Responding to a ‘Fabulous Takedown’ of My Work

Twitter has an endless supply of credentialed academics whose ignorance is only exceeded by their confidence.

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Last week, on Easter, the Wall Street Journal published my essay titled “A Biologist Explains Why Sex Is Binary.” In this essay, I clarified the meaning of the sex binary, emphasized its importance, and highlighted ways to avoid falling into rhetorical traps set by activists.

The article is extremely straightforward. It explains that the sex binary means “there are only two sexes” because there are only two types of gametes (sperm and ova) that an individual can have the function to produce. People with rare intersex conditions do not undermine the sex binary because sex ambiguity is not a third sex. It is important not to conflate intersex conditions with identifying as transgender. Therefore, to protect female sports and spaces from males who identify as female, one need only specify that inclusion requires being recorded female at birth.

There really isn’t much to argue with here. However, Twitter never ceases to amaze with its ability to bring forth credentialed academics whose ignorance is only exceeded by their confidence.

One such academic is Agustin Fuentes, a tenured professor of anthropology at Princeton University, who wrote a critical Twitter thread about my essay. Fuentes claimed that it was a prime example of “why people really need to learn about biology in general and human biology in particular.” After briefly skimming his thread and observing the same tired misunderstandings of sex that have become routine, I decided not to respond unless the thread gained significant traction. I don't have the time to respond to every Twitter thread that is wrong about sex.

However, the Grand Poobah of sex pseudoscience, Anne Fausto-Sterling, decided to join in and christen Fuentes’ thread as a “fabulous takedown” of my essay and an example of “what biologists really say about binary sex.” She also included a backhanded comment, doubting that I would bother to read any of the articles Fuentes listed at the end of his thread for “those truly interested” in reading “some actual research work in this area of biology and human variation.”

For readers unaware, Anne Fausto-Sterling is the originator of the influential claim that there are five sexes in humans. Additionally, she is responsible for the widely circulated and exaggerated statistic claiming that intersex people comprise 1.7 percent or more of the human population. Later, when interrogated about her claim that there are five human sexes, Fausto-Sterling dismissed it by stating that she was being “tongue-in-cheek” and “ironic” when she made that assertion. She even scolded me on Twitter for calling her out on this!

When confronted about her wildly inaccurate statistic on intersex conditions in a Letter to the Editor by Carrie Hull at the University of Toronto, Fausto-Sterling appeared to accept the critique in her short three paragraph response, yet has done nothing to counter the spread of her false statistic, which is regularly being used by gender activists to argue that “sex is a spectrum.”

Since Fausto-Sterling decided to throw her hat in the ring, I believe a detailed response to Fuentes’ “fabulous takedown” of my work is now warranted. I will also go through all 10 of the articles he linked in his thread that Fausto-Sterling dared me to read to see whether they contain any significant rebuke to the sex binary.

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Fuentes begins his critique by highlighting and responding to what he views as “a few lines that are factually incorrect” in my essay. The first line he takes issue with was my claim that, “There are only two sexes. This is true throughout the plant and animal kingdoms.” Fuentes says this is “Totally untrue” because while “Most animals have two sexes…some have more than two, and some have only one. Let's not even get started on plants.”

Fuentes is very confused here and appears not to understand what sexes (i.e. males and females) are. Sexes are a phenomenon that exists only within anisogamous species, which are species that reproduce via the fusion of two differently-sized gametes (an organism’s reproductive cells). Males are universally defined as the sex that produces small sex cells (sperm), and females produce large sex cells (ova). Because there is no species that has evolved gametes that come in three different sizes, there are only two sexes that exist in nature—males and females.

My statement that “there are only two sexes…throughout the plant and animal kingdoms” is correct because every anisogamous species has only two gamete types—sperm and ova—and therefore has two and only two sexes. It is simply not true that some animals and plants “have more than two” sexes, because no species has ever been discovered that has evolved to produce gametes of three different sizes.

