‘Biology’ Is Not Binary, But ‘Sex’ Is
A new open letter signed by over 350 academics and clinicians criticizes the British government for acknowledging the reality of biological sex.
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About the Author
Dr. Colin Wright is an evolutionary biology PhD, Manhattan Institute Fellow, and CEO/Editor-in-Chief of Reality’s Last Stand. His writing has appeared in The Wall Street Journal, The Times, the New York Post, Newsweek, City Journal, Quillette, Queer Majority, and other major news outlets and peer-reviewed journals.
Last month, over 350 academics, clinicians, and activists signed an open letter titled “Biology is not binary.” Addressed to Bridget Phillipson, the United Kingdom’s Minister for Women and Equalities, its purpose was to denounce recent legal and policy developments in the U.K. that reassert the biological basis of sex in law.
The letter frames these developments as a threat to people who identify as transgender or non-binary. But in doing so, the signatories advance an anti-scientific ideology at odds with reality.
The letter was signed by some of the most influential proponents of sex denialism—the belief that “male” and “female” are socially constructed concepts rather than objective biological classifications. Among them were Sari van Anders, Canada 150 Research Chair at Queen’s University; Simón(e) Sun, a postdoctoral fellow at Cold Spring Harbor Laboratory; and Anne Fausto-Sterling, an emerita professor at Brown University best known for claiming that humans come in “five sexes.” These are not fringe figures—they are credentialed authorities, denying one of biology’s most fundamental truths.
The signatories express opposition to the U.K. Supreme Court’s April 2025 ruling in For Women Scotland v. The Scottish Ministers, which affirmed that the words “woman” and “sex” in the 2010 Equality Act refer to biological sex, not gender identity. Following the ruling, the nation’s Equality and Human Rights Commission (EHRC) updated its guidance to clarify that institutions should not admit trans-identifying males (so-called “trans women”) into female-only spaces such as bathrooms, hospital wards, and prisons.
The letter’s problems begin with its title: “Biology is not binary.” By using the word “biology” instead of “sex,” the authors obscure the actual subject under discussion. Biology is indeed full of variation—traits like height, skin tone, and hormone levels span a spectrum. But some biological traits really are binary. Sex, defined by the type of gamete an organism has the function to produce—sperm or ova—is one of them. There is no third gamete and therefore no third sex.
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By using the broad term “biology,” the authors hope to divert attention from this reproductive foundation and toward traits that correlate with but do not define sex. Their strategy is to conflate sexual dimorphism (physical and behavioral differences between males and females) with sex itself. But these are not the same thing. Sex-related traits exist on a continuum, but sex does not.
The authors write: “An individual’s ‘sex’ is in fact made up of a collection of characteristics, including external genitalia, secondary sex characteristics, gonads, chromosomes, and hormones, and may be better described as bimodal.” This is a textbook example of what I call the “polythetic fallacy.” A polythetic category is one in which no single trait is necessary or sufficient for membership—a category like “game” or “furniture.”
But sex is not polythetic. It is a functional classification defined by one—and only one—criterion: the type of gamete an individual has the biological function to produce. All other traits—genitalia, hormones, chromosomes—are either causes or consequences of this primary reproductive distinction.
The listed characteristics reflect this fact. People consider testosterone a “male” hormone because it is primarily produced by testes—the organs in humans and animals that have the function to produce sperm. An XY karyotype is considered “male”-typical because genes on the Y chromosome trigger the development of a reproductive system with the function to produce sperm. The signatories’ notion of male and female “characteristics” relies on the gamete-based definition of sex. The only reason we associate these characteristics with an individual’s sex is because they track the binary gamete roles.
From here, the signatories advance three arguments. The first is that binary sex categorization is too “simplistic” because some people are born with differences of sex development—congenital conditions that cause atypical chromosomal, gonadal, or genital development—and because medical interventions can alter sex-related traits. They state: “Binary categorisations at birth are simplified rules and do not precisely capture biological variation . . . . Primary and secondary sex characteristics change during life and through medical interventions.”
This is a bait-and-switch. It is true that sex-related traits can vary. But these variations do not define a person’s sex. A male with Complete Androgen Insensitivity Syndrome, for instance, may have a female-typical appearance due to his body’s inability to respond to androgens like testosterone—but he is still male. Why? Because he has testes, which have the biological function to produce sperm, even if that function is disrupted. CAIS is a male-only condition.
Medical interventions like puberty blockers, cross-sex hormones, and surgeries can change a person’s appearance, but they do not change his sex. You can remove a man’s testes and construct a “neovagina,” but he remains male. These interventions are cosmetic, not functional in a reproductive sense.
Most non-human species look very different from us. Many don’t have external genitals, the vast majority don’t grow breasts or facial hair, and some don’t even possess sex chromosomes. Yet, we can still reliably identify these animals’ sex by their reproductive roles.
Some reptiles’ sex is developmentally determined based on incubation temperature. Some fish can change sex during their lives. Yet we still categorize them as male or female based on the type of gamete they have the function to produce. Humans are no different.
The signatories’ second claim is that, because hormones and physiology are not binary, sex must not be, either. “Hormones, gene expression and physiology are not binary,” they write. “Many trans[-identifying] people take hormones during medical transition, and this process is generally responsible for extensive biological changes.”
This is true—and entirely irrelevant. Hormones are not sex. Fat distribution is not sex. Lung capacity is not sex. These are sex-related traits, not sex itself. This is a category error so basic that any first-year biology student should be able to spot it. The existence of variability in sex-influenced traits does not imply that sex itself is a spectrum.
Trans-identifying individuals certainly undergo significant physical changes during “transition.” But unless this results in the production of the opposite type of gamete—and it doesn’t—their sex has not changed.
Far from causing harm, acknowledging biological sex in medicine is essential for accurate diagnosis and treatment. A trans-identified male taking estrogen still needs to be screened for male-specific conditions like prostate cancer. Denying biological sex in favor of affirming identity not only creates confusion—it also risks lives.
The third argument made in the letter is perhaps the most dishonest: that sex may be binary in a narrow reproductive sense but is irrelevant to most aspects of daily life and therefore should not inform public policy. The authors write: “Statements that biological sex is binary are only approximately accurate in the context of reproduction or fertility, which are largely irrelevant to daily life.”
That might be true if human beings were amoebas. But in reality, humans are a sexually dimorphic species. On average, men are larger, stronger, faster, and more physically and sexually aggressive than women. These differences are not limited to sports—they show up in crime statistics, occupational-injury risks, and medicine. Sex matters for everything from pharmacology to prison safety.
The law recognizes many distinctions that are “irrelevant to daily life” for most people but relevant in specific contexts, such as age or felony-conviction status when purchasing a firearm. Sex is no different. We need the law to recognize the distinction between men and women not because sex matters every moment, but because when it does, the consequences of ignoring it are serious.
Finally, the authors accuse defenders of sex realism of engaging in “biological essentialism,” an antiquated concept that biologists now consider pseudoscience. But the biology of sex is not about essences. There is no metaphysical “maleness” or “femaleness” floating in the ether—just real, observable patterns in anatomy and function.
The signatories claim that the U.K. government and EHRC are promoting an “oversimplified” view of biology that harms people. In truth, what harms people is substituting ideology for science. We can uphold people’s rights without pretending that sex is a socially constructed fiction. And we certainly don’t need to erode women’s protections, rewrite biology textbooks, or abandon evidence-based medicine to do it.
The U.K. Supreme Court was right to affirm the reality of sex, and the EHRC was right to align its guidance accordingly. Public policy must reflect truths about the world, not what’s politically fashionable. There are two sexes, male and female, and this truth remains intact no matter how many open letters are signed to the contrary.
This article was originally published in City Journal on June 11, 2025.
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It is sad that some people who are educated will go along with an ideology that doesn’t reflect basic reality. It would be interesting to find out the motives . They must know that they are not being honest. So why do they perpetuate the falsehood of the gender cult?
Thank you for this very clear statement if the facts.