In a recent post on his site “Reality’s Last Stand,” Colin Wright reprints an article he published a few months ago in Archives of Sexual behavior, outlining why there are exactly two sexes and dismantling five common arguments that biological sexes actually comprise either more than two types or a spectrum. Click below to read it or find the free pdf here.
If you’re already familiar with the rebuttals of the more-than-two-sexes arguments, you may want to skip this, but it’s a very short piece and worth refreshing yourself. And it was, of course, peer-reviewed.
I’ll give a few excerpts, listing the five arguments supposedly fatal to the only-two-biological sex view and making a few of my own comments. Wright’s excerpts are indented, and my own commentary is flush left.
He begins with what we all know is true: the two-sex definition (really a “recognition of reality”), based on differences in gamete size, was the accepted view until recently, when gender transformation became common, making people want to redefine biology to conform to their own views or identity.
In recent years, however, this previously uncontroversial fact has been challenged in popular discourse (Fuentes, 2023; Kralick, 2018; Viloria & Nieto, 2020) and now increasingly in scholarly scientific publications (Ainsworth, 2015; Fuentes, 2025; McLaughlin et al., 2023; Velocci, 2024), seemingly driven by cultural and political debates surrounding the concept of “gender identity” and transgender rights. Popular outlets now routinely publish articles asserting that there are more than two sexes or that sex is a nonbinary “spectrum” conceived as a continuum or as a multivariate cluster of traits. Scholarly articles have amplified this framing by characterizing the sex binary as overly simplistic, outdated, and even oppressive, urging its replacement with broader and putatively more nuanced models (Ainsworth, 2015).
Here I synthesize evolutionary and developmental evidence to demonstrate that sex is binary (i.e., there are only two sexes) in all anisogamous species and that males and females are defined universally by the type of gamete they have the biological function to produce—not by karyotypes, secondary sexual characteristics, or other correlates.
“Anisogamous” species are those having different sizes of gametes, and comprise all animals and vascular plants. That of course includes humans. And again, although the sperm vs. egg dichotomy is called a “definition” of biological sex, it recally should be called a concept because, like biological species, it simply recognizes an existing dichotomy and does not impose arbitrary human views onto nature.
Wright goes on to define biological sex, which has evolved several times independently. But isn’t it curious that each time it does—no matter what determines sex—there are only two classes that result? That’s an insight that has led to the creation of good theories for why the sexes are always two. Here’s what Wright sees as the most common attempts to refute the sex dichotomy, and why they fail. Bold headlines are my characterizations.
a.) There are more than two “sexes” in organisms that have gametes of equal size (“isogamous species”), including some fungi and slime molds
. . . . sexes in anisogamous taxa are defined by gametic dimorphism—the production of small gametes (sperm) versus large gametes (ova). Some anisogamous species may also possess mating-type systems layered on top of male and female functions, but isogamous species, by definition, lack sexes.
Claims of hundreds or thousands of sexes thus refer to many mating types in isogamous systems, not to sexes. Where reproduction is anisogamous, the number of sexes remains two—male and female—defined by gamete type (Lehtonen, 2021).
This may seem like a slippery definitional ploy, but in fact biological sexes were recognized as being of only two types in anisogamous species, not isogamous species. Still, if someone insists on saying that there are many sees in isogamous species, I am not going to argue with them too vehemently. The claim of more than two sexes, of course, is invariably used to apply to animals and plants, especially humans. And there we see only two mating types.
b.) If you define sex by chromosome types, there could be more than two sexes. (Note, though, that many species with two sexes do not have them determined by chromosomes: in turtles sex can be determined by temperature, and in some fish by social hierarchies. ) In humans, for example, the typical XX (female) and XY (male karyotype) are supplemented by rare karuyotypes like XO, XYY, XXY, and others.
