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

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 $\frac{1}{3}$.

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.

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.