct answer, provided you recognize you are using the "one particular child" interpretation. But, since this is a puzzle or riddle, the smallest assumption ("at least one child"), which gives the counter-intuitive answer, is the riddler's preferred answer. Do you agree?
                                    – Jonas Kibelbek
                                    Dec 24 '10 at 16:34














                                  up vote
                                  4
                                  down vote













                                  It really depends on what knowledge you have to restrict the probability space. (The original question is a little ambiguous, but I agree that 1/3 is the best answer.)



                                  The marble example and the example of the firstborn daughter are the same situation-- the second trial is independent, and the odds are 1/2. In these cases you know the first trial is independent, and you can ignore it.



                                  The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter. For example, suppose there is a social science study on 2 child families with at least 1 daughter-- in this situation, about 1/3 of the families will be daughter-daughter, 1/3 will be daughter-son, and 1/3 will be son-daughter. You have to consider the full probability space of two trials (d-d,d-s,s-d,s-s) and eliminate the s-s possibility. This leaves a 1/3 chance of 2 daughters.



                                  For other variations on this question, you have to decide the appropriate probability space before answering. (One common, confusing variation is to suppose the family has two children, one of which is a son born on a Tuesday-- what are the odds of two sons?)



                                  In your example where the first child in the room is a daughter, if we suppose that the order the children enter has nothing to do with sex, then the probability the other child is a daughter is 1/2. However, maybe the family encourages their daughters to play inside and their sons outside-- then you could assume the other child is a son, since he hasn't come in yet. Maybe the culture has a tradition that the oldest daughter in the household (if there is a daughter) offers guests drinks-- then you know the family has at least one daughter, and the odds that they have another daughter is 1/3. Once you've chosen your assumptions, you can define the probability space and compute the answer.






                                  share|cite|improve this answer
















                                  • 1




                                    @Jonas It isn't at all clear that "The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter." Instead I imagined I hadn't seen my nephew in a decade, and we hadn't kept in touch. But I had heard that he had 2 kids and one was a girl. I was going to visit him soon and was wondering about the other child. Girl? or boy? I figured the chances of a girl was 1 in 2, and I still do. I found the Wikipedia article linked to by Rawling enlightening.
                                    – NotSuper
                                    Dec 21 '10 at 15:15











                                  • @NotSuper: Yes, I agree it's ambiguous. It is clear that I gave Mlodinow's intended interpretation only because he gave the answer 1/3. But your nephew scenario is ambiguous as well-- how did you hear that one child is a girl? You could learn that among their children is at least one daughter (maybe they send an old dollhouse to your grandchildren), which gives 1/3. Or, you could specify one of their children (oldest, first to answer the phone, etc., as long as it's independent of sex) and learn she is a girl, which gives 1/2.
                                    – Jonas Kibelbek
                                    Dec 21 '10 at 15:42










                                  • These correspond to the two probability spaces described in my answer and in the Wikipedia article. When you hear that one child is a girl, can you cross off son-son and son-daughter, or can you just cross off son-son? (With other information and messier probability models, we could get other answers than 1/2 and 1/3 as well. But these are the two most sensible models to apply.)
                                    – Jonas Kibelbek
                                    Dec 21 '10 at 15:46











                                  • @Jonas "How did you hear that one child is a girl?" I wrote "I had heard that he had 2 kids and one was a girl." I think that implies that someone told me that (or possibly wrote that to me in a letter or email), and that's all I know about my nephew's children. Isn't that the standard interpretation?
                                    – NotSuper
                                    Dec 24 '10 at 13:34











                                  • @NotSuper: To quote from Carl's answer, "one child" can be interpreted several ways in English: "one particular child," "at least one child," or "exactly one child." If the letter or e-mail gives you enough context to pick one interpretation, then it's easy to compute the probability. Your intuitive answer of 1/2 is a correct answer, provided you recognize you are using the "one particular child" interpretation. But, since this is a puzzle or riddle, the smallest assumption ("at least one child"), which gives the counter-intuitive answer, is the riddler's preferred answer. Do you agree?
                                    – Jonas Kibelbek
                                    Dec 24 '10 at 16:34












                                  up vote
                                  4
                                  down vote










                                  up vote
                                  4
                                  down vote









                                  It really depends on what knowledge you have to restrict the probability space. (The original question is a little ambiguous, but I agree that 1/3 is the best answer.)



                                  The marble example and the example of the firstborn daughter are the same situation-- the second trial is independent, and the odds are 1/2. In these cases you know the first trial is independent, and you can ignore it.



                                  The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter. For example, suppose there is a social science study on 2 child families with at least 1 daughter-- in this situation, about 1/3 of the families will be daughter-daughter, 1/3 will be daughter-son, and 1/3 will be son-daughter. You have to consider the full probability space of two trials (d-d,d-s,s-d,s-s) and eliminate the s-s possibility. This leaves a 1/3 chance of 2 daughters.



                                  For other variations on this question, you have to decide the appropriate probability space before answering. (One common, confusing variation is to suppose the family has two children, one of which is a son born on a Tuesday-- what are the odds of two sons?)



                                  In your example where the first child in the room is a daughter, if we suppose that the order the children enter has nothing to do with sex, then the probability the other child is a daughter is 1/2. However, maybe the family encourages their daughters to play inside and their sons outside-- then you could assume the other child is a son, since he hasn't come in yet. Maybe the culture has a tradition that the oldest daughter in the household (if there is a daughter) offers guests drinks-- then you know the family has at least one daughter, and the odds that they have another daughter is 1/3. Once you've chosen your assumptions, you can define the probability space and compute the answer.






                                  share|cite|improve this answer












                                  It really depends on what knowledge you have to restrict the probability space. (The original question is a little ambiguous, but I agree that 1/3 is the best answer.)



                                  The marble example and the example of the firstborn daughter are the same situation-- the second trial is independent, and the odds are 1/2. In these cases you know the first trial is independent, and you can ignore it.



                                  The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter. For example, suppose there is a social science study on 2 child families with at least 1 daughter-- in this situation, about 1/3 of the families will be daughter-daughter, 1/3 will be daughter-son, and 1/3 will be son-daughter. You have to consider the full probability space of two trials (d-d,d-s,s-d,s-s) and eliminate the s-s possibility. This leaves a 1/3 chance of 2 daughters.



                                  For other variations on this question, you have to decide the appropriate probability space before answering. (One common, confusing variation is to suppose the family has two children, one of which is a son born on a Tuesday-- what are the odds of two sons?)



                                  In your example where the first child in the room is a daughter, if we suppose that the order the children enter has nothing to do with sex, then the probability the other child is a daughter is 1/2. However, maybe the family encourages their daughters to play inside and their sons outside-- then you could assume the other child is a son, since he hasn't come in yet. Maybe the culture has a tradition that the oldest daughter in the household (if there is a daughter) offers guests drinks-- then you know the family has at least one daughter, and the odds that they have another daughter is 1/3. Once you've chosen your assumptions, you can define the probability space and compute the answer.







                                  share|cite|improve this answer












                                  share|cite|improve this answer



                                  share|cite|improve this answer










                                  answered Dec 21 '10 at 14:03









                                  Jonas Kibelbek

                                  5,10912228




                                  5,10912228







                                  • 1




                                    @Jonas It isn't at all clear that "The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter." Instead I imagined I hadn't seen my nephew in a decade, and we hadn't kept in touch. But I had heard that he had 2 kids and one was a girl. I was going to visit him soon and was wondering about the other child. Girl? or boy? I figured the chances of a girl was 1 in 2, and I still do. I found the Wikipedia article linked to by Rawling enlightening.
                                    – NotSuper
                                    Dec 21 '10 at 15:15











                                  • @NotSuper: Yes, I agree it's ambiguous. It is clear that I gave Mlodinow's intended interpretation only because he gave the answer 1/3. But your nephew scenario is ambiguous as well-- how did you hear that one child is a girl? You could learn that among their children is at least one daughter (maybe they send an old dollhouse to your grandchildren), which gives 1/3. Or, you could specify one of their children (oldest, first to answer the phone, etc., as long as it's independent of sex) and learn she is a girl, which gives 1/2.
                                    – Jonas Kibelbek
                                    Dec 21 '10 at 15:42










                                  • These correspond to the two probability spaces described in my answer and in the Wikipedia article. When you hear that one child is a girl, can you cross off son-son and son-daughter, or can you just cross off son-son? (With other information and messier probability models, we could get other answers than 1/2 and 1/3 as well. But these are the two most sensible models to apply.)
                                    – Jonas Kibelbek
                                    Dec 21 '10 at 15:46











                                  • @Jonas "How did you hear that one child is a girl?" I wrote "I had heard that he had 2 kids and one was a girl." I think that implies that someone told me that (or possibly wrote that to me in a letter or email), and that's all I know about my nephew's children. Isn't that the standard interpretation?
                                    – NotSuper
                                    Dec 24 '10 at 13:34











                                  • @NotSuper: To quote from Carl's answer, "one child" can be interpreted several ways in English: "one particular child," "at least one child," or "exactly one child." If the letter or e-mail gives you enough context to pick one interpretation, then it's easy to compute the probability. Your intuitive answer of 1/2 is a correct answer, provided you recognize you are using the "one particular child" interpretation. But, since this is a puzzle or riddle, the smallest assumption ("at least one child"), which gives the counter-intuitive answer, is the riddler's preferred answer. Do you agree?
                                    – Jonas Kibelbek
                                    Dec 24 '10 at 16:34












