Boy and girl paradox is driving me crazy
Then the father called (one of) the boy(s) here
Why did the father do that? It matters.
(A) If the father called over a child at random, and it happened to be one of the boys you had been talking about, then the probability that the remaining child is a boy is $1/2$.
(B) If you specifically asked the father to call over a boy, and he obliged, then you've learned nothing new, and the probability that the remaining child is a boy is still $1/3$.
Let's model both scenarios with the following probability space:
- BB1 BB2 BG1 BG2 GB1 GB2 GG1 GG2
The first letter is the sex of the older child, the second letter is the sex of the younger child, and the numeral is the favorite child, whom the father will call over if given the opportunity. Assume that all three variables are independent coin flips.
This is the event that the father has at least one boy:
- BB1 BB2 BG1 BG2 GB1 GB2
This is the event that the father calls over a boy in scenario (A):
- BB1 BB2 BG1 GB2
This is the event that the father calls over a boy in scenario (B):
- BB1 BB2 BG1 BG2 GB1 GB2
The apparent paradox lies in the information difference between "at least one" and "this one".
Suppose it's Halloween and the father is accompanied by two children in bulky costumes; so you cannot tell which is what. Let $L$ be the event of a boy being inside the costume on the left, and $R$ be the event of a boy being inside the costume on the right.
Prior to learning anything else, the probability that they are both boys is $\mathsf P(L\cap R)=1/4$, if we assume an independent 50:50 chance of boyhood for each.
If it comes up in conversation that at least one of them is a boy, then you only know $L\cup R$ and the conditional probability is: $$\mathsf P(L\cap R\mid L\cup R) = 1/3$$
If you then learn that the one on the left is a boy, then you know $L$ and the conditional probability is: $$\mathsf P(L\cap R\mid L) = 1/2$$
This is something you have to watch for as it is somewhat counter intuitive and can be subtle.
Our intuition is that if we know that "at least one child is a boy" , then the probability that "both children are boys" is the probability that "the other child is a boy". However, our intuition is wrong we don't know which child is the other child.
When told that "at least one a boy", that information could be given when just this child, just that child, or both children are boys.
When we are told that "that one a boy", that the information can only be given when either just this child, or both children, are boys. We know something extra (whether it's position, age, or whatever) about the identity of the boy.
It's not so much a paradox as an illustration that the way you pick the child matters. Let's look at it in the large:
If you take a list of all two-child families that have at least one boy, you'll find that one-third of them have two boys.
If you take a list of all boys with exactly one sibling, you'll find that half of them have a brother.
Why does this happen? In the second case, you're double-counting families with two boys.