Colin:
The fundamental flaw is conflating how sex is determined with how it is defined (Capel, 2017; Griffiths, 2021; Hilton & Wright, 2023). In developmental biology, sex determination refers to the mechanisms that trigger and regulate sexual development. These mechanisms vary widely across taxa (Bachtrog, 2014). Examples include chromosomal (e.g., SRY gene on Y chromosome in mammals), temperature-dependent (e.g., higher temperatures produce males in many reptiles), haplodiploidy (e.g., unfertilized haploid eggs yield males in most Hymenoptera insects), or environmental (e.g., chemical cues in Bonellia viridis).
Yet, regardless of the mechanism by which sex is determined, an individual’s sex—male or female—is universally defined by the type of gamete (sperm or ova) their reproductive system has the biological function to produce (Goymann et al., 2023). Sex chromosome aneuploidies therefore represent variations within the two sexes, not additional sexes.
c.) Sex is a spectrum because individuals have a continuity of male and female traits, as exemplified by individuals having DSDs (differences/disorders of sex development. As I’ve noted before, the frequency of “intersex” individuals, which supposedly cause the spectrum, is quite low: about 1/5600 individuals—close to the probability that if you toss a nickel in the air, it will land on its edge. Yet we don’t see people flipping coins saying, “Call it: heads, tails, or edge.”
. . .The primary evidence invoked to support the spectrum model is the existence of disorders/differences in sex development (DSDs) (Sax, 2002), including forms of genital or gonadal atypicality, often presented visually along a continuum from “typical female” to “typical male.”
However, the existence of such conditions does not undermine the binary nature of sex, because the sex binary does not entail that every individual can be unambiguously categorized as male or female. Rather, the claim is that in anisogamous organisms there are only two gamete types, sperm and ova, and thus only two sexes. Sexual ambiguity is not a third or intermediate sex because developmental variation does not correspond to producing new gamete types.
These next two objections are those I see most often in the literature, and they both have the problem that they don’t set out criteria for defining or recognizing someone as male or female.
d.) In reality, sex is a “polythetic” category, which Colin defines as “one in which members share overlapping characteristics, with no single feature necessary or sufficient for membership. Inclusion is based on “family resemblance”. This is the objection raised, for example, by people like Steve Novella and Agustín Fuentes.
Proponents of a polythetic sex model draw on this idea to portray sex as multivariate (rather than univariate, as in a simple “spectrum”). On this view, “sex” is an aggregate of traits—chromosomes, gonads, gametes, hormones, neuroanatomy, secondary sex characteristics, and other sexually dimorphic traits—and individuals are assigned degrees of maleness or femaleness according to how their overall profile aligns with what is considered male-typical or female-typical (Dreger, 2000; Fausto-Sterling, 2000).
However, male and female are not polythetic categories. They are reproductive classes defined by a single criterion: The type of gamete (sperm or ova) an organism’s reproductive system has the biological function tomproduce. All other traits—karyotype, genital morphology, hormone profiles, neurological and somatic dimorphisms—are typically causes, proxies, or consequences of that functional distinction. Treating those correlates as jointly definitional blurs the determinants and downstream effects of sex with sex itself.
e.) You can be a member of different sexes depending on which trait you’re looking at (chromosomes, genitalia, hormones, and so on).
As articulated by McLaughlin et al. (2023), sex is framed as “a constructed category operating at multiple biological levels,” with four focal levels: genetic, endocrine, morphological, and behavioral. This framing conflates the determinants and correlates of sex with sex itself (Bachtrog, 2014; Capel, 2017). Genes and gene networks initiate and regulate sexual differentiation; hormones mediate downstream development and phenotypic dimorphisms; morphology and many behaviors are influenced by an organism’s sex. Yet none of these traits defines sex. Sex is an organism-level reproductive class anchored to the type of gamete that organism has the biological function to produce. Treating upstream regulators (e.g., SRY activity, hormonal milieu) or downstream outcomes (e.g., dimorphic morphology, behavior) as coequal “levels” of sex is a level-of-analysis error.
And the kickerm which shows the fact that critics really do recognize two sexes (and use them in their own scientific papers!):
Moreover, the multilevel account inherits the same circularity as the polythetic model. Traits are labeled “male-typical” or “female-typical” only because they correlate with organisms already identified as male or female—an identification that, in anisogamous species, is made ultimately by reference to gametes. Once that reference is removed, the typology loses its interpretive footing. As a descriptive framework to integrate genetic, endocrine, and morphological findings in clinical differential diagnosis, the multilevel schema has pragmatic value; as a definition of sex, it is incoherent.
