The question of exactly what sex differences exist and whether they have a biological foundation has been one of our culture's favorite enduring discussions. It should. After a baby is born, a parent's first concern is for its physical health. The next concern is its sex. Only in the most modern societies does sex not virtually guarantee the type of future life a new human being will have. Even in modern societies, one's sex usually plays a large role in the path a life follows.

Scientists have published thousands of papers on the subject, with the general conclusion being that men and...