A Biologist Explains Why Sex Is Binary

In an effort to confuse the issue, gender ideologues cite rare ambiguous ‘intersex’ cases.

Image Credit: PhonlamaiPhoto on iStock

This essay was originally published in The Wall Street Journal on April 9, 2023.

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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.”

In reality, the existence of borderline cases no more raises questions about everyone else’s sex than the existence of dawn and dusk casts doubt on day and night. For the vast majority of people, their sex is obvious. And our society isn’t experiencing a sudden dramatic surge in people born with ambiguous genitalia. We are experiencing a surge in people who are unambiguously one sex claiming to “identify” as the opposite sex or as something other than male or female.

Gender ideology seeks to portray sex as so incomprehensibly complex and multivariable that our traditional practice of classifying people as simply either male or female is grossly outdated and should be abandoned for a revolutionary concept of “gender identity.” This entails that males wouldn’t be barred from female sports, women’s prisons or any other space previously segregated according to our supposedly antiquated notions of “biological sex,” so long as they “identify” as female.

But “intersex” and “transgender” mean entirely different things. Intersex people have rare developmental conditions that result in apparent sex ambiguity. Most transgender people aren’t sexually ambiguous at all but merely “identify” as something other than their biological sex.

Once you’re conscious of this distinction, you will begin to notice gender ideologues attempting to steer discussions away from whether men who identify as women should be allowed to compete in female sports toward prominent intersex athletes like South African runner Caster Semenya. Why? Because so long as they’ve got you on your heels making difficult judgment calls on a slew of complex intersex conditions, they’ve succeeded in drawing your attention away from easy calls on unquestionably male athletes like 2022 NCAA Division I women’s swimming and diving champion Lia Thomas. They shift the focus to intersex to distract from transgender.

Acknowledging the existence of rare difficult cases doesn’t weaken the position or arguments against allowing males in female sports, prisons, restrooms and other female-only spaces. In fact, it’s a much stronger approach because it makes a crucial distinction that the ideologues are at pains to obscure.

Crafting policy to exclude males who identify as women, or “trans women,” from female sports, prisons and other female-only spaces isn’t complicated. Trans women are unambiguously male, so the chances that a doctor incorrectly recorded their sex at birth is zero. Any “transgender policy” designed to protect female spaces need only specify that participants must have been recorded (or “assigned,” in the current jargon) female at birth.

Crafting effective intersex policies is more complicated, but the problem of intersex athletes in female sports is less pressing than that of males in female sports, and there seem to be no current concerns arising from intersex people using female spaces. It should be up to individual organizations to decide which criteria or cut-offs should be used to keep female spaces safe and, in the context of sports, safe and fair. It is imperative, however, that such policies be rooted in properties of bodies, not “identity.” Identity alone is irrelevant to issues of fairness and safety.

Ideologues are wrong to insist that the biology of sex is so complex as to defy all categorization. They’re also wrong to represent the sex binary in an overly simplistic way. The biology of sex isn’t quite as simple as common sense, but common sense will get you a long way in understanding it.

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