In his acclaimed new book “The Evolution of Beauty” (Doubleday, 428 pages, $30), Richard O. Prum theorizes that evolution is not just about what we need but what we want. And that has profound implications for gender issues, including what I describe on this blog as “the beauty trap.”

The hourglass shape and youthful facial features, such as large eyes, of the female and the particular characteristics of male genitalia (boneless phallus, large scrotum, small testes) are beyond what is necessary for reproduction, Prum writes. Rather, he says, these features are what each desires in the other.

But it is not a level playing field ...