Two Americans

One of my favorite "lateral thinking puzzles" that I pose to young children is this:
Two Americans are standing in front of the main library in London; one of them is the son of the other one, but the second one is not the father of the first one -- how could this be?

Surprisingly, the fraction of girls that solve this is roughly the same as the fraction of boys that solve this, and in both cases, the fraction is low.

This article by Gloria Steinem in the New York Times revisits this bias in the context of the elections for the Democratic party's Presidential candidate, and includes this gem of a sentence: "...he is seen as unifying by his race while she is seen as divisive by her sex".