                                  • 1




                                    @Jonas It isn't at all clear that "The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter." Instead I imagined I hadn't seen my nephew in a decade, and we hadn't kept in touch. But I had heard that he had 2 kids and one was a girl. I was going to visit him soon and was wondering about the other child. Girl? or boy? I figured the chances of a girl was 1 in 2, and I still do. I found the Wikipedia article linked to by Rawling enlightening.
                                    – NotSuper
                                    Dec 21 '10 at 15:15











                                  • @NotSuper: Yes, I agree it's ambiguous. It is clear that I gave Mlodinow's intended interpretation only because he gave the answer 1/3. But your nephew scenario is ambiguous as well-- how did you hear that one child is a girl? You could learn that among their children is at least one daughter (maybe they send an old dollhouse to your grandchildren), which gives 1/3. Or, you could specify one of their children (oldest, first to answer the phone, etc., as long as it's independent of sex) and learn she is a girl, which gives 1/2.
                                    – Jonas Kibelbek
                                    Dec 21 '10 at 15:42










                                  • These correspond to the two probability spaces described in my answer and in the Wikipedia article. When you hear that one child is a girl, can you cross off son-son and son-daughter, or can you just cross off son-son? (With other information and messier probability models, we could get other answers than 1/2 and 1/3 as well. But these are the two most sensible models to apply.)
                                    – Jonas Kibelbek
                                    Dec 21 '10 at 15:46











                                  • @Jonas "How did you hear that one child is a girl?" I wrote "I had heard that he had 2 kids and one was a girl." I think that implies that someone told me that (or possibly wrote that to me in a letter or email), and that's all I know about my nephew's children. Isn't that the standard interpretation?
                                    – NotSuper
                                    Dec 24 '10 at 13:34











                                  • @NotSuper: To quote from Carl's answer, "one child" can be interpreted several ways in English: "one particular child," "at least one child," or "exactly one child." If the letter or e-mail gives you enough context to pick one interpretation, then it's easy to compute the probability. Your intuitive answer of 1/2 is a correct answer, provided you recognize you are using the "one particular child" interpretation. But, since this is a puzzle or riddle, the smallest assumption ("at least one child"), which gives the counter-intuitive answer, is the riddler's preferred answer. Do you agree?
                                    – Jonas Kibelbek
                                    Dec 24 '10 at 16:34







                                  1




                                  1




                                  @Jonas It isn't at all clear that "The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter." Instead I imagined I hadn't seen my nephew in a decade, and we hadn't kept in touch. But I had heard that he had 2 kids and one was a girl. I was going to visit him soon and was wondering about the other child. Girl? or boy? I figured the chances of a girl was 1 in 2, and I still do. I found the Wikipedia article linked to by Rawling enlightening.
                                  – NotSuper
                                  Dec 21 '10 at 15:15





                                  @Jonas It isn't at all clear that "The original question intends you to imagine a randomly chosen 2 child family from among all families with at least 1 daughter." Instead I imagined I hadn't seen my nephew in a decade, and we hadn't kept in touch. But I had heard that he had 2 kids and one was a girl. I was going to visit him soon and was wondering about the other child. Girl? or boy? I figured the chances of a girl was 1 in 2, and I still do. I found the Wikipedia article linked to by Rawling enlightening.
                                  – NotSuper
                                  Dec 21 '10 at 15:15













                                  @NotSuper: Yes, I agree it's ambiguous. It is clear that I gave Mlodinow's intended interpretation only because he gave the answer 1/3. But your nephew scenario is ambiguous as well-- how did you hear that one child is a girl? You could learn that among their children is at least one daughter (maybe they send an old dollhouse to your grandchildren), which gives 1/3. Or, you could specify one of their children (oldest, first to answer the phone, etc., as long as it's independent of sex) and learn she is a girl, which gives 1/2.
                                  – Jonas Kibelbek
                                  Dec 21 '10 at 15:42




                                  @NotSuper: Yes, I agree it's ambiguous. It is clear that I gave Mlodinow's intended interpretation only because he gave the answer 1/3. But your nephew scenario is ambiguous as well-- how did you hear that one child is a girl? You could learn that among their children is at least one daughter (maybe they send an old dollhouse to your grandchildren), which gives 1/3. Or, you could specify one of their children (oldest, first to answer the phone, etc., as long as it's independent of sex) and learn she is a girl, which gives 1/2.
                                  – Jonas Kibelbek
                                  Dec 21 '10 at 15:42












                                  These correspond to the two probability spaces described in my answer and in the Wikipedia article. When you hear that one child is a girl, can you cross off son-son and son-daughter, or can you just cross off son-son? (With other information and messier probability models, we could get other answers than 1/2 and 1/3 as well. But these are the two most sensible models to apply.)
                                  – Jonas Kibelbek
                                  Dec 21 '10 at 15:46





                                  These correspond to the two probability spaces described in my answer and in the Wikipedia article. When you hear that one child is a girl, can you cross off son-son and son-daughter, or can you just cross off son-son? (With other information and messier probability models, we could get other answers than 1/2 and 1/3 as well. But these are the two most sensible models to apply.)
                                  – Jonas Kibelbek
                                  Dec 21 '10 at 15:46













                                  @Jonas "How did you hear that one child is a girl?" I wrote "I had heard that he had 2 kids and one was a girl." I think that implies that someone told me that (or possibly wrote that to me in a letter or email), and that's all I know about my nephew's children. Isn't that the standard interpretation?
                                  – NotSuper
                                  Dec 24 '10 at 13:34





                                  @Jonas "How did you hear that one child is a girl?" I wrote "I had heard that he had 2 kids and one was a girl." I think that implies that someone told me that (or possibly wrote that to me in a letter or email), and that's all I know about my nephew's children. Isn't that the standard interpretation?
                                  – NotSuper
                                  Dec 24 '10 at 13:34













                                  @NotSuper: To quote from Carl's answer, "one child" can be interpreted several ways in English: "one particular child," "at least one child," or "exactly one child." If the letter or e-mail gives you enough context to pick one interpretation, then it's easy to compute the probability. Your intuitive answer of 1/2 is a correct answer, provided you recognize you are using the "one particular child" interpretation. But, since this is a puzzle or riddle, the smallest assumption ("at least one child"), which gives the counter-intuitive answer, is the riddler's preferred answer. Do you agree?
                                  – Jonas Kibelbek
                                  Dec 24 '10 at 16:34




                                  @NotSuper: To quote from Carl's answer, "one child" can be interpreted several ways in English: "one particular child," "at least one child," or "exactly one child." If the letter or e-mail gives you enough context to pick one interpretation, then it's easy to compute the probability. Your intuitive answer of 1/2 is a correct answer, provided you recognize you are using the "one particular child" interpretation. But, since this is a puzzle or riddle, the smallest assumption ("at least one child"), which gives the counter-intuitive answer, is the riddler's preferred answer. Do you agree?
                                  – Jonas Kibelbek
                                  Dec 24 '10 at 16:34










                                  up vote
                                  2
                                  down vote













                                  This is really a continuation of BlueRaja's answer, where I'm changing the question slightly to give another illustration of the underlying point. Consider the following two procedures:



                                  Procedure A: I flip two coins, a nickel and a penny. After the flip, I
                                  pick one at random, and report its value to you.



                                  Procedure B: I flip two coins, a nickel and a penny. If exactly one lands
                                  heads, I report that one's value to you, i.e. I say either "the nickel
                                  landed heads" or "the penny landed heads" as appropriate. If both land
                                  heads, I report on one of them chosen at random. If neither lands
                                  heads, I say "both landed tails."



                                  Now, if I follow Procedure A and say "the nickel landed
                                  heads", then the probability that the penny landed heads is 50/50. But
                                  if I say the very same thing after following Procedure B, the
                                  probability that the penny landed heads is 1 in 3.



                                  Now take the following problem: I flip two coins and say to you "the
                                  nickel landed heads". What is the probability that the penny landed
                                  heads? Clearly there's no way to answer without knowing or assuming something about the procedure I'm following.



                                  (All this having been said, I agree that the best answer to the original question is 1/3.)






                                  share|cite|improve this answer
























                                    up vote
                                    2
                                    down vote













                                    This is really a continuation of BlueRaja's answer, where I'm changing the question slightly to give another illustration of the underlying point. Consider the following two procedures:



                                    Procedure A: I flip two coins, a nickel and a penny. After the flip, I
                                    pick one at random, and report its value to you.



                                    Procedure B: I flip two coins, a nickel and a penny. If exactly one lands
                                    heads, I report that one's value to you, i.e. I say either "the nickel
                                    landed heads" or "the penny landed heads" as appropriate. If both land
                                    heads, I report on one of them chosen at random. If neither lands
                                    heads, I say "both landed tails."



                                    Now, if I follow Procedure A and say "the nickel landed
                                    heads", then the probability that the penny landed heads is 50/50. But
                                    if I say the very same thing after following Procedure B, the
                                    probability that the penny landed heads is 1 in 3.



                                    Now take the following problem: I flip two coins and say to you "the
                                    nickel landed heads". What is the probability that the penny landed
                                    heads? Clearly there's no way to answer without knowing or assuming something about the procedure I'm following.



                                    (All this having been said, I agree that the best answer to the original question is 1/3.)






