Who's Counting: Do Summer Births Mean More Boy Births?

New probability puzzle from columnist John Allen Paulos.

ByABC News
May 20, 2010, 8:34 PM

Oct. 3, 2010 &#151; -- During my summer hiatus from Who's Counting, a fascinating new paradox, first noticed by puzzle-meister Gary Foshee, became widely known. The following variant of it suggests that summer births may be associated with a much increased likelihood of boy births. Before I get to this paradoxical result, let me put it into context with two simple probability problems.

The first is very straightforward. Assume you know that a woman has two children, the older of whom is a boy. Given this knowledge, what is the probability that she has two boys? Let's count the possibilities. The only two are an older boy and younger girl (B-G) or an older boy and a younger boy (B-B). Since these are equally likely, the probability that the woman has two boys is 1/2.

No problem there, but now consider this second scenario. Assume that you know that a woman has two children, at least one of whom is a boy. You know nothing about this boy except his sex. Given this knowledge, what is the probability that she has two boys?

You might jump to the conclusion that the answer is again 1/2, reasoning that the sex of one child has no bearing on the sex of the other. This conclusion is incorrect, however, since you don't know whether the boy you know about is the older or the younger child.

So let's look at the possibilities. Listing two children in the order in which they might be born, we note four possibilities: B-B, B-G, G-B, G-G. Since you know that at least one of the two children is a boy, the G-G possibility is eliminated. Of the three remaining equally likely possibilities (B-B, B-G, and G-B) only one results in two boys. Therefore the correct conclusion in this case is that the probability the woman has two boys is 1/3, not 1/2.

This much has long been understood, but the paradox mentioned above is considerably less intuitive.

Now for the odd result. Suppose that when children are born in a certain large city, the season of their birth, whether spring, summer, fall, or winter, is noted prominently on their birth certificate. The question is: Assume you know that a lifetime resident of the city has two children, at least one of whom is a boy born in summer. Given this knowledge, what is the probability she has two boys?