When biologists claim that sex is binary, we mean something straightforward: There are only two sexes. This is true throughout the plant and animal kingdoms. An organism’s sex is defined by the type of gamete (sperm or ova) it has the function of producing. Males have the function of producing sperm, or small gametes; females, ova, or large ones. Because there is no third gamete type, there are only two sexes. Sex is binary.

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.)

When biologists claim that sex is binary, we mean something straightforward: There are only two sexes. This is true throughout the plant and animal kingdoms. An organism’s sex is defined by the type of gamete (sperm or ova) it has the function of producing. Males have the function of producing sperm, or small gametes; females, ova, or large ones. Because there is no third gamete type, there are only two sexes. Sex is binary.

Intersex people, whose genitalia appear ambiguous or mixed, don’t undermine the sex binary. Many gender ideologues, however, falsely claim the existence of intersex conditions renders the categories “male” and “female” arbitrary and meaningless. In “Hermaphrodites and the Medical Invention of Sex” (1998), the historian of science Alice Dreger writes: “Hermaphroditism causes a great deal of confusion, more than one might at first appreciate, because—as we will see again and again—the discovery of a ‘hermaphroditic’ body raises doubts not just about the particular body in question, but about all bodies. The questioned body forces us to ask what exactly it is—if anything—that makes the rest of us unquestionable.”

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