                                    share|cite|improve this answer






















                                      up vote
                                      2
                                      down vote










                                      up vote
                                      2
                                      down vote









                                      This is really a continuation of BlueRaja's answer, where I'm changing the question slightly to give another illustration of the underlying point. Consider the following two procedures:



                                      Procedure A: I flip two coins, a nickel and a penny. After the flip, I
                                      pick one at random, and report its value to you.



                                      Procedure B: I flip two coins, a nickel and a penny. If exactly one lands
                                      heads, I report that one's value to you, i.e. I say either "the nickel
                                      landed heads" or "the penny landed heads" as appropriate. If both land
                                      heads, I report on one of them chosen at random. If neither lands
                                      heads, I say "both landed tails."



                                      Now, if I follow Procedure A and say "the nickel landed
                                      heads", then the probability that the penny landed heads is 50/50. But
                                      if I say the very same thing after following Procedure B, the
                                      probability that the penny landed heads is 1 in 3.



                                      Now take the following problem: I flip two coins and say to you "the
                                      nickel landed heads". What is the probability that the penny landed
                                      heads? Clearly there's no way to answer without knowing or assuming something about the procedure I'm following.



                                      (All this having been said, I agree that the best answer to the original question is 1/3.)






                                      share|cite|improve this answer












                                      This is really a continuation of BlueRaja's answer, where I'm changing the question slightly to give another illustration of the underlying point. Consider the following two procedures:



                                      Procedure A: I flip two coins, a nickel and a penny. After the flip, I
                                      pick one at random, and report its value to you.



                                      Procedure B: I flip two coins, a nickel and a penny. If exactly one lands
                                      heads, I report that one's value to you, i.e. I say either "the nickel
                                      landed heads" or "the penny landed heads" as appropriate. If both land
                                      heads, I report on one of them chosen at random. If neither lands
                                      heads, I say "both landed tails."



                                      Now, if I follow Procedure A and say "the nickel landed
                                      heads", then the probability that the penny landed heads is 50/50. But
                                      if I say the very same thing after following Procedure B, the
                                      probability that the penny landed heads is 1 in 3.



                                      Now take the following problem: I flip two coins and say to you "the
                                      nickel landed heads". What is the probability that the penny landed
                                      heads? Clearly there's no way to answer without knowing or assuming something about the procedure I'm following.



                                      (All this having been said, I agree that the best answer to the original question is 1/3.)







                                      share|cite|improve this answer












                                      share|cite|improve this answer



                                      share|cite|improve this answer










                                      answered Dec 22 '10 at 22:47









                                      Larry Denenberg

                                      656310




                                      656310




















                                          up vote
                                          2
                                          down vote













                                          In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?



                                          0%. You already said that ONE is a girl. The other must be a boy.






                                          share|cite|improve this answer
















                                          • 5




                                            If a family has two children and both are girls, it is not incorrect to say "one of the children is a girl." "One" doesn't mean "exactly one."
                                            – manthanomen
                                            May 6 '13 at 23:11







                                          • 4




                                            Does one mean at least one, exactly one, at most one, a specific one? I think Judie's point was that the parameters of the problem are not clearly stated -- you have to make an assumption about the intent. The given answers generally assume that in this case one was intended to mean one or two, but maybe we don't know that from the question by itself unless we recognize it due to prior exposure to this type of problem.
                                            – nobar
                                            May 11 '13 at 22:34














                                          up vote
                                          2
                                          down vote













                                          In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?



                                          0%. You already said that ONE is a girl. The other must be a boy.






                                          share|cite|improve this answer
















                                          • 5




                                            If a family has two children and both are girls, it is not incorrect to say "one of the children is a girl." "One" doesn't mean "exactly one."
                                            – manthanomen
                                            May 6 '13 at 23:11







                                          • 4




                                            Does one mean at least one, exactly one, at most one, a specific one? I think Judie's point was that the parameters of the problem are not clearly stated -- you have to make an assumption about the intent. The given answers generally assume that in this case one was intended to mean one or two, but maybe we don't know that from the question by itself unless we recognize it due to prior exposure to this type of problem.
                                            – nobar
                                            May 11 '13 at 22:34












                                          up vote
                                          2
                                          down vote










                                          up vote
                                          2
                                          down vote









                                          In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?



                                          0%. You already said that ONE is a girl. The other must be a boy.






                                          share|cite|improve this answer












                                          In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?



                                          0%. You already said that ONE is a girl. The other must be a boy.







                                          share|cite|improve this answer












                                          share|cite|improve this answer



                                          share|cite|improve this answer










                                          answered May 6 '13 at 23:01









                                          Judie

                                          311




                                          311







                                          • 5




                                            If a family has two children and both are girls, it is not incorrect to say "one of the children is a girl." "One" doesn't mean "exactly one."
                                            – manthanomen
                                            May 6 '13 at 23:11







                                          • 4




                                            Does one mean at least one, exactly one, at most one, a specific one? I think Judie's point was that the parameters of the problem are not clearly stated -- you have to make an assumption about the intent. The given answers generally assume that in this case one was intended to mean one or two, but maybe we don't know that from the question by itself unless we recognize it due to prior exposure to this type of problem.
                                            – nobar
                                            May 11 '13 at 22:34












                                          • 5




                                            If a family has two children and both are girls, it is not incorrect to say "one of the children is a girl." "One" doesn't mean "exactly one."
                                            – manthanomen
                                            May 6 '13 at 23:11







                                          • 4




                                            Does one mean at least one, exactly one, at most one, a specific one? I think Judie's point was that the parameters of the problem are not clearly stated -- you have to make an assumption about the intent. The given answers generally assume that in this case one was intended to mean one or two, but maybe we don't know that from the question by itself unless we recognize it due to prior exposure to this type of problem.
                                            – nobar
                                            May 11 '13 at 22:34







                                          5




                                          5




                                          If a family has two children and both are girls, it is not incorrect to say "one of the children is a girl." "One" doesn't mean "exactly one."
                                          – manthanomen
                                          May 6 '13 at 23:11





                                          If a family has two children and both are girls, it is not incorrect to say "one of the children is a girl." "One" doesn't mean "exactly one."
                                          – manthanomen
                                          May 6 '13 at 23:11





                                          4




                                          4




                                          Does one mean at least one, exactly one, at most one, a specific one? I think Judie's point was that the parameters of the problem are not clearly stated -- you have to make an assumption about the intent. The given answers generally assume that in this case one was intended to mean one or two, but maybe we don't know that from the question by itself unless we recognize it due to prior exposure to this type of problem.
                                          – nobar
                                          May 11 '13 at 22:34




                                          Does one mean at least one, exactly one, at most one, a specific one? I think Judie's point was that the parameters of the problem are not clearly stated -- you have to make an assumption about the intent. The given answers generally assume that in this case one was intended to mean one or two, but maybe we don't know that from the question by itself unless we recognize it due to prior exposure to this type of problem.
                                          – nobar
                                          May 11 '13 at 22:34










                                          up vote
                                          1
                                          down vote













                                          If you are given that the first child is a girl or that the second child is a girl, then the obvious answer of 50% is correct.



                                          In this case we are given that one or both of the children is a girl, but not given any information about which one(s) -- so a simple evaluation of the 4 possible scenarios (for a two-child family) reveals the answer of 33%.



                                          The confusion stems from the slightly ambiguous language used. The phrase one of the children was used to mean one or both of the children. Based on that language, our brains trick us into thinking that a specific one of the children is given and therefore we can evaluate the other child independently.






                                          share|cite|improve this answer




















                                          • A link to this Wikipedia article was given in the main comments, but I wanted to point out that the article addresses the psychology of this problem, in an attempt to expose the seeming "paradox" of common intuition.
                                            – nobar
                                            May 13 '13 at 2:20











                                          • youtu.be/be2wuOaglFY?t=37m0s "This problem is famous for being ill-posed."
                                            – nobar
                                            Mar 3 '17 at 1:43










                                          • There is some ambiguity about whether I should have said "4 possible scenarios" or "3 possible scenarios".
                                            – nobar
                                            Mar 3 '17 at 1:46














                                          up vote
                                          1
                                          down vote













                                          If you are given that the first child is a girl or that the second child is a girl, then the obvious answer of 50% is correct.



                                          In this case we are given that one or both of the children is a girl, but not given any information about which one(s) -- so a simple evaluation of the 4 possible scenarios (for a two-child family) reveals the answer of 33%.



                                          The confusion stems from the slightly ambiguous language used. The phrase one of the children was used to mean one or both of the children. Based on that language, our brains trick us into thinking that a specific one of the children is given and therefore we can evaluate the other child independently.






                                          share|cite|improve this answer




















                                          • A link to this Wikipedia article was given in the main comments, but I wanted to point out that the article addresses the psychology of this problem, in an attempt to expose the seeming "paradox" of common intuition.
                                            – nobar
                                            May 13 '13 at 2:20











                                          • youtu.be/be2wuOaglFY?t=37m0s "This problem is famous for being ill-posed."
                                            – nobar
                                            Mar 3 '17 at 1:43










                                          • There is some ambiguity about whether I should have said "4 possible scenarios" or "3 possible scenarios".
                                            – nobar
                                            Mar 3 '17 at 1:46












                                          up vote
                                          1
                                          down vote










                                          up vote
                                          1
                                          down vote









                                          If you are given that the first child is a girl or that the second child is a girl, then the obvious answer of 50% is correct.



                                          In this case we are given that one or both of the children is a girl, but not given any information about which one(s) -- so a simple evaluation of the 4 possible scenarios (for a two-child family) reveals the answer of 33%.



