B. D. Earp, M. Carpenter, and S. Porsdam Mann attempt to answer the question posed by the Archives of Sexual Behavior, “How many sexes are there?”, in their recent paper (Earp, et al., 2026). They explicitly state that they set aside the related question, “How many genders are there?”, because they want to focus on the biological issue.

From the outset, they make it clear that they endorse an ambiguity thesis concerning the word “sex” in a scientific context. This is somewhat surprising in itself, because ambiguity theses in the philosophy of trans issues are usually advanced with respect to the semantics of the words “woman” and “man” in ordinary language, where one possible interpretation is biological, i.e. expressing the concept of sex, and the other is social, i.e. expressing the concept of gender. The case for such an ambiguity thesis is often fairly strong, since one might expect the richness and flexibility required for everyday conversation to clash with the demand for simplicity in ordinary language, resulting in a compromise at the expense of clarity. Scientific language, by contrast, is generally driven toward reducing ambiguity by assigning different terms to different concepts wherever possible. This should already make one skeptical of the linguistic picture of biology presented in On Whether Sex is Binary. And indeed, as I will argue, that picture is rather peculiar.

The authors claim that there are two possible semantic values for the word “sex” in biology: “[1] an abstract evolutionary model based on idealized body plans (interpreted functionally in terms of hypothetical gamete production), and [2] the diverse ways that actual human bodies are organized in practice.” Their explanation of the first meaning, however, leaves the scope of the word “plan” rather vague. On the most charitable reading, it could be interpreted as a “plan to grow tissue that produces a certain type of gametes.” Yet this restricted interpretation is clearly not what they have in mind, as becomes apparent later in the paper when they apply the notion and contrast it with the second interpretation. There they write: “The existence of people who have what are sometimes called intersex traits, DSDs, or innate variations in sex characteristics is, again, relevant to the point we are making. So, too, is the existence of transgender people—perhaps especially those who have undergone hormonal or surgical interventions to alter their biology in significant ways.“ Thus, they do not appear to restrict the notion of a “plan” to the development of potentially gamete-producing tissue, but rather to the body as a whole.

This also helps clarify what the authors mean by “abstract” and “idealized.” One might reasonably wonder what is supposed to be abstract or idealized about gamete-producing tissue, given that such tissue and the corresponding gametes differentiate in the fetus by the sixth week of pregnancy. A charitable interpretation would be that gamete or gamete-producing tissue cannot be studied in isolation from the complex system of the human body in which it exists. On this reading, their “abstract evolutionary theory” could refer to a population model that reduces the complexity of the human body as a whole to a vector encoding gametes producing tissue within a mathematical structure that interacts with other such vectors. Since not all features of the human body can be represented within such a model, some degree of idealization is unavoidable, and one may therefore work with statistical averages across male and female populations. Yet this kind of abstraction would not stand in much contrast to what the authors present as the second meaning of “sex.” Consequently, what they seem to be offering as an abstract evolutionary model based on idealized body plans is not merely a model of gamete-producing tissue, but of the body as a whole. By this I mean—and I take it that their somewhat vague language points in this direction—idealized women and men in the manner of figures found in a neoclassical painting.

This is less a straw man than an instance of wishful thinking: scientific theories are reconstructed in such a way that a preferred critique becomes applicable. There is some literature on idealized archetypes that function in a manner analogous to Platonic forms within the biological sciences (Daston & Galison, 1992). More generally, it is a recurring feature of contemporary academic debates about sex and gender that substantive analysis of the topic often becomes secondary to the intellectual tradition one wishes to advance.

Yet the evolutionary theory of sex, if it is abstract at all, does not abstract away from real women toward archetypal women, nor does it idealize actual bodies into something resembling the human figures depicted on the Pioneer 10 plaque. 

However, the sex binary in evolutionary biology can—and in some cases must—engage with the statistical distribution of visually recognizable bodily features in order to explain certain phenomena. For this purpose, a (hypothetical) female who looks exactly like Arnold Schwarzenegger in Conan the Barbarian is no further removed from the relevant abstract entity representing femaleness than a Playboy cover model. This is because any account of visual recognition in sexual behavior must explain why the former is less likely to be recognized as female by males, despite being fully female.

For some, it may be difficult to see what a female is, according to the sex binary. When people in the past pointed to an ideally feminine-looking woman as an example of femaleness (assuming such a person would even resemble our contemporary stereotype of a woman, given that she lived in a society characterized by starvation, poor dental care, and widespread disease), they were nevertheless singling out a property entirely independent of visual appearance—as we now know. Because biology eventually discovered that this property is the potential production of a particular type of gamete, associated with one reproductive strategy rather than others that have evolved across species, and not a slighter physique or the presence of breasts. 

This is bad news for those who believe that biology contains a second meaning of the word “sex,” namely something like “the diverse ways that actual human bodies are organized in practice,” because these are simply not features of sex itself. Rather, they are features that may help the sexes to identify each other more reliably. But the fact that sometimes it is more difficult to identify a sex does not make it a different thing. 

Once we begin analyzing the reliability of such indicators (in another way than estimating the probability of successful mating) we leave the realm of sex and enter the realm of gender. The fact that the presence of breasts increases the probability that the person before us is female explains why many people infer sex from breasts and why breasts, together with styles of clothing that emphasize them, are culturally regarded as feminine. But analyzing that connection means analyzing the construction of gender norms, not analyzing femaleness itself. Something the authors did not want to do. But perhaps they should have. Because the broader debate about trans and intersex people is marked by a certain degree of cherry-picking. Many participants are eager to treat genital shape in the case of intersex people, or notions of a “female soul” in the case of trans women, as evidence of a departure from the sex binary. Yet the same willingness largely disappears when it comes to the brains of gay males, which, on average, appear to resemble female brains more closely than those of heterosexual males (and perhaps some gay males have brains closer to a female brain than some trans women). Perhaps what is needed is a critical examination of why some people take certain biological properties to be more reliable indicators of a person's sex than others. 

In this sense, there is no need to bang one's fist on the table and insist that there are only two sexes and that some people are simply disordered. Evolutionary biology does not operate with a category of the “disordered”. To say that there are only two sexes is to say precisely that there are only two sexes. To claim that some people are disordered with respect to sex would imply that they deviate and therefore contradict the very claim being made. Once we recognize that maleness is determined by the potential production of a gamete type rather than by having a penis, a person with internal testicles but no penis is not a disordered male. Such a person is simply male. To describe that person as a disordered male is to confuse expectations about appearance with a more fundamental biological property.

Daston, L. & Galison, P., 1992. The Image of Objectivity. Representations, 40 (Special Issue: Seeing Science), pp. 81-128.

Earp, B. D., Carpenter, M. & Porsdam Mann, S., 2026. On Whether Sex Is Binary. Archives of Sexual Behavior, 55, pp. 489-494.