One of the famous intuitive mistakes in probability comes from the simple question, "Do boys have more sisters than girls do?" A quick analysis of the situation may prompt you to say yes. A more in-depth look might change your mind.

I don't have a sister. I have to say, I don't feel much deprived, except when I think of the millions I might make if I were to get my sister to drive across America with me, or volunteer in Bangladesh with me, or meditate with me every morning for a year, and then write a sappy best-selling memoir about it. The question is, did being born female decrease my odds of having a sister?

This is the set-up for a well-known question in probability. A quick look at the idea might give you the impression that, yes, boys have more sisters than girls do. A two-child family that includes a boy and a girl gives the boy a sister, and the girl a nothing but a contemptible brother. Even if the parents had another child, and provided their daughter with a sister, the boy would now have two sisters to the girl's one. Boys have the advantage, it seems.