                                          The confusion stems from the slightly ambiguous language used. The phrase one of the children was used to mean one or both of the children. Based on that language, our brains trick us into thinking that a specific one of the children is given and therefore we can evaluate the other child independently.






                                          share|cite|improve this answer












                                          If you are given that the first child is a girl or that the second child is a girl, then the obvious answer of 50% is correct.



                                          In this case we are given that one or both of the children is a girl, but not given any information about which one(s) -- so a simple evaluation of the 4 possible scenarios (for a two-child family) reveals the answer of 33%.



                                          The confusion stems from the slightly ambiguous language used. The phrase one of the children was used to mean one or both of the children. Based on that language, our brains trick us into thinking that a specific one of the children is given and therefore we can evaluate the other child independently.







                                          share|cite|improve this answer












                                          share|cite|improve this answer



                                          share|cite|improve this answer










                                          answered May 6 '13 at 4:24









                                          nobar

                                          1113




                                          1113











                                          • A link to this Wikipedia article was given in the main comments, but I wanted to point out that the article addresses the psychology of this problem, in an attempt to expose the seeming "paradox" of common intuition.
                                            – nobar
                                            May 13 '13 at 2:20











                                          • youtu.be/be2wuOaglFY?t=37m0s "This problem is famous for being ill-posed."
                                            – nobar
                                            Mar 3 '17 at 1:43










                                          • There is some ambiguity about whether I should have said "4 possible scenarios" or "3 possible scenarios".
                                            – nobar
                                            Mar 3 '17 at 1:46
















                                          • A link to this Wikipedia article was given in the main comments, but I wanted to point out that the article addresses the psychology of this problem, in an attempt to expose the seeming "paradox" of common intuition.
                                            – nobar
                                            May 13 '13 at 2:20











                                          • youtu.be/be2wuOaglFY?t=37m0s "This problem is famous for being ill-posed."
                                            – nobar
                                            Mar 3 '17 at 1:43










                                          • There is some ambiguity about whether I should have said "4 possible scenarios" or "3 possible scenarios".
                                            – nobar
                                            Mar 3 '17 at 1:46















                                          A link to this Wikipedia article was given in the main comments, but I wanted to point out that the article addresses the psychology of this problem, in an attempt to expose the seeming "paradox" of common intuition.
                                          – nobar
                                          May 13 '13 at 2:20





                                          A link to this Wikipedia article was given in the main comments, but I wanted to point out that the article addresses the psychology of this problem, in an attempt to expose the seeming "paradox" of common intuition.
                                          – nobar
                                          May 13 '13 at 2:20













                                          youtu.be/be2wuOaglFY?t=37m0s "This problem is famous for being ill-posed."
                                          – nobar
                                          Mar 3 '17 at 1:43




                                          youtu.be/be2wuOaglFY?t=37m0s "This problem is famous for being ill-posed."
                                          – nobar
                                          Mar 3 '17 at 1:43












                                          There is some ambiguity about whether I should have said "4 possible scenarios" or "3 possible scenarios".
                                          – nobar
                                          Mar 3 '17 at 1:46




                                          There is some ambiguity about whether I should have said "4 possible scenarios" or "3 possible scenarios".
                                          – nobar
                                          Mar 3 '17 at 1:46










                                          up vote
                                          1
                                          down vote













                                          We can solve this from first principles (maybe they're technically "second" principles, who knows).



                                          Define:



                                          1. $p_G$ be the probability that a child is a girl

                                          2. $p_B$ be the probability that a child is a boy

                                          3. $N_G$ be the number of girls in a family

                                          4. $N_B$ be the number of boys in a family

                                          5. $N_T = N_G + N_B$

                                          We want to find: $mathbbP(N_G=2 | N_G geq 1 , N_T=2)$



                                          $$mathbbP(N_G=2 | N_G geq 1 , N_T=2) = fracmathbbP(N_G=2 , N_G geq 1 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



                                          $$=fracmathbbP(N_G=2 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



                                          $$=fracp_G^2p_G^2 + 2p_Gp_B$$



                                          If we take $p_G = p_B = frac12$ then we get the book's answer of $frac1over4frac14+frac12$



                                          $$=frac1over43over4 = frac13$$



                                          NOTE: In the above, the second factor in the denominator is derived since the order in which the children are born does matter.






                                          share|cite|improve this answer






















                                          • The question does not say "$N_G geq 1$", though, it just says "one of the children is a girl". Once we have interpreted this to mean $N_g geq 1$, we have already solved the problem, as sketched here. The issue is not whether $P(N_G = 2| N_G geq 1, N_T = 2) = 1/3$ - the issue is what probability the English is asking for in the first place.
                                            – Carl Mummert
                                            Aug 21 at 20:51














                                          up vote
                                          1
                                          down vote













                                          We can solve this from first principles (maybe they're technically "second" principles, who knows).



                                          Define:



                                          1. $p_G$ be the probability that a child is a girl

                                          2. $p_B$ be the probability that a child is a boy

                                          3. $N_G$ be the number of girls in a family

                                          4. $N_B$ be the number of boys in a family

                                          5. $N_T = N_G + N_B$

                                          We want to find: $mathbbP(N_G=2 | N_G geq 1 , N_T=2)$



                                          $$mathbbP(N_G=2 | N_G geq 1 , N_T=2) = fracmathbbP(N_G=2 , N_G geq 1 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



                                          $$=fracmathbbP(N_G=2 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



                                          $$=fracp_G^2p_G^2 + 2p_Gp_B$$



                                          If we take $p_G = p_B = frac12$ then we get the book's answer of $frac1over4frac14+frac12$



                                          $$=frac1over43over4 = frac13$$



                                          NOTE: In the above, the second factor in the denominator is derived since the order in which the children are born does matter.






                                          share|cite|improve this answer






















                                          • The question does not say "$N_G geq 1$", though, it just says "one of the children is a girl". Once we have interpreted this to mean $N_g geq 1$, we have already solved the problem, as sketched here. The issue is not whether $P(N_G = 2| N_G geq 1, N_T = 2) = 1/3$ - the issue is what probability the English is asking for in the first place.
                                            – Carl Mummert
                                            Aug 21 at 20:51












                                          up vote
                                          1
                                          down vote










                                          up vote
                                          1
                                          down vote









                                          We can solve this from first principles (maybe they're technically "second" principles, who knows).



                                          Define:



                                          1. $p_G$ be the probability that a child is a girl

                                          2. $p_B$ be the probability that a child is a boy

                                          3. $N_G$ be the number of girls in a family

                                          4. $N_B$ be the number of boys in a family

                                          5. $N_T = N_G + N_B$

                                          We want to find: $mathbbP(N_G=2 | N_G geq 1 , N_T=2)$



                                          $$mathbbP(N_G=2 | N_G geq 1 , N_T=2) = fracmathbbP(N_G=2 , N_G geq 1 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



                                          $$=fracmathbbP(N_G=2 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



                                          $$=fracp_G^2p_G^2 + 2p_Gp_B$$



                                          If we take $p_G = p_B = frac12$ then we get the book's answer of $frac1over4frac14+frac12$



                                          $$=frac1over43over4 = frac13$$



                                          NOTE: In the above, the second factor in the denominator is derived since the order in which the children are born does matter.






                                          share|cite|improve this answer














                                          We can solve this from first principles (maybe they're technically "second" principles, who knows).



                                          Define:



                                          1. $p_G$ be the probability that a child is a girl

                                          2. $p_B$ be the probability that a child is a boy

                                          3. $N_G$ be the number of girls in a family

                                          4. $N_B$ be the number of boys in a family

                                          5. $N_T = N_G + N_B$

                                          We want to find: $mathbbP(N_G=2 | N_G geq 1 , N_T=2)$



                                          $$mathbbP(N_G=2 | N_G geq 1 , N_T=2) = fracmathbbP(N_G=2 , N_G geq 1 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



                                          $$=fracmathbbP(N_G=2 , N_T=2)mathbbP(N_G geq 1 , N_T=2)$$



                                          $$=fracp_G^2p_G^2 + 2p_Gp_B$$



                                          If we take $p_G = p_B = frac12$ then we get the book's answer of $frac1over4frac14+frac12$



                                          $$=frac1over43over4 = frac13$$



                                          NOTE: In the above, the second factor in the denominator is derived since the order in which the children are born does matter.







                                          share|cite|improve this answer














                                          share|cite|improve this answer



                                          share|cite|improve this answer








                                          edited Jan 31 '14 at 17:38

























                                          answered Jan 31 '14 at 17:32









                                          Patrick

                                          1,51821219




                                          1,51821219











                                          • The question does not say "$N_G geq 1$", though, it just says "one of the children is a girl". Once we have interpreted this to mean $N_g geq 1$, we have already solved the problem, as sketched here. The issue is not whether $P(N_G = 2| N_G geq 1, N_T = 2) = 1/3$ - the issue is what probability the English is asking for in the first place.
                                            – Carl Mummert
                                            Aug 21 at 20:51
















                                          • The question does not say "$N_G geq 1$", though, it just says "one of the children is a girl". Once we have interpreted this to mean $N_g geq 1$, we have already solved the problem, as sketched here. The issue is not whether $P(N_G = 2| N_G geq 1, N_T = 2) = 1/3$ - the issue is what probability the English is asking for in the first place.
                                            – Carl Mummert
                                            Aug 21 at 20:51















                                          The question does not say "$N_G geq 1$", though, it just says "one of the children is a girl". Once we have interpreted this to mean $N_g geq 1$, we have already solved the problem, as sketched here. The issue is not whether $P(N_G = 2| N_G geq 1, N_T = 2) = 1/3$ - the issue is what probability the English is asking for in the first place.
                                          – Carl Mummert
                                          Aug 21 at 20:51




                                          The question does not say "$N_G geq 1$", though, it just says "one of the children is a girl". Once we have interpreted this to mean $N_g geq 1$, we have already solved the problem, as sketched here. The issue is not whether $P(N_G = 2| N_G geq 1, N_T = 2) = 1/3$ - the issue is what probability the English is asking for in the first place.
                                          – Carl Mummert
                                          Aug 21 at 20:51










                                          up vote
                                          1
                                          down vote













                                          I feel like people are always left with a feeling of "sure, I see how you can obtain that result but why isn't my interpretation correct?" Here's my latest attempt to address this issue.