Fuentes then takes issue with my claim that “An organism’s sex is defined by the type of gamete (sperm or ova) it has the function of producing.” Below is his full (slightly cleaned up) response to this statement:

This is a very limited definition of sex, but even if one uses it there are a lot of variations. Some animals produce only one type of gamete, others produce two types, others alternate across the lifetime producing one or the other. Plus, producing the same type of gamete does not equal the same selection pressures, physiology or morphology on a given body/species—hyenas, whiptail lizards, humans and cave beetles all have morphs that produce large gametes but the lives and ecologies of those “females” (defined by large gamete production) are radically different due to different evolutionary trajectories, processes and dynamics.

So you can call large gamete producers “female” and go not further, but if that is the entirety of your definition, then it is a poor representation of the actual biology and evolutionary dynamics of any given organism. It is sloppy (lazy) science. Plus, and to the entire point of the piece: for humans an ova is not a woman and an sperm is not a man...to assert that gametes are the only relevant feature in the complex development, cultural, and evolutionary relevance of human sex biology is factually incorrect.

Fuentes claims my definition is “very limited,” and then goes on to provide a list of things that are totally irrelevant to an individual’s sex such as “selection pressures, physiology or morphology on a given body/species.” Let me be clear, nothing about males and females being fundamentally and universally defined by the gametes they produce necessitates that the selection pressures, physiology, morphology, “evolutionary trajectories,” or “lives and ecologies” must be identical across species.

This claim is nonsensical, and is the result of a fake ideological opponent activists have constructed out of thin air. This strawman was perhaps most clearly articulated by the biologist Joan Roughgarden, who said that “the biggest error in biology today is uncritically assuming that the gamete size binary implies a corresponding binary in body type, behavior, and life history.” I am, however, not aware of any biologist having ever made such a claim. If this truly is “the biggest error in biology today,” then surely there must be many examples of biologists saying this, but somehow I’ve yet to see one.

Fuentes says that stopping the definition of “female” at “large gamete producers” is “a poor representation of the actual biology and evolutionary dynamics of any given organism” and an example of “sloppy (lazy) science.” There are two main issues with this claim. The first is that the terms male and female were never meant to represent anything beyond fundamental reproductive strategies. Secondly, understanding the universal definition of male and female rooted in gamete size provides a much deeper insight into evolutionary dynamics than we could ever hope to have without it. It is a central organizing principle that helps explain the general pattern as well as interesting exceptions. This is the opposite of “sloppy” or “lazy” science—it is precise and illuminating.

Fuentes’ then claims that defining the sexes in terms of gamete size entails that “gametes are the only relevant feature in the complex development, cultural, and evolutionary relevance of human sex biology.” This is a truly bizare take, because I never claimed that gamete size is the only relevant feature of an organism. It is, however, the only relevant feature for deciding an organism’s sex.

Next, Fuentes claims that “no one who knows anything about biology is arguing that ‘the biology of sex is so complex as to defy all categorization.’”

This simply isn’t true. In 2018, Nature—the most prestigious scientific journal in the world—published an editorial claiming that classifying people’s sex “on the basis of anatomy or genetics should be abandoned” and “has no basis in science” and that “the research and medical community now sees sex as more complex than male and female.” It further claimed that “The idea that science can make definitive conclusions about a person’s sex or gender is fundamentally flawed.” Below is an excerpt from the piece that is clearly arguing for the futility in categorizing people according to sex.

Fuentes’ final argument asserts that my essay ignores “the reality of transexual and transgender lives in the current moment.”

I am not sure what this actually means. I understand that people who identify as transexual or transgender claim that they “feel like” or “identify as” the opposite sex, but subjective feelings and identity do not define one’s sex. While people are free to disagree with how the biology of sex should inform policy, people are not free to arbitrarily redefine what it means to be male and female for political reasons.