Why is this important biologically? Colin explains:
The scientific value of clear and precise definitions is enormous (Dawkins, 2025). A gamete-based definition prevents error propagation across comparative biology, physiology, ecology, and medicine. It preserves the interpretability of sex-linked phenomena—sexual selection, dimorphism, and life-history trade-offs—and maintains conceptual discipline by keeping determination mechanisms (e.g., SRY pathways, ZW systems, temperature-dependent determination, social cues) in their proper explanatory lane. It also secures cross-taxon coherence: Whether a species is gonochoric or hermaphroditic, and whether determination is chromosomal, environmental, or social, “male” and “female” remain meaningfully comparable because those terms are anchored to reproductive function rather than to a bundle of traits that shift widely from taxa to taxa.
I like to summarize this by saying that the biological sex definition/concept is both universal and explanatory. No other concept of sex, for example, can explain sexual selection and the differences in behavior and phenotype that appear in animals.
It’s important to recognize that the recent reframing of the two sexes as needing revision did not result from any new discoveries about biology. All the things about sex determination and differentiation have been known for a long time. What has changed is not biology but ideology. It is perfectly clear that arguing that there are more than two sexes is derived from the desire to give solace to those who don’t feel or identify as male or female, But there’s no need to change your view of nature to bring such solace. As Wright says:
The societal and ethical stakes are also significant. Accurate biology is distinct from questions of dignity, rights, and how we treat one another. Policy disputes should not be adjudicated by redefining—or defining away—the reproductive realities that make sex a useful scientific concept in the first place. When categories are blurred for nonscientific reasons, we invite downstream harms: muddled clinical protocols, compromised epidemiology, eroding and/or conflicting legal protections, and diminished public trust in science.
It is not transphobic to recognize the two sexes that biologists have known for decades, but, unfortunately, we are dealing with ideologues who are largely impervious to both facts and reason, and so the five points above are aimed largely at those who don’t know a lot about the way biologists conceive of sex.
Behmenism is the practice of the mystic Jakob Böhme (1575-1624) :
Gender in Mystical and Occult Thought – Behmenism and its Development in England
B. J. Gibbons
Cambridge University Press
1996
So yes indeed,
“ “[..] and so the dialectic continues.” ”
-as quoted by Delgado and Stefancic in
Critical Race Theory – An Introduction, p.66, 3rd Ed., 2017
… same Emperor of the Dialectic, wearing transformed, modern clothing.
Good piece. Activists will undoubtedly still conflate the sex definition (binary based on gametes) with sex determinants and sex-correlated characteristics simply because they insist on preserving their narrative—whether or not it’s true. Nonetheless, it is important to get articles like this published in the major peer-reviewed journals. These are still where young scientists go to learn and to engage the profession—at least I think that’s still so.
Why don’t they just focus on gender and leave sex alone? Because it won’t get them the legitimacy they desire.
The “right” that trans activists are claiming has been violated is an individual’s right to have their sex substituted in society and law by their gender identity. This right is supposed to be so taken for granted that it’s easy to miss — unless you’re transgender.
If we grant that sex is both legitimate and binary, however, that right basically rests on an argumentum ad misercordiam — that if people are sad enough, then they must be what they think they are. We are now morally obligated to treat them as whatever “gender” they see themself as, so they may be happy. You’re a woman if you really, really need to be a woman.
That sort of appeal to emotion, however, isn’t going to stand up to any kind of robust disagreement. For one thing, it doesn’t— can’t— define “woman.” So go on the attack: you can’t define “woman” by sex, either.
Confuse; conflate; create. Since there’s no objective ground to stand on, it’s all subjective now.
Colin Wright may be wrong about Claire Ainsworth. Tweet from her “No, not at all. Two sexes, with a continuum of variation in anatomy/physiology.” She also wrote “THE IDEA OF TWO SEXES IS SIMPLISTIC. BIOLOGISTS NOW THINK THERE IS A
WIDER SPECTRUM THAN THAT”.
