Two sexes have never been enough to describe human variety. Not in biblical times and not now. Before we knew much about biology, we made social rules to administer sexual diversity. The ancient Jewish rabbinical code known as the Tosefta, for example, sometimes treated people who had male and female parts (such as testes and a vagina) as women — they could not inherit property or serve as priests; at other times, as men — forbidding them to shave or be secluded with women. More brutally, the Romans, seeing people of mixed sex as a bad omen, might kill a person whose body and mind did not conform to a binary sexual classification.

Today, some governments seem to be following the Roman model, if not killing people who do not fit into one of two sex-labeled bins, then at least trying to deny their existence. This month, Prime Minister Viktor Orban of Hungary banned university-level gender studies programs, declaring that “people are born either male or female” and that it is unacceptable “to talk about socially constructed genders, rather than biological sexes.” Now the Trump administration’s Department of Health and Human Services wants to follow suit by legally defining sex as “a person’s status as male or female based on immutable biological traits identifiable by or before birth.”

This is wrong in so many ways, morally as well as scientifically. Others will explain the human damage wrought by such a ruling. I will stick to the biological error.

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It has long been known that there is no single biological measure that unassailably places each and every human into one of two categories — male or female. In the 1950s the psychologist John Money and his colleagues studied people born with unusual combinations of sex markers (ovaries and a penis, testes and a vagina, two X chromosomes and a scrotum, and more). Thinking about these people, whom today we would call intersex, Dr. Money developed a multilayered model of sexual development.