I am performing my duty as a scientist by stating how the world is, but it is up to everyone to collectively decide what we ought to do about it.

To conclude his thread, Fuentes provided 10 links to various articles and books that he claims represent the “actual research work in this area of biology and human variation.” Fausto-Sterling said she doubted I would read any of these articles about “what biologists really say about binary sex.”

I will now go through each of these links to see whether they substantiate a significant rebuke to the sex binary.

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List of “What Biologists Really Say about Binary Sex.”

1. J. F. McLaughlin et al. “Multimodal models of animal sex: breaking binaries leads to a better understanding of ecology and evolution.” bioRxiv (2023).

It’s amazing how quickly this pre-print has been soaked up by academics set on debunking the sex binary. Knowing this was likely to happen, I already wrote a lengthy rebuttal to this paper here on Reality’s Last Stand. For a shorter version of my critique, see my article in City Journal.

It is not an exaggeration to say that this manuscript is perhaps the most confused and amateurish paper I have ever read attempting to debunk the sex binary. It relies on constructing an absurd strawman that insists the sex binary requires a corresponding binary in every sex-related trait imaginable, such as body shape, behavior, life history, hormones, etc. However, no biologist to my knowledge has ever claimed that this is or must be the case.

In short, this paper blatantly confuses sexual dimorphism with the sex binary. This would be a forgivable mistake for an undergraduate to make, but not for graduate students, postdocs, or university professors.

2. Ah-King, Malin. On anisogamy and the evolution of ‘sex roles’. Trends Ecol. Evol 28, no. 1-2 (2013): 492.

This article does not undermine the sex binary at all. It simply argues that it is not always possible to claim with certainty that observed “sex roles,” by which the author means traits like “predominant male–male competition (i), female choosiness (ii), and conventional care patterns (iii),” are caused by anisogamy (different gamete sizes). While I believe the authors of the article downplay the importance of anisogamy for explaining general patterns in nature, their point that physiology, morphology, and environmental factors can also be a proximate cause of sex roles is well taken.

Importantly, the authors of this paper clearly understand that sex is rooted in gamete size. This is evident because of their ability to track instances of “conventional” verses “reversed” sex roles across species. If sex were not defined according to gametes, then “conventional” and “reversed” sex roles would be impossible to identify.

This paper unequivocally affirms the sex binary.

3. Cooke, Lucy. Bitch: On the Female of the Species. Basic Books, 2022.

This third link is a book by Lucy Cooke titled Bitch: On the Female of the Species. I don’t have time to read this entire book, so I will analyze the book’s Amazon description.

Studying zoology made Lucy Cooke feel like a sad freak. Not because she loved spiders or would root around in animal feces: all her friends shared the same curious kinks. The problem was her sex. Being female meant she was, by nature, a loser. 

Since Charles Darwin, evolutionary biologists have been convinced that the males of the animal kingdom are the interesting ones—dominating and promiscuous, while females are dull, passive, and devoted. 
 
In Bitch, Cooke tells a new story. Whether investigating same-sex female albatross couples that raise chicks, murderous mother meerkats, or the titanic battle of the sexes waged by ducks, Cooke shows us a new evolutionary biology, one where females can be as dynamic as any male. This isn‘t your grandfather’s evolutionary biology. It’s more inclusive, truer to life, and, simply, more fun.

It is clear from this description that Cooke is not disputing the definition of males and females as relating to gametes, but is simply trying to combat sexist stereotypes that depict females are less important than males. The description says she covers “same-sex female albatross couples that raise chicks” and examples in nature “where females can be as dynamic as any male.”

Nothing about this undermines the gametic definition of males and females. In fact, the author would not be able to know whether an albatross couple were both female, or highlight examples of male-like females without understanding the sexes in terms of the gametes they produce.

4. Todd, Erica V., Hui Liu, Simon Muncaster, and Neil J. Gemmell. "Bending genders: the biology of natural sex change in fish." Sexual Development 10, no. 5-6 (2016): 223-241.