For those who would like to verify Ainsworth’s Tweet.
https://x.com/ClaireAinsworth/status/888365994577735680
I think the second quote was the headline; Ainsworth probably didn’t write it.
Very useful, especially the piece about chromosomal variations. Thanks.
Strategically, I wouldn’t try to claim that it is not transphobic to say that there are only two sexes. The trans activists reply that it is transphobic to so erase them. So there. To them, my saying I’m not transphobic is just what transphobes would say. “The oppressor doesn’t control the language, bigot! We do.” Because racists today deny being racists, you must be a racist if someone says you are. I could say that I’m not transphobic because I’m not afraid of trans people (as long as they don’t have guns or have the ear of the HR lady at work) but the activists long ago co-opted “-phobia” to mean hate, not fear. The oppressed control the discourse, remember? Their activist goal is to criminalize hate and erasure because it sounds more noble and pressing. Fear can be addressed with re-education.
If I had to engage with transphobia at all, such as if someone accused me of it for supporting the sex binary, I would say instead, “Meh, so what if it is? It’s the truth. Why are you afraid of the truth?” That way I don’t concede to them that transphobia is even a thing I should make any effort to not be. It’s like being accused or racism for doubting that there are bodies of children secretly buried under former Indian residential schools. We just have to let the accusation slide for the activist slur that it is and stick with the facts. Contesting the slur would mean we care that a motivated activist thinks we’re racist.
Agreed. Accusations of “transphobia”, “Islamophobia” and “racism”, etc are just ways of trying to shut people up, trying to imply that it’s morally wrong to disagree with them.
Thus, denying that you are those things is just playing their game. If anyone doesn’t have opinions that would get them so labelled then they are not following the evidence.
“The oppressor doesn’t control the language, bigot! We do.”
Oppressors control language to the extent that opponents of oppression must walk on eggshells to avoid using words, phrases, and tactics favored by the oppressors. Bullying and dehumanization are such tactics. The above quotation employs both. It tries to bully. And calling someone a “bigot” dehumanizes. The correct, non-dehumanizing phrase is “You are acting like a bigot.” A person who “acts” is capable of change and improvement. A person who “is” can do neither. By not walking on eggshells, trans activists embrace the oppression they claim to oppose. Playing king of the hill does not a democracy create.
A significant component of “ideology” is not philosophical at all but rather sartorial: chosen by adherents not because of logical coherence but rather because they ask themselves “how will it look on me?”. I suggest that “ideological fashion taste”, rather than “ideology”, is the accurate descriptor.
The contemporary fashion of transgenderwang, incoherent to the point of parody, is a prime example. Similarly, the rhetoric of “decolonializing” this or that (e.g., mathematics, Chemistry, the names of birds, etc. ) appeals to individuals who wish to wear the colors of “progressivism”. The same with the buzzword “social justice” applied to something as nonsensical as “citation justice”. A 1940s form of sartorial “progressivism” was apologetics for the murderous Soviet police state by some Liberals of Henry Wallace’s gullibility .
Yes, and there is seldom accountability for one’s choices. At most, you get a sheepish, “Well, we all looked a little silly back then.”
Sure, but bell bottoms never killed anyone, and mullets didn’t mutilate children.
Henry Wallace was quite gullible about the USSR for a while. He later repented. From Wikipedia ‘In 1952, he published an article, “Where I Was Wrong”, in which he repudiated his earlier foreign policy positions and declared the Soviet Union to be “utterly evil”.’
Precisely. In fact, I have some sympathy for Wallace’s
post-gullible period, when he broke decisively with the remnants of the 1948 Progressive Party.
Perhaps we can look forward to some future Democratic Party politicians walking back their more wokely assertions. For example, Mayor Mamdani will soon reveal that he never
really thought all NYC policemen were so awful.
I appreciate the piece and responses. Activists both for and against trans rights seem to think that the binary or rejection of it is important (at least rhetorically). Biologists just want to be clearly understood on the issue of sex determination. Some suggