More on the “binarity” of biological sex

August 24, 2023 • 11:30 am

I guess today is a Sex Day, as I think every post will be on sex and gender. Right now I just want to call your attention to an excellent discussion of sex versus gender published at the end of last year. Had I known of it, it would have been cited in the “sex is a binary” section of the paper Luana and I wrote for The Skeptical Inquirer. Well, here’s my chance to let you know about this paper now.

Perhaps you’ve already had the gamete-based biological definition of “male” and “female” drilled into you on this site, but in case you didn’t, or even if you did but want to read about purported exceptions to this binary, the paper below in BioEssays is essential reading. It’s the paper you want to give to your friends who doubt that there is a sex binary. (It’s accessible to laypeople.)

Click below to read it, or you can find the pdf here.

I’ve written enough about the sex binary that I don’t want to say more, but I do want to give some nice quotes from this paper to give you an idea of its contents. If nothing else, they provide a review of what I and many other biologists think about sex and gender.  So here they are, indented (I’ve omitted most of the references, which are given in the original paper.

Biomedical and social scientists are increasingly calling the biological sex into question, arguing that sex is a graded spectrum rather than a binary trait. Leading science journals have been adopting this relativist view, thereby opposing fundamental biological facts. While we fully endorse efforts to create a more inclusive environment for gender-diverse people, this does not require denying biological sex. On the contrary, the rejection of biological sex seems to be based on a lack of knowledge about evolution and it champions species chauvinism, inasmuch as it imposes human identity notions on millions of other species. We argue that the biological definition of the sexes remains central to recognising the diversity of life. Humans with their unique combination of biological sex and gender are different from non-human animals and plants in this respect. Denying the concept of biological sex, for whatever cause, ultimately erodes scientific progress and may open the flood gates to “alternative truths.”

. . . Yet, the attempt of influential science journals to re-define sex is done for a laudable cause: namely, they wish to promote a more inclusive environment for gender-diverse people in academia and beyond. However, there is no need to deny the biological concept of sex to endorse the rights of gender-diverse people, because biological sex and gender are two entirely separate issues.  The gist of the problem seems to be that the definitions of sex and gender and their relationship are not generally appreciated, promoting the spread of flawed notions among readers of high-impact journals.

. . . Biological sex is defined as a binary variable in every sexually reproducing plant and animal species. With a few exceptions, all sexually reproducing organisms generate exactly two types of gametes that are distinguished by their difference in size: females, by definition, produce large gametes (eggs) and males, by definition, produce small and usually motile gametes (sperm).  This distinct dichotomy in the size of female and male gametes is termed “anisogamy” and refers to a fundamental principle in biology (Figure 1).

. . . .Biological sex reflects two distinct evolutionary strategies to produce offspring: the female strategy is to produce few large gametes and the male strategy is to produce many small (and often motile) gametes. This fundamental definition is valid for all sexually reproducing organisms. Sex-associated genotypes or phenotypes (including sex chromosomes, primary and secondary sexual characteristics and sex hormones), sex roles and sexual differentiation are consequences of the biological sex. Genotypic and phenotypic features, as well as sex roles are often used as operational criteria to define sex, but since these traits differ vastly between sexually reproducing species, they only work for selected species.

This biological definition of the two sexes is, however, not based on an essential “maleness” or “femaleness” of individuals, but it merely refers to two distinct evolutionary strategies that sexually reproducing organisms use to produce offspring. Sexual reproduction does not require the existence of separate male and female individuals, though. While in the majority of animals, female and male gametes are produced by different individuals, they can also be produced by the same individual, either simultaneously or at different times. For instance, many corals, worms, octopuses, snails and almost all flowering plants are simultaneous hermaphrodites, combining the production of male and female gametes and functions in the same individual at the same time. Many fish species, on the other hand, are sequential hermaphrodites, that is, they change their biological sex during their lifetime. Clownfish (Walt Disney’s Nemo), for example, start their reproductive career as males and only the largest individual of a group turns into a female. Some cleaner fish, on the other hand, are initially all females and later the largest individuals convert to males.

. . . Lest we are misread, we fully endorse the endeavor to create a more inclusive environment for women and gender-diverse people. Gender equity is a humanistic matter of course and it will also benefit science, which – for much too long – has been dominated by a male perspective. It appears, however, that the rejection or the disregard of the biological definition of sex by some philosophers, biomedical scientists and influential science journals is founded in a short-sighted perspective that only considers humans (or mammals) and neglects all other species.

. . . A widespread misconception among philosophers, biomedical scientists and gender theorists – and now also among some authors and editors of influential science journals – is that the definition of the biological sex is based on chromosomes, genes, hormones, vulvas, or penises, etc. or that biological sex is a social construct.  These notions very much reflect our own anthropocentric view. In fact, femaleness or maleness is not defined by any of these features that can, but do not need to be associated with the biological or gametic sex.

