Sex is supposed to be simple—at least at the molecular level. The biological explanations that appear in textbooks amount to X + X = and X + Y = . Venus or Mars, pink or blue. As science looks more closely, however, it becomes increasingly clear that a pair of chromosomes do not always suffice to distinguish girl/boy—either from the standpoint of sex (biological traits) or of gender (social identity).

In the cultural realm, this shift in perspective has already received a wide embrace. “Nonbinary” definitions of gender—transfeminine, genderqueer, hijra—have entered the vernacular. Less visible perhaps are the changes taking place in the biological sciences. The emerging picture that denotes “girlness” or “boyness” reveals the involvement of complex gene networks—and the entire process appears to extend far beyond a specific moment six weeks after gestation when the gonads begin to form.

To varying extents, many of us are biological hybrids on a male-female continuum. Researchers have found XY cells in a 94-year-old woman, and surgeons discovered a womb in a 70-year-old man, a father of four. New evidence suggests that the brain consists of a […]