The transgender movement has left many intelligent Americans confused about sex. Asked to define the word “woman” during her Supreme Court confirmation hearings last year, Ketanji Brown Jackson demurred, saying “I’m not a biologist.” I am a biologist, and I’m here to help.

Are sex categories in humans empirically real, immutable and binary, or are they mere “social constructs”? The question has public-policy implications related to sex-based legal protections and medicine, including whether males should be allowed in female sports, prisons and other spaces that have historically been segregated by sex for reasons of fairness and safety.

Chase Strangio of the American Civil Liberties Union frequently claims that the binary concept of sex is a recent invention “exclusively for the purposes of excluding trans people from legal protections.” Scottish politician Maggie Chapman asserted in December that her rejection of the “binary and immutable” nature of sex was her motivation for pursuing “comprehensive gender recognition for nonbinary people in Scotland.” (“Nonbinary” people are those who “identify” as neither male nor female.)