                                          https://math.stackexchange.com/a/1801261/342573.






                                          share|cite|improve this answer


























                                            up vote
                                            1
                                            down vote













                                            I feel like people are always left with a feeling of "sure, I see how you can obtain that result but why isn't my interpretation correct?" Here's my latest attempt to address this issue.



                                            https://math.stackexchange.com/a/1801261/342573.






                                            share|cite|improve this answer
























                                              up vote
                                              1
                                              down vote










                                              up vote
                                              1
                                              down vote









                                              I feel like people are always left with a feeling of "sure, I see how you can obtain that result but why isn't my interpretation correct?" Here's my latest attempt to address this issue.



                                              https://math.stackexchange.com/a/1801261/342573.






                                              share|cite|improve this answer














                                              I feel like people are always left with a feeling of "sure, I see how you can obtain that result but why isn't my interpretation correct?" Here's my latest attempt to address this issue.



                                              https://math.stackexchange.com/a/1801261/342573.







                                              share|cite|improve this answer














                                              share|cite|improve this answer



                                              share|cite|improve this answer








                                              edited Apr 13 '17 at 12:21









                                              Community♦

                                              1




                                              1










                                              answered May 27 '16 at 1:17









                                              Karla H.

                                              503210




                                              503210




















                                                  up vote
                                                  1
                                                  down vote













                                                  There are at least three interpretations of the statement that "one child is a
                                                  girl". Two of them lead to the 1/2 and 1/3 answers which have been discussed
                                                  at length, but I found only one brief
                                                  answer covering the third
                                                  interpretation (though I did only skim).



                                                  The meaning of the statement, and consequently the correct answer, can be
                                                  altered by considering it to answer different questions.



                                                  1. "How many of your children are girls?" "One of my children is a girl."

                                                  2. "Is one of your children a girl?" "Yes, one of my children is a girl."

                                                  3. "Is the child in the stroller a boy or a girl?" "This one is a girl."

                                                  Suppose you've just had the first discussion; what do you think the other child
                                                  will be in that case? There is no chance the other is a girl.



                                                  The second question would prompt most people to search their set of children
                                                  for a girl and answer "yes" if they can find one or "no" otherwise.
                                                  Consequently, out of the 3/4 of two-child parents who can answer yes, about 1/3
                                                  of those will have another girl.



                                                  However it's conceivable that some kind of contrarian may resolve "one of your
                                                  children" even before they have considered the rest of the question, choosing
                                                  one at random regardless of its gender, and answering the rest of the question
                                                  with respect to that random choice. This renders the situation equivalent to
                                                  the third case.



                                                  In the third case, obviously, the gender of the other child is independent of
                                                  all the information we have. So it's a 1/2 chance.






                                                  share|cite|improve this answer






















                                                  • This is entirely incorrect. You are misunderstanding what "one of" means.
                                                    – The Count
                                                    Feb 22 '17 at 19:00










                                                  • @TheCount, who's the authority on the meaning of the words, here?
                                                    – sh1
                                                    Feb 22 '17 at 19:07










                                                  • Fair point, but I would argue that the term "one of them" does not imply the exclusion of "both of them" as in the famous and idiotic puzzle "I have two coins that add to 30 cents and one is not a nickel". The question was clearly not intended to be taken that way.
                                                    – The Count
                                                    Feb 22 '17 at 19:08






                                                  • 1




                                                    @TheCount, the point I'm trying to make is that the confusion between 1/2 and 1/3 comes about because we don't exactly know what we've been told. There's a third interpretation; not necessarily more correct, but also not invalid. It's a more "it couldn't possibly mean that!" case which is meant to highlight that the question is not well framed.
                                                    – sh1
                                                    Feb 22 '17 at 19:13










                                                  • I see. Well, you make a fair point, but I suppose my confusion comes about from the fact that your answer seemingly declares your interpretation to be the correct one, instead of explaining it to me as you just did. I think including that info in the answer itself would be helpful.
                                                    – The Count
                                                    Feb 22 '17 at 19:14














                                                  up vote
                                                  1
                                                  down vote













                                                  There are at least three interpretations of the statement that "one child is a
                                                  girl". Two of them lead to the 1/2 and 1/3 answers which have been discussed
                                                  at length, but I found only one brief
                                                  answer covering the third
                                                  interpretation (though I did only skim).



                                                  The meaning of the statement, and consequently the correct answer, can be
                                                  altered by considering it to answer different questions.



                                                  1. "How many of your children are girls?" "One of my children is a girl."

                                                  2. "Is one of your children a girl?" "Yes, one of my children is a girl."

                                                  3. "Is the child in the stroller a boy or a girl?" "This one is a girl."

                                                  Suppose you've just had the first discussion; what do you think the other child
                                                  will be in that case? There is no chance the other is a girl.



                                                  The second question would prompt most people to search their set of children
                                                  for a girl and answer "yes" if they can find one or "no" otherwise.
                                                  Consequently, out of the 3/4 of two-child parents who can answer yes, about 1/3
                                                  of those will have another girl.



                                                  However it's conceivable that some kind of contrarian may resolve "one of your
                                                  children" even before they have considered the rest of the question, choosing
                                                  one at random regardless of its gender, and answering the rest of the question
                                                  with respect to that random choice. This renders the situation equivalent to
                                                  the third case.



                                                  In the third case, obviously, the gender of the other child is independent of
                                                  all the information we have. So it's a 1/2 chance.






                                                  share|cite|improve this answer






















                                                  • This is entirely incorrect. You are misunderstanding what "one of" means.
                                                    – The Count
                                                    Feb 22 '17 at 19:00










                                                  • @TheCount, who's the authority on the meaning of the words, here?
                                                    – sh1
                                                    Feb 22 '17 at 19:07










                                                  • Fair point, but I would argue that the term "one of them" does not imply the exclusion of "both of them" as in the famous and idiotic puzzle "I have two coins that add to 30 cents and one is not a nickel". The question was clearly not intended to be taken that way.
                                                    – The Count
                                                    Feb 22 '17 at 19:08






                                                  • 1




                                                    @TheCount, the point I'm trying to make is that the confusion between 1/2 and 1/3 comes about because we don't exactly know what we've been told. There's a third interpretation; not necessarily more correct, but also not invalid. It's a more "it couldn't possibly mean that!" case which is meant to highlight that the question is not well framed.
                                                    – sh1
                                                    Feb 22 '17 at 19:13










                                                  • I see. Well, you make a fair point, but I suppose my confusion comes about from the fact that your answer seemingly declares your interpretation to be the correct one, instead of explaining it to me as you just did. I think including that info in the answer itself would be helpful.
                                                    – The Count
                                                    Feb 22 '17 at 19:14












                                                  up vote
                                                  1
                                                  down vote










                                                  up vote
                                                  1
                                                  down vote









                                                  There are at least three interpretations of the statement that "one child is a
                                                  girl". Two of them lead to the 1/2 and 1/3 answers which have been discussed
                                                  at length, but I found only one brief
                                                  answer covering the third
                                                  interpretation (though I did only skim).



                                                  The meaning of the statement, and consequently the correct answer, can be
                                                  altered by considering it to answer different questions.



                                                  1. "How many of your children are girls?" "One of my children is a girl."

                                                  2. "Is one of your children a girl?" "Yes, one of my children is a girl."

                                                  3. "Is the child in the stroller a boy or a girl?" "This one is a girl."

                                                  Suppose you've just had the first discussion; what do you think the other child
                                                  will be in that case? There is no chance the other is a girl.



                                                  The second question would prompt most people to search their set of children
                                                  for a girl and answer "yes" if they can find one or "no" otherwise.
                                                  Consequently, out of the 3/4 of two-child parents who can answer yes, about 1/3
                                                  of those will have another girl.



                                                  However it's conceivable that some kind of contrarian may resolve "one of your
                                                  children" even before they have considered the rest of the question, choosing
                                                  one at random regardless of its gender, and answering the rest of the question
                                                  with respect to that random choice. This renders the situation equivalent to
                                                  the third case.



                                                  In the third case, obviously, the gender of the other child is independent of
                                                  all the information we have. So it's a 1/2 chance.






                                                  share|cite|improve this answer














                                                  There are at least three interpretations of the statement that "one child is a
                                                  girl". Two of them lead to the 1/2 and 1/3 answers which have been discussed
                                                  at length, but I found only one brief
                                                  answer covering the third
                                                  interpretation (though I did only skim).



                                                  The meaning of the statement, and consequently the correct answer, can be
                                                  altered by considering it to answer different questions.



                                                  1. "How many of your children are girls?" "One of my children is a girl."

                                                  2. "Is one of your children a girl?" "Yes, one of my children is a girl."

