On April 4, 2022, Congressman Madison Cawthorn of North Carolina, a conservative firebrand, delivered a speech intended to lecture Democrats on the meaning of “woman.” A woman, he said, is “XX chromosomes, no tallywhacker. It’s so simple.”

Chromosomal sex is of course one facet of human sexual biology. Ordinarily, someone with a large X and a small Y will be born male, and someone with two large X chromosomes will be born female. By “born” male or female, we mean that they will be assessed as one or the other, usually on the basis of possession of a tallywhacker (i.e., a penis, to adults), or lack thereof; and will be raised and socialized accordingly. Chromosomal sex, however, is hardly uniform across species. In birds, for example, the system is essentially reversed, so that the female is the one with two different sex chromosomes and the male is the one with two similar sex chromosomes.