. . . One reason for this misconception of the biological sex lies in biomedical practices, in which mammalian sex chromosomes or sex-associated phenotypes are widely used to define sex . . . It is this definition that is targeted by critics of the fact that there are only two discrete sexes. However, sex chromosomes or sex-associated phenotypes do not qualify to define biological sex, as there are many species that do not have sex chromosomes at all. Whereas in mammals, birds, or butterflies sex chromosomes trigger sexual differentiation, in many other organisms, environmental factors, such as temperature or social regulators, initiate sex determination or sex change. Hence, sex chromosomes or other sex-determining systems cannot generally define sex. Instead, as the philosopher Paul Griffiths pointed out, “they are operational criteria for sex determination underpinned by the gametic definition of sex and valid only for one species or group of species”. Sex chromosomes, temperature gradients or social cues from group members can all be ways of making a sex, but they do not define it.

. . . Especially in biomedicine, many people are simply unaware of how evolutionary biologists define sex as biological sex. Another set of academics are fully aware of what biological sex is, but are blurring it on account of a political will to treat all people fairly. This stance seems to be motivated by a naturalistic fallacy (the mistake of a moral judgment based on natural properties), or an appeal-to-nature argument (proposing that something is good because it is natural)*, thereby overlooking that “being natural” is irrelevant for ethics. If these misconceptions are spread by scientists it may lead directly to people rejecting science in general, which will be most damaging for progress in society. Our main aim here is to draw attention to the dangers of scientific journals ignoring scientific facts, and to clarify the concept of biological sex.

. . . It is clear that the biological definition of the sexes cannot be the basis for defining social genders of people, as forcefully pointed out by the philosopher Paul Griffiths.  Likewise, the socio-cultural, and thus anthropocentric, construct of gender cannot be applied to non-human organisms.  There is a red line that separates humans with their unique combination of biological sex and gender from non-human animals and plants, which only have two distinct sexes – both of which are either expressed in the same or in different individuals. As much as the concept of biological sex remains central to recognize the diversity of life, it is also crucial for those interested in a profound understanding of the nature of gender in humans. Denying the biological sex, for whatever noble cause, erodes scientific progress. In addition, and probably even worse, by rejecting simple biological facts influential science journals may open the flood gates for “alternative truths.”

In our paper, Luana and I attribute the blurring of biological sex, and the claims that animals in nature don’t have a sex binary, to a reverse appeal to nature: the equally mistaken view that “what we see as good in human society (a sex spectrum) must be also what we see in nature.”

17 thoughts on “More on the “binarity” of biological sex

  1. Very good piece! Yes indeed, post-modernists and Trump loyalists seem to be enamored with “alternative facts.” There are no “alternative facts.” There are facts and there are fallacies. That’s it.

  2. This scientific fact of Nature has reached punishing levels of clarity.


    Never underestimate “supremely intelligent writing” about sex.

    (Go ahead – search for it … should still be there… yep – try the WorldCat, or Harvard results…)

  3. Good stuff. But this brought me up short:

    “[M]any corals, worms, octopuses, snails and almost all flowering plants are simultaneous hermaphrodites, combining the production of male and female gametes and functions in the same individual at the same time.”

    Octopuses are gonochoristic and have some famous sexual dimorphisms (the modified male arm or hectocotylus used for sperm transfer to the female). I don’t think there are any hermaphrodite cephalopods.

    1. Good catch. Another one in the Abstract that puzzled me was the statement that humans uniquely combine biological sex and gender. I’m not sure what was meant there, but I could see where males and females of other species have gender roles besides their contribution of gametes. Especially for social species where males and females play different parts in the group. Don’t lionesses do most of the hunting and cub rearing? And in some species the gender roles are reversed from the usual.

    2. Agree that’s ambiguous in the Abstract. I thought the authors meant “gender identity” not “gender role”. They unpack this in the first paragraph of SEX AND GENDER IN HUMANS. They introduce gender roles, but then explain that gender roles

      “…refer to how people perceive themselves, with or without a mismatch between this assessment and their biological sex. Because we have no way of knowing whether animals have any notion of gender, and no way of questioning them about it,[7] gender is uniquely human and the term should therefore not be used to refer to non-human animals.”

      I think that line in the Abstract is about self-perception (which we can only know in humans by asking them), not sex differences in behaviour like lions. But I’m not sure.