                                                  3. "Is the child in the stroller a boy or a girl?" "This one is a girl."

                                                  Suppose you've just had the first discussion; what do you think the other child
                                                  will be in that case? There is no chance the other is a girl.



                                                  The second question would prompt most people to search their set of children
                                                  for a girl and answer "yes" if they can find one or "no" otherwise.
                                                  Consequently, out of the 3/4 of two-child parents who can answer yes, about 1/3
                                                  of those will have another girl.



                                                  However it's conceivable that some kind of contrarian may resolve "one of your
                                                  children" even before they have considered the rest of the question, choosing
                                                  one at random regardless of its gender, and answering the rest of the question
                                                  with respect to that random choice. This renders the situation equivalent to
                                                  the third case.



                                                  In the third case, obviously, the gender of the other child is independent of
                                                  all the information we have. So it's a 1/2 chance.







                                                  share|cite|improve this answer














                                                  share|cite|improve this answer



                                                  share|cite|improve this answer








                                                  edited Apr 13 '17 at 12:20









                                                  Community♦

                                                  1




                                                  1










                                                  answered Feb 22 '17 at 18:54









                                                  sh1

                                                  1273




                                                  1273











                                                  • This is entirely incorrect. You are misunderstanding what "one of" means.
                                                    – The Count
                                                    Feb 22 '17 at 19:00










                                                  • @TheCount, who's the authority on the meaning of the words, here?
                                                    – sh1
                                                    Feb 22 '17 at 19:07










                                                  • Fair point, but I would argue that the term "one of them" does not imply the exclusion of "both of them" as in the famous and idiotic puzzle "I have two coins that add to 30 cents and one is not a nickel". The question was clearly not intended to be taken that way.
                                                    – The Count
                                                    Feb 22 '17 at 19:08






                                                  • 1




                                                    @TheCount, the point I'm trying to make is that the confusion between 1/2 and 1/3 comes about because we don't exactly know what we've been told. There's a third interpretation; not necessarily more correct, but also not invalid. It's a more "it couldn't possibly mean that!" case which is meant to highlight that the question is not well framed.
                                                    – sh1
                                                    Feb 22 '17 at 19:13










                                                  • I see. Well, you make a fair point, but I suppose my confusion comes about from the fact that your answer seemingly declares your interpretation to be the correct one, instead of explaining it to me as you just did. I think including that info in the answer itself would be helpful.
                                                    – The Count
                                                    Feb 22 '17 at 19:14
















                                                  • This is entirely incorrect. You are misunderstanding what "one of" means.
                                                    – The Count
                                                    Feb 22 '17 at 19:00










                                                  • @TheCount, who's the authority on the meaning of the words, here?
                                                    – sh1
                                                    Feb 22 '17 at 19:07










                                                  • Fair point, but I would argue that the term "one of them" does not imply the exclusion of "both of them" as in the famous and idiotic puzzle "I have two coins that add to 30 cents and one is not a nickel". The question was clearly not intended to be taken that way.
                                                    – The Count
                                                    Feb 22 '17 at 19:08






                                                  • 1




                                                    @TheCount, the point I'm trying to make is that the confusion between 1/2 and 1/3 comes about because we don't exactly know what we've been told. There's a third interpretation; not necessarily more correct, but also not invalid. It's a more "it couldn't possibly mean that!" case which is meant to highlight that the question is not well framed.
                                                    – sh1
                                                    Feb 22 '17 at 19:13










                                                  • I see. Well, you make a fair point, but I suppose my confusion comes about from the fact that your answer seemingly declares your interpretation to be the correct one, instead of explaining it to me as you just did. I think including that info in the answer itself would be helpful.
                                                    – The Count
                                                    Feb 22 '17 at 19:14















                                                  This is entirely incorrect. You are misunderstanding what "one of" means.
                                                  – The Count
                                                  Feb 22 '17 at 19:00




                                                  This is entirely incorrect. You are misunderstanding what "one of" means.
                                                  – The Count
                                                  Feb 22 '17 at 19:00












                                                  @TheCount, who's the authority on the meaning of the words, here?
                                                  – sh1
                                                  Feb 22 '17 at 19:07




                                                  @TheCount, who's the authority on the meaning of the words, here?
                                                  – sh1
                                                  Feb 22 '17 at 19:07












                                                  Fair point, but I would argue that the term "one of them" does not imply the exclusion of "both of them" as in the famous and idiotic puzzle "I have two coins that add to 30 cents and one is not a nickel". The question was clearly not intended to be taken that way.
                                                  – The Count
                                                  Feb 22 '17 at 19:08




                                                  Fair point, but I would argue that the term "one of them" does not imply the exclusion of "both of them" as in the famous and idiotic puzzle "I have two coins that add to 30 cents and one is not a nickel". The question was clearly not intended to be taken that way.
                                                  – The Count
                                                  Feb 22 '17 at 19:08




                                                  1




                                                  1




                                                  @TheCount, the point I'm trying to make is that the confusion between 1/2 and 1/3 comes about because we don't exactly know what we've been told. There's a third interpretation; not necessarily more correct, but also not invalid. It's a more "it couldn't possibly mean that!" case which is meant to highlight that the question is not well framed.
                                                  – sh1
                                                  Feb 22 '17 at 19:13




                                                  @TheCount, the point I'm trying to make is that the confusion between 1/2 and 1/3 comes about because we don't exactly know what we've been told. There's a third interpretation; not necessarily more correct, but also not invalid. It's a more "it couldn't possibly mean that!" case which is meant to highlight that the question is not well framed.
                                                  – sh1
                                                  Feb 22 '17 at 19:13












                                                  I see. Well, you make a fair point, but I suppose my confusion comes about from the fact that your answer seemingly declares your interpretation to be the correct one, instead of explaining it to me as you just did. I think including that info in the answer itself would be helpful.
                                                  – The Count
                                                  Feb 22 '17 at 19:14




                                                  I see. Well, you make a fair point, but I suppose my confusion comes about from the fact that your answer seemingly declares your interpretation to be the correct one, instead of explaining it to me as you just did. I think including that info in the answer itself would be helpful.
                                                  – The Count
                                                  Feb 22 '17 at 19:14










                                                  up vote
                                                  0
                                                  down vote













                                                  Let $X$ be the event "one of the children is a girl" Let $BG$ refer to the event the family has one boy and one girl (I'm not distinguishing $BG$ and $GB$), and so forth. Everything is conditioned on the family having two children.



                                                  $$ P(GG | X) = fracP(GG wedge X)P(X)
                                                  = fracP(XGG) P(GG) + P(X
                                                  = frac1 cdot frac14
                                                  1 cdot frac14 + P(X
                                                  = frac1BG)
                                                  $$



                                                  (note that I am assuming this isn't taken as a "trick" question, so that $P(X|GG) = 1$ is meant, rather than $P(X|GG)=0$)



                                                  So everything boils down to the probability of "one of the children is a girl" given that the family has a boy and a girl.



                                                  Now, what does the phrase "one of the children is a girl" mean? There are two typical options



                                                  • This phrase simply means that this is not an all-boy family. In this case, $P(X|BG) = 1$, and $P(GG|X) = 1/3$.

                                                  • The phrase means that a particular child was chosen and it could just has easily have been the boy, so that $P(X|BG) = 1/2$, and $P(GG|X) = 1/2$.

                                                  In my opinion, an English speaker would almost always interpret the phrase the first way rather than the second way.



                                                  However, there is a nasty linguistic trap: even meaning it the first way, we may often pick a girl to be the referent of "one" (e.g. so that we can speak of the "other" child). And since we've picked a girl, we get stuck in the second interpretation. This is exacerbated by the fact the second interpretation has a very appealing shortcut for reasoning out the solution.






                                                  share|cite|improve this answer


























                                                    up vote
                                                    0
                                                    down vote













                                                    Let $X$ be the event "one of the children is a girl" Let $BG$ refer to the event the family has one boy and one girl (I'm not distinguishing $BG$ and $GB$), and so forth. Everything is conditioned on the family having two children.



                                                    $$ P(GG | X) = fracP(GG wedge X)P(X)
                                                    = fracP(XGG) P(GG) + P(X
                                                    = frac1 cdot frac14
                                                    1 cdot frac14 + P(X
                                                    = frac1BG)
                                                    $$



                                                    (note that I am assuming this isn't taken as a "trick" question, so that $P(X|GG) = 1$ is meant, rather than $P(X|GG)=0$)



                                                    So everything boils down to the probability of "one of the children is a girl" given that the family has a boy and a girl.



                                                    Now, what does the phrase "one of the children is a girl" mean? There are two typical options



                                                    • This phrase simply means that this is not an all-boy family. In this case, $P(X|BG) = 1$, and $P(GG|X) = 1/3$.

                                                    • The phrase means that a particular child was chosen and it could just has easily have been the boy, so that $P(X|BG) = 1/2$, and $P(GG|X) = 1/2$.

                                                    In my opinion, an English speaker would almost always interpret the phrase the first way rather than the second way.



                                                    However, there is a nasty linguistic trap: even meaning it the first way, we may often pick a girl to be the referent of "one" (e.g. so that we can speak of the "other" child). And since we've picked a girl, we get stuck in the second interpretation. This is exacerbated by the fact the second interpretation has a very appealing shortcut for reasoning out the solution.






