Why sex is binary

Last week the Wall Street Journal published an article ‘The Dangerous Denial of Sex’ by two scientists, Colin Wright and Emma Hilton. Wright is an evolutionary biologist at Penn State and Hilton is a developmental biologist at the University of Manchester. It is well worth reading the article in full. Here are some excerpts from it where they debunk the anti-scientific trend toward the outright denial of biological sex:

“The idea of two sexes is simplistic,” an article in the scientific journal Nature declared in 2015. “Biologists now think there is a wider spectrum than that.” A 2018 Scientific American piece asserted that “biologists now think there is a larger spectrum than just binary female and male.” And an October 2018 New York Times headline promised to explain “Why Sex Is Not Binary.”

The argument is that because some people are intersex—they have developmental conditions resulting in ambiguous sex characteristics—the categories male and female exist on a “spectrum,” and are therefore no more than “social constructs.”

Wright and Hilton point out that this is false on all levels:

If male and female are merely arbitrary groupings, it follows that everyone, regardless of genetics or anatomy should be free to choose to identify as male or female, or to reject sex entirely in favor of a new bespoke “gender identity.”

In humans, as in most animals or plants, an organism’s biological sex corresponds to one of two distinct types of reproductive anatomy that develop for the production of small or large sex cells—sperm and eggs, respectively—and associated biological functions in sexual reproduction. In humans, reproductive anatomy is unambiguously male or female at birth more than 99.98% of the time. The evolutionary function of these two anatomies is to aid in reproduction via the fusion of sperm and ova. No third type of sex cell exists in humans, and therefore there is no sex “spectrum” or additional sexes beyond male and female. Sex is binary.

There is a difference, however, between the statements that there are only two sexes (true) and that everyone can be neatly categorized as either male or female (false). The existence of only two sexes does not mean sex is never ambiguous. But intersex individuals are extremely rare, and they are neither a third sex nor proof that sex is a “spectrum” or a “social construct.” Not everyone needs to be discretely assignable to one or the other sex in order for biological sex to be functionally binary. To assume otherwise—to confuse secondary sexual traits with biological sex itself—is a category error.

The authors also argue that women are entitled to keep their hard fought for sex-based legal protections:

Female-only spaces are necessary due to the pervasive threat of male violence and sexual assault. Separate sporting categories are also necessary to ensure that women and girls don’t have to face competitors who have acquired the irreversible performance-enhancing effects conferred by male puberty. The different reproductive roles of males and females require laws to safeguard women from discrimination in the workplace and elsewhere. The falsehood that sex is rooted in subjective identity instead of objective biology renders all these sex-based rights impossible to enforce.

This ideological attack also negates the existence of homosexuals. Without biological sex and the distinction between the sexes same-sex attraction is meaningless. Wright and Hilton are critical of the attacks on lesbians who have been called bigots for not being attracted to males who identify as women.

The most vulnerable in this sex denialism are children.  This is an important point at a time when organisations like InsideOUT are in schools telling children they can be born into the wrong body and it is possible to change sex. It’s not only unscientific; it is resurrecting sex stereotypes.

When they’re taught that sex is grounded in identity instead of biology, sex categories can easily become conflated with regressive stereotypes of masculinity and femininity. Masculine girls and feminine boys may become confused about their own sex. The dramatic rise of “gender dysphoric” adolescents—especially young girls—in clinics likely reflects this new cultural confusion.

The large majority of gender-dysphoric youths eventually outgrow their feelings of dysphoria during puberty, and many end up identifying as homosexual adults. “Affirmation” therapies, which insist a child’s cross-sex identity should never be questioned, and puberty-blocking drugs, advertised as a way for children to “buy time” to sort out their identities, may only solidify feelings of dysphoria, setting them on a pathway to more invasive medical interventions and permanent infertility. This pathologizing of sex-atypical behavior is extremely worrying and regressive. It is similar to gay “conversion” therapy, except that it’s now bodies instead of minds that are being converted to bring children into “proper” alignment with themselves.

They call on biologists and medical professionals to stand up for the empirical reality of biological sex.

When authoritative scientific institutions ignore or deny empirical fact in the name of social accommodation, it is an egregious betrayal to the scientific community they represent. It undermines public trust in science, and it is dangerously harmful to those most vulnerable.

Emma Hilton established  Project Nettie, an online record of scientists, medics and those in related disciplines who assert the material reality of biological sex and reject attempts to reframe it as a malleable social construct.  It is regularly updated with signatures.


  1. The argument that the existence of intersex people does not matter sounds convincing in theory, but fails in actual practice. Intersex is not a theoretical concept, it is real living breathing people whose social place and even legal status depends on how society and the law interpret their bodies. To say “they are just exceptions” ignores the fact that the whole issue is about What Does Society Do With The Exceptions. THAT becomes very complicated indeed when those exceptions get complex, contradictory, or even frankly beyond the full knowledge of even the experts. What do you do with a human composed of one male and one female embryo that fused, leaving a patchwork body of cells of each sex? The cases where sex seems certain at birth only to change drastically at puberty, as with Alpha reductase Deficiency Syndrome? What about when Drs make one choice at birth only to find they were very wrong? The argument that sex is not a strict binary is expressly to give these people meaningful lives instead of being shamed into invisibility or mutilated to allow society to maintain the illusion that sex is more straightforward than biological reality.

    • The article explains that intersex conditions do exist but that doesn’t negate the binary nature of sex.

      The example you give of the intersex condition Alpha reductase Defiicency Syndrome is actually more proof too that there are only two sexes. That syndrome *only* occurs in males. Similarly Klinefelter syndrome only occurs in males. Turner syndrome is another intersex condition, and that only occurs in females.

      What your comment shows is not the complexity of sex but the cynical use of intersex by gender identity ideologues.

    • Intersex is a specific set of individual genetic disorders of sexual development –
      Between 0.5% and 1.7% of the global population are born with intersex traits.

      If someone had trisonomy 21, we don’t say they are on a human spectrum of any sort, we classify them as having Down syndrome.

      Without changing biological facts, some countries have a “third option” tick to accommodate ppl with intersex traits.

      On January 1, [2019] Germany will become the first country in the European Union to offer a “third gender” option on birth certificates.
      Intersex people — and parents of intersex babies — will be able to register as “divers,” or miscellaneous, on birth certificates.
      Under the new law, people wanting to change their birth certificate to read “divers” will only be able to do so with a medical certificate to prove it.

      Several other countries have provided gender-neutral options on passports and official documents such as the census or ID cards, including Argentina, Bangladesh, Canada, Denmark, India, Malta, Nepal, the Netherlands, New Zealand and Pakistan.

  2. I agree with the general conclusion of this article, and the various concerns it raises, but I do not think sex is binary because sex is detained by genetics/genome (DNA), and that is not binary.

    See https://en.wikipedia.org/wiki/Sex
    “In genetic sex-determination systems, an organism’s sex is determined by the genome it inherits.”

    And https://en.wikipedia.org/wiki/Intersex
    “Intersex people are individuals born with any of several variations in sex characteristics including chromosomes, …”

    • Nevertheless the *result* of the genetic expression of sex is sexual dimorphism, and its evolutionary purpose is sexual reproduction. Further, when we talk of male and female we are using scientific classifications. It is necessary language to describe the distinct categories.

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