  4. Excellent. I imagine a formalized debate between two well-spoken and quick-witted people who nevertheless sit on opposite sides of this debate. The one who argues that sex is binary will lean heavily on aspects of biology, and will reel out fact after fact. They will invoke the history of how the consensus over biological sex was established over some centuries, leading to the terms we use today. They will take care to bring up how all of this has no effect on the humanistic values regarding the rights of LGBTQ persons. But the main body of their argument will be the observations and history that led to the meaning of the words “sex”, “male”, and “female”.
    Then the person who argues that sex is a spectrum will take the stage. They will quickly invoke various fallacies, and will also make great use of “chromosomal sex”, “anatomical sex”, and “social construct sex”. They will mix the words sex and gender interchangeably. Male with man. Female with women. They will create a view where biological sex is murky and undefinable because it’s a milieu of factors. But that is not where they will spend most of their time. Most of their time will be spent on impassioned appeals to emotion. Social justice. And feelings.

    1. Yup. The word salads spewed forth by (some of) the social justice advocates either reflect a muddled understanding of the facts or represent a purposeful effort to mislead or confuse. Or it could be both. Choose your poison.

    2. I would be happier if you would not force-team LGB with TQ in discussions about rights, Mark. You and I both know what are “humanistic values regarding the rights of LGBTQ persons.” But we (especially women) don’t want anyone in the alphabet, especially the male ones, entering spaces reserved for the sex they aren’t, (the familiar bathrooms, shelters, sports, and prisons business.) LGB people have no problem with this constraint on their “rights” because they would have little reason to want to enter opposite-sex spaces*. But the official TQ advocacy position is that entering those opposite-sex spaces just is part of the humanistic values regarding their rights, especially if they are men presenting (and wanting access) as women.

      We say, magnanimously, that TQ people should have all the usual civil rights except the one we don’t want to give them, that is, the right to enter spaces according to their self-claimed gender. They say we have no right to limit their rights that way, because doing so is erasure and genocide.

      While we must resolutely defend the human biological reality of the binary and immutable nature of sex, the controversy has moved on. The TQ activists don’t really say their sex is on a spectrum. They say sex doesn’t matter. It’s gender that trumps everything. (I think that as they start to lose ground, they stake out a shriller position.)
      The definition of a woman becomes anyone who adopts a female gender. They refuse to acknowledge that we have any rights to keep them, as women, out of women’s spaces.

      There are two public battles:
      1) Keeping men with female gender out of women’s spaces, as above.
      2) Protecting children from being mutilated by ideologues who see the need to bring the child’s body into cosmetic conformity with some stereotypic appearance of the opposite sex. Sex might not matter but it does provide a “serving suggestion.”
      In neither case does the binary nature of sex anymore squelch the ideologic goals. It is simply ignored in favour of gender.
      * I’m told that in gay nightclubs where everyone in the place is one sex, the opposite-sex bathroom is likely to be used equally. No pee-able place goes unused when people are drinking beer with no one around to complain or pose a threat.

      1. Humanistic values must imply there are dehumanizing values that antagonize them. Who might embody those dehumanizing values? The bourgeoise.

        Not necessarily, of course, but as Gayle Rubin makes crystal clear in Thinking Sex, Queer Theory is normativity Marxism. The “rights” she elaborates upon are breathtaking – “eroticism [that] transgresses generational boundaries” – ooo, Gayle, what might that be? Ooo, those grumpy mean conservatives, prosecuting ” ‘child pornography’ “… yes, she puts “child pornography” in scare quotes. Read it for yourself (search the Internet for it).

        1. Late edit: I put “rights” in quotes but I didn’t mean Rubin wrote that word. I just meant it as my own scare quotes.

  5. Are there any exceptions to the rule in sexual reproduction that (i) there are exactly two fertilizing gametes (rather than three or more) and that (ii) one of them is always much larger than the other?

    I once found a table of the sperm/egg size ratios across many species, but I can’t seem to find it now.

    1. In animals no this doesn’t happen. In flowering plants, two fertilization events happen (involving 5 cells – 2 sperm from the male gametophyte that hatches from the pollen grain, an ovule from the female gametophyte, plus the two polar bodies that are tiny cells left over from meiosis in the female). The sperm + ovule form the plant embryo, and the other sperm plus the two polar bodies form the endosperm that nourishes the developing offspring.

      The wiki page is pretty good.

      1. Thanks for the link!

        If you know a reference for the range in egg/sperm size ratios across species, I’d be very interested in that too.

  6. Sex to a biologist is short for sexual reproduction, a binary system. Sex to the rest of of is shorthand for a lot of stuff. It is a slangy term for us, not a precise term as used by biologists. Add in ideological agendas that wish to load up a term their own ends and you have a complete mess with people around each other.

    1. Last sentence meant to read: ‘Add in people with ideological agendas who wish to load up a term their own ends and you have a complete mess with people talking around each other’

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