We’ve all said it, or at least heard someone say it. It happened to be the day after Christmas and I got to wondering where the saying came from. Purely coincidental, of course.

A piece in the New York Times that day opened with this sentence: “Tell us a few details about you and we’ll guess which political party you belong to.” Like a person’s gender. Male or female? Mars or Venus? Comparing data from the decade 1968-1978 to the recent past (2008-2016) reveals that the gap between youn...