                                                    share|cite|improve this answer
























                                                      up vote
                                                      0
                                                      down vote










                                                      up vote
                                                      0
                                                      down vote









                                                      Let $X$ be the event "one of the children is a girl" Let $BG$ refer to the event the family has one boy and one girl (I'm not distinguishing $BG$ and $GB$), and so forth. Everything is conditioned on the family having two children.



                                                      $$ P(GG | X) = fracP(GG wedge X)P(X)
                                                      = fracP(XGG) P(GG) + P(X
                                                      = frac1 cdot frac14
                                                      1 cdot frac14 + P(X
                                                      = frac1BG)
                                                      $$



                                                      (note that I am assuming this isn't taken as a "trick" question, so that $P(X|GG) = 1$ is meant, rather than $P(X|GG)=0$)



                                                      So everything boils down to the probability of "one of the children is a girl" given that the family has a boy and a girl.



                                                      Now, what does the phrase "one of the children is a girl" mean? There are two typical options



                                                      • This phrase simply means that this is not an all-boy family. In this case, $P(X|BG) = 1$, and $P(GG|X) = 1/3$.

                                                      • The phrase means that a particular child was chosen and it could just has easily have been the boy, so that $P(X|BG) = 1/2$, and $P(GG|X) = 1/2$.

                                                      In my opinion, an English speaker would almost always interpret the phrase the first way rather than the second way.



                                                      However, there is a nasty linguistic trap: even meaning it the first way, we may often pick a girl to be the referent of "one" (e.g. so that we can speak of the "other" child). And since we've picked a girl, we get stuck in the second interpretation. This is exacerbated by the fact the second interpretation has a very appealing shortcut for reasoning out the solution.






                                                      share|cite|improve this answer














                                                      Let $X$ be the event "one of the children is a girl" Let $BG$ refer to the event the family has one boy and one girl (I'm not distinguishing $BG$ and $GB$), and so forth. Everything is conditioned on the family having two children.



                                                      $$ P(GG | X) = fracP(GG wedge X)P(X)
                                                      = fracP(XGG) P(GG) + P(X
                                                      = frac1 cdot frac14
                                                      1 cdot frac14 + P(X
                                                      = frac1BG)
                                                      $$



                                                      (note that I am assuming this isn't taken as a "trick" question, so that $P(X|GG) = 1$ is meant, rather than $P(X|GG)=0$)



                                                      So everything boils down to the probability of "one of the children is a girl" given that the family has a boy and a girl.



                                                      Now, what does the phrase "one of the children is a girl" mean? There are two typical options



                                                      • This phrase simply means that this is not an all-boy family. In this case, $P(X|BG) = 1$, and $P(GG|X) = 1/3$.

                                                      • The phrase means that a particular child was chosen and it could just has easily have been the boy, so that $P(X|BG) = 1/2$, and $P(GG|X) = 1/2$.

                                                      In my opinion, an English speaker would almost always interpret the phrase the first way rather than the second way.



                                                      However, there is a nasty linguistic trap: even meaning it the first way, we may often pick a girl to be the referent of "one" (e.g. so that we can speak of the "other" child). And since we've picked a girl, we get stuck in the second interpretation. This is exacerbated by the fact the second interpretation has a very appealing shortcut for reasoning out the solution.







                                                      share|cite|improve this answer














                                                      share|cite|improve this answer



                                                      share|cite|improve this answer








                                                      edited Oct 17 '15 at 3:51

























                                                      answered Oct 17 '15 at 3:37









                                                      Hurkyl

                                                      109k9113254




                                                      109k9113254




















                                                          up vote
                                                          0
                                                          down vote













                                                          You have to read carefully. Here, the probability of a girl means the probability of at least one girl. You have
                                                          $$P(hboxone child is a girl) = 1 - P(hboxno child is a girl)
                                                          = 1 - P(hboxtwo boys) = frac14. $$
                                                          Now you are calculating the conditional probabilty
                                                          $$P(hboxtwo girls| hboxat least 1 girl = P(hboxtwo girls)over P(hboxat least one girl) = 1over 3.$$






                                                          share|cite|improve this answer
























                                                            up vote
                                                            0
                                                            down vote













                                                            You have to read carefully. Here, the probability of a girl means the probability of at least one girl. You have
                                                            $$P(hboxone child is a girl) = 1 - P(hboxno child is a girl)
                                                            = 1 - P(hboxtwo boys) = frac14. $$
                                                            Now you are calculating the conditional probabilty
                                                            $$P(hboxtwo girls| hboxat least 1 girl = P(hboxtwo girls)over P(hboxat least one girl) = 1over 3.$$






                                                            share|cite|improve this answer






















                                                              up vote
                                                              0
                                                              down vote










                                                              up vote
                                                              0
                                                              down vote









                                                              You have to read carefully. Here, the probability of a girl means the probability of at least one girl. You have
                                                              $$P(hboxone child is a girl) = 1 - P(hboxno child is a girl)
                                                              = 1 - P(hboxtwo boys) = frac14. $$
                                                              Now you are calculating the conditional probabilty
                                                              $$P(hboxtwo girls| hboxat least 1 girl = P(hboxtwo girls)over P(hboxat least one girl) = 1over 3.$$






                                                              share|cite|improve this answer












                                                              You have to read carefully. Here, the probability of a girl means the probability of at least one girl. You have
                                                              $$P(hboxone child is a girl) = 1 - P(hboxno child is a girl)
                                                              = 1 - P(hboxtwo boys) = frac14. $$
                                                              Now you are calculating the conditional probabilty
                                                              $$P(hboxtwo girls| hboxat least 1 girl = P(hboxtwo girls)over P(hboxat least one girl) = 1over 3.$$







                                                              share|cite|improve this answer












                                                              share|cite|improve this answer



                                                              share|cite|improve this answer










                                                              answered May 27 '16 at 1:27









                                                              ncmathsadist

                                                              41.4k257100




                                                              41.4k257100




















                                                                  up vote
                                                                  0
                                                                  down vote













                                                                  You reach into a bag and pull two marbles. You look at the first and see that it is black. There is a 1/2 chance that the other is red.



                                                                  You reach into a bag and pull two marbles. Your friend tells you whether or not you have 2 red marbles. He says you don't have 2 red marbles. The chance you have 2 black marbles isn't 1/2 or 1/4, it's 1/3.



                                                                  The assertion that at least one child is a girl is equivalent to saying that you don't have 2 boys. It is analogous to the second case above, not the first.






                                                                  share|cite|improve this answer
























                                                                    up vote
                                                                    0
                                                                    down vote













                                                                    You reach into a bag and pull two marbles. You look at the first and see that it is black. There is a 1/2 chance that the other is red.



                                                                    You reach into a bag and pull two marbles. Your friend tells you whether or not you have 2 red marbles. He says you don't have 2 red marbles. The chance you have 2 black marbles isn't 1/2 or 1/4, it's 1/3.



                                                                    The assertion that at least one child is a girl is equivalent to saying that you don't have 2 boys. It is analogous to the second case above, not the first.






                                                                    share|cite|improve this answer






















                                                                      up vote
                                                                      0
                                                                      down vote










                                                                      up vote
                                                                      0
                                                                      down vote









                                                                      You reach into a bag and pull two marbles. You look at the first and see that it is black. There is a 1/2 chance that the other is red.



                                                                      You reach into a bag and pull two marbles. Your friend tells you whether or not you have 2 red marbles. He says you don't have 2 red marbles. The chance you have 2 black marbles isn't 1/2 or 1/4, it's 1/3.



                                                                      The assertion that at least one child is a girl is equivalent to saying that you don't have 2 boys. It is analogous to the second case above, not the first.






                                                                      share|cite|improve this answer












                                                                      You reach into a bag and pull two marbles. You look at the first and see that it is black. There is a 1/2 chance that the other is red.



                                                                      You reach into a bag and pull two marbles. Your friend tells you whether or not you have 2 red marbles. He says you don't have 2 red marbles. The chance you have 2 black marbles isn't 1/2 or 1/4, it's 1/3.



                                                                      The assertion that at least one child is a girl is equivalent to saying that you don't have 2 boys. It is analogous to the second case above, not the first.







                                                                      share|cite|improve this answer












                                                                      share|cite|improve this answer



                                                                      share|cite|improve this answer










                                                                      answered May 27 '16 at 3:42









                                                                      DanielV

                                                                      17.4k42651




                                                                      17.4k42651




















                                                                          up vote
                                                                          0
                                                                          down vote













                                                                          There are many ways to explain this, but let me share with you how I do it in my class. I set $q=P(mboxYou say that he has at least one daughter mid mboxHe has $B,G$)$.



                                                                          If you sample at family level, then you ask the father if he has at least a daughter or you check official records. In that case $q=1$.



                                                                          If you sample at child level, by for instance meeting the father in the company of one child or otherwise happening to find out about one child, then $q=1/2$. In this case you will half the time miss this family because you happen to see the son first.



                                                                          Using Bayes' formula, we get
                                                                          $$
                                                                          P(B,G) mid mboxYou say that he has at least one daughter)
                                                                          = fracP(mboxYou say that he has at least one daughter mid B,G) P(B,G)
                                                                          P(mboxYou say that he has at least one daughter).
                                                                          $$



                                                                          Since
                                                                          $$P(mboxYou say that he has at least one daughter) = P(mboxYou say that he has at least one daughter mid B,G) P(B,G) + P(mboxYou say that he has at least one daughter mid G, G) P(G, G),
                                                                          $$



                                                                          we get
                                                                          $$
                                                                          P(B,G mid mboxYou say that he has at least one daughter) = (q times 1/2)/(q times 1/2 + 1 times 1/4) = 2q/(1 + 2q),
                                                                          $$



                                                                          and it follows that
                                                                          $$
                                                                          P(G, G mid mboxYou say that he has at least one daughter)=1/(1 + 2q).
                                                                          $$



                                                                          If $q=1/2$, we get $P=1/2$, and $q=1$ gives $P=1/3$.