This article is a review of the biological processes underlying sex changes in fish at the molecular level. Here is an excerpt from the abstract:

Sequential hermaphrodites begin life as one sex, changing sometime later to the other, and include species capable of protandrous (male-to-female), protogynous (female-to-male), or serial (bidirectional) sex change. Natural sex change involves coordinated transformations across multiple biological systems, including behavioural, anatomical, neuroendocrine, and molecular axes.

Nothing here undermines the sex binary. The way we know that some fish can change sex is because we observe individuals who produce one gamete type undergo changes that cause them to produce the other gamete type. This can happen in either direction—male to female, or female to male. The mechanisms driving these sex changes can be diverse. Without understanding that gametes define one’s sex, the very concept of a sequential hermaphrodite would be meaningless.

Far from challenging the sex binary, sequential hermaphrodites affirm it. That’s because we only ever observe males turning into females and females turning into males. No sequential hermaphrodite has ever changed into a third sex.

Throughout the review the authors only ever discuss males and females and switching between them. They never mention a third sex.

5. Zajitschek, Susanne RK et al. "Sexual dimorphism in trait variability and its eco-evolutionary and statistical implications." elife 9 (2020): e63170.

The title of this paper already makes it clear that it is about “sexual dimorphism,” which refers to sex-related differences in various traits like body size, anatomy, and behavior. Sexual dimorphism is not the same as the sex binary, although dimorphism is often a result of different selection pressures experienced by males and females.

Here is an excerpt from the paper’s abstract:

Biomedical and clinical sciences are experiencing a renewed interest in the fact that males and females differ in many anatomic, physiological, and behavioural traits. Sex differences in trait variability, however, are yet to receive similar recognition. In medical science, mammalian females are assumed to have higher trait variability due to estrous cycles (the ‘estrus-mediated variability hypothesis’); historically in biomedical research, females have been excluded for this reason. Contrastingly, evolutionary theory and associated data support the ‘greater male variability hypothesis’. Here, we test these competing hypotheses in 218 traits measured in >26,900 mice, using meta-analysis methods.

Here we see that the paper refers to “estrous cycles” as being unique to “mammalian females.” This is because the authors understand that only females produce eggs.

Here is one figure from the paper: “Comparing two groups”

What are those two groups? Males and females. In fact, this paper relies entirely on the sex binary. Nothing in this paper would make sense without it.

6. Glickman, Stephen E., Roger V. Short, and Marilyn B. Renfree. "Sexual differentiation in three unconventional mammals: spotted hyenas, elephants and tammar wallabies." Hormones and Behavior 48, no. 4 (2005): 403-417.

This review looks at “sexual differentiation in three non-conventional species: the spotted hyena, the elephant and the tammar wallaby, selected because of the natural challenges they present for contemporary understanding of sexual differentiation.” It’s important to note here that “sexual differentiation” refers to how male and female phenotypes develop from an undifferentiated initial state. Sexual differentiation typically refers to reproductive anatomy, but is sometimes expanded to secondary sexual characteristics and behavior as well.

The review selected spotted hyenas, elephants, and the tammar wallaby because they challenged a widely held assumption that “secretion of androgen and anti-Mullerian hormone (AMH) by the fetal testes during critical stages of development accounts for the full range of sexually dimorphic urogenital traits observed at birth.” The review concludes that “not all aspects of sexual differentiation have been delegated to testicular hormones in these mammals.”

This review does not challenge the sex binary at all, but simply explains how different species regulate the way males and females differentiate during development. Like many of the above articles, it would be impossible to know that “female spotted hyenas and elephants initially develop male-type external genitalia prior to gonadal differentiation” if you didn’t already understand what a female was apart from their external genitalia.

This article, like all the ones previously covered, relies on the sex binary.