                                                                          share|cite|improve this answer




















                                                                          • The issue with the question is that it does not say "at least one daughter", and instead says "one of the children is a girl". Once we re-interpret the problem to say 'at least one is a girl' we have already solved it.
                                                                            – Carl Mummert
                                                                            Aug 21 at 20:48














                                                                          up vote
                                                                          0
                                                                          down vote













                                                                          There are many ways to explain this, but let me share with you how I do it in my class. I set $q=P(mboxYou say that he has at least one daughter mid mboxHe has $B,G$)$.



                                                                          If you sample at family level, then you ask the father if he has at least a daughter or you check official records. In that case $q=1$.



                                                                          If you sample at child level, by for instance meeting the father in the company of one child or otherwise happening to find out about one child, then $q=1/2$. In this case you will half the time miss this family because you happen to see the son first.



                                                                          Using Bayes' formula, we get
                                                                          $$
                                                                          P(B,G) mid mboxYou say that he has at least one daughter)
                                                                          = fracP(mboxYou say that he has at least one daughter mid B,G) P(B,G)
                                                                          P(mboxYou say that he has at least one daughter).
                                                                          $$



                                                                          Since
                                                                          $$P(mboxYou say that he has at least one daughter) = P(mboxYou say that he has at least one daughter mid B,G) P(B,G) + P(mboxYou say that he has at least one daughter mid G, G) P(G, G),
                                                                          $$



                                                                          we get
                                                                          $$
                                                                          P(B,G mid mboxYou say that he has at least one daughter) = (q times 1/2)/(q times 1/2 + 1 times 1/4) = 2q/(1 + 2q),
                                                                          $$



                                                                          and it follows that
                                                                          $$
                                                                          P(G, G mid mboxYou say that he has at least one daughter)=1/(1 + 2q).
                                                                          $$



                                                                          If $q=1/2$, we get $P=1/2$, and $q=1$ gives $P=1/3$.






                                                                          share|cite|improve this answer




















                                                                          • The issue with the question is that it does not say "at least one daughter", and instead says "one of the children is a girl". Once we re-interpret the problem to say 'at least one is a girl' we have already solved it.
                                                                            – Carl Mummert
                                                                            Aug 21 at 20:48












                                                                          up vote
                                                                          0
                                                                          down vote










                                                                          up vote
                                                                          0
                                                                          down vote









                                                                          There are many ways to explain this, but let me share with you how I do it in my class. I set $q=P(mboxYou say that he has at least one daughter mid mboxHe has $B,G$)$.



                                                                          If you sample at family level, then you ask the father if he has at least a daughter or you check official records. In that case $q=1$.



                                                                          If you sample at child level, by for instance meeting the father in the company of one child or otherwise happening to find out about one child, then $q=1/2$. In this case you will half the time miss this family because you happen to see the son first.



                                                                          Using Bayes' formula, we get
                                                                          $$
                                                                          P(B,G) mid mboxYou say that he has at least one daughter)
                                                                          = fracP(mboxYou say that he has at least one daughter mid B,G) P(B,G)
                                                                          P(mboxYou say that he has at least one daughter).
                                                                          $$



                                                                          Since
                                                                          $$P(mboxYou say that he has at least one daughter) = P(mboxYou say that he has at least one daughter mid B,G) P(B,G) + P(mboxYou say that he has at least one daughter mid G, G) P(G, G),
                                                                          $$



                                                                          we get
                                                                          $$
                                                                          P(B,G mid mboxYou say that he has at least one daughter) = (q times 1/2)/(q times 1/2 + 1 times 1/4) = 2q/(1 + 2q),
                                                                          $$



                                                                          and it follows that
                                                                          $$
                                                                          P(G, G mid mboxYou say that he has at least one daughter)=1/(1 + 2q).
                                                                          $$



                                                                          If $q=1/2$, we get $P=1/2$, and $q=1$ gives $P=1/3$.






                                                                          share|cite|improve this answer












                                                                          There are many ways to explain this, but let me share with you how I do it in my class. I set $q=P(mboxYou say that he has at least one daughter mid mboxHe has $B,G$)$.



                                                                          If you sample at family level, then you ask the father if he has at least a daughter or you check official records. In that case $q=1$.



                                                                          If you sample at child level, by for instance meeting the father in the company of one child or otherwise happening to find out about one child, then $q=1/2$. In this case you will half the time miss this family because you happen to see the son first.



                                                                          Using Bayes' formula, we get
                                                                          $$
                                                                          P(B,G) mid mboxYou say that he has at least one daughter)
                                                                          = fracP(mboxYou say that he has at least one daughter mid B,G) P(B,G)
                                                                          P(mboxYou say that he has at least one daughter).
                                                                          $$



                                                                          Since
                                                                          $$P(mboxYou say that he has at least one daughter) = P(mboxYou say that he has at least one daughter mid B,G) P(B,G) + P(mboxYou say that he has at least one daughter mid G, G) P(G, G),
                                                                          $$



                                                                          we get
                                                                          $$
                                                                          P(B,G mid mboxYou say that he has at least one daughter) = (q times 1/2)/(q times 1/2 + 1 times 1/4) = 2q/(1 + 2q),
                                                                          $$



                                                                          and it follows that
                                                                          $$
                                                                          P(G, G mid mboxYou say that he has at least one daughter)=1/(1 + 2q).
                                                                          $$



                                                                          If $q=1/2$, we get $P=1/2$, and $q=1$ gives $P=1/3$.







                                                                          share|cite|improve this answer












                                                                          share|cite|improve this answer



                                                                          share|cite|improve this answer










                                                                          answered Apr 2 '17 at 18:23









                                                                          Helmer.Aslaksen

                                                                          212




                                                                          212











                                                                          • The issue with the question is that it does not say "at least one daughter", and instead says "one of the children is a girl". Once we re-interpret the problem to say 'at least one is a girl' we have already solved it.
                                                                            – Carl Mummert
                                                                            Aug 21 at 20:48
















                                                                          • The issue with the question is that it does not say "at least one daughter", and instead says "one of the children is a girl". Once we re-interpret the problem to say 'at least one is a girl' we have already solved it.
                                                                            – Carl Mummert
                                                                            Aug 21 at 20:48















                                                                          The issue with the question is that it does not say "at least one daughter", and instead says "one of the children is a girl". Once we re-interpret the problem to say 'at least one is a girl' we have already solved it.
                                                                          – Carl Mummert
                                                                          Aug 21 at 20:48




                                                                          The issue with the question is that it does not say "at least one daughter", and instead says "one of the children is a girl". Once we re-interpret the problem to say 'at least one is a girl' we have already solved it.
                                                                          – Carl Mummert
                                                                          Aug 21 at 20:48










                                                                          up vote
                                                                          -1
                                                                          down vote













                                                                          I think that the answer is going to be 1/3



                                                                          As the ways in which your neighbor can have children are as follow:



                                                                          (where B= Boy; G= Girl)



                                                                          BB, GG, BG, GB where BB is not possible as one of them is a girl. So the probability of the other child being a G is GG out of the sample space= GG, BG, GB



                                                                          I know some of you might argue that BG and GB are similar things but we don’t know whether the Sister is the elder one or the smaller one. So, it’s gonna work like that.






                                                                          share|cite|improve this answer
























                                                                            up vote
                                                                            -1
                                                                            down vote













                                                                            I think that the answer is going to be 1/3



                                                                            As the ways in which your neighbor can have children are as follow:



                                                                            (where B= Boy; G= Girl)



                                                                            BB, GG, BG, GB where BB is not possible as one of them is a girl. So the probability of the other child being a G is GG out of the sample space= GG, BG, GB



                                                                            I know some of you might argue that BG and GB are similar things but we don’t know whether the Sister is the elder one or the smaller one. So, it’s gonna work like that.






                                                                            share|cite|improve this answer






















                                                                              up vote
                                                                              -1
                                                                              down vote










                                                                              up vote
                                                                              -1
                                                                              down vote









                                                                              I think that the answer is going to be 1/3



                                                                              As the ways in which your neighbor can have children are as follow:



                                                                              (where B= Boy; G= Girl)



                                                                              BB, GG, BG, GB where BB is not possible as one of them is a girl. So the probability of the other child being a G is GG out of the sample space= GG, BG, GB



                                                                              I know some of you might argue that BG and GB are similar things but we don’t know whether the Sister is the elder one or the smaller one. So, it’s gonna work like that.






                                                                              share|cite|improve this answer












                                                                              I think that the answer is going to be 1/3



                                                                              As the ways in which your neighbor can have children are as follow:



                                                                              (where B= Boy; G= Girl)



                                                                              BB, GG, BG, GB where BB is not possible as one of them is a girl. So the probability of the other child being a G is GG out of the sample space= GG, BG, GB



                                                                              I know some of you might argue that BG and GB are similar things but we don’t know whether the Sister is the elder one or the smaller one. So, it’s gonna work like that.







                                                                              share|cite|improve this answer












                                                                              share|cite|improve this answer



                                                                              share|cite|improve this answer










                                                                              answered May 2 at 20:19









                                                                              Divy Rangan

                                                                              12




                                                                              12















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