7. Hyde, Janet Shibley, Rebecca S. Bigler, Daphna Joel, Charlotte Chucky Tate, and Sari M. van Anders. "The future of sex and gender in psychology: Five challenges to the gender binary." American Psychologist 74, no. 2 (2019): 171.

This review paper is confusing as it claims to debunk the “gender binary,” which is typically viewed as the set of social roles and norms society associates with males and females, yet uses examples of overlapping distributions in sex-related traits like brain structures and hormone profiles as evidence against it. Even if they meant “sex binary,” using these traits still would not make sense, as the sex binary only refers to the fact that there are only two sexes, and no biologist has ever claimed that the sex binary must entail a corresponding binary in all sex-related traits.

Once again, the authors of this paper appear to be confusing the sex binary with sexual dimorphism. Further, the paper rests on the assumption of the sex binary being real because all the studies used compare males and females across various traits. But they wouldn’t have been able to select these males and females for comparison if they didn’t already understand what males and females are apart from these other traits. Their own methods demonstrate their conclusion must be false.

8. Fausto-Sterling, Anne. "Gender/sex, sexual orientation, and identity are in the body: How did they get there?." The Journal of Sex Research 56, no. 4-5 (2019): 529-555.

This is a very confused paper by Anne Fausto-Sterling that proposed the idea that sex, which it never defines, and gender, which it also doesn’t define although she mentioned that others have defined it as “a person’s self-representation as male or female or how that person is responded to by social institutions on the basis of the individual’s gender presentation,” are completely intertwined and inseparable. However, even this definition of gender assumes sex is real and separate from gender, because otherwise what could it mean to represent yourself as male or female? What do the words male and female refer to in that definition of gender?

Fausto-Sterling also uses overlapping distributions of sex-related traits as evidence against the sex binary, which fails to understand what the sex binary actually means.

This paper has nothing whatsoever to do with what sex is, and instead focuses on a slew of sex-related traits, norms, sexual orientations, and how identities form. It certainly does not debunk the binary nature of biological sex.

9. DuBois, L. Zachary, and Heather Shattuck‐Heidorn. "Challenging the binary: Gender/sex and the bio‐logics of normalcy." American Journal of Human Biology 33, no. 5 (2021): e23623.

This paper also commits the fallacy of insisting that the sex binary is invalidated if any trait between the sexes can be found to overlap. It says that “binary categories” and the use of “group means to represent typical biologies” are “problematic” because they “reinforce stigma and inequality regarding gender/sex, gender identity, and sexuality.”

Once again, the fact that not all sex differences conform to a binary does not undermine the sex binary, which simply refers to the fact that there are only two sexes defined by the type of gamete they have the function to produce. While it may be true that we should not make normative claims about people based on group averages, this is totally unrelated to how we classify people according to biological sex.

10. van Anders, Sari M. "Gender/sex/ual diversity and biobehavioral research." Psychology of Sexual Orientation and Gender Diversity (2022).

This monograph by Sari van Anders article is attempting to do away with the sex binary by claiming, like Fausto-Sterling, that sex and gender cannot be separated and we should therefore use the term “sex/gender” or “gender/sex” when describing biology and identity. van Anders defines “gender, sex, and gender/sex in dynamic and multifaceted ways” and purports to outline a new “biobehavioral research framework” that “can be a feminist and queer successor science that moves beyond binaries to more empirical, accurate, and just knowledge.”

Despite claiming to offer clarity on matters of sex and gender, the paper uses meaningless circular definitions of both terms and is saturated with dense social justice jargon. In the end, van Anders asserts that “there is no ‘getting sex right’” and that “attempts to do so are a typological wild goose chase.” It is hard to imagine a claim more detached from reality given that doctors have no trouble “getting sex right” in humans over 99.98 percent of the time.

This article doesn’t debunk the sex binary. It is simply so detached from biological reality that it’s difficult to critique. But please stay tuned, as I am currently working on a detailed rebuttal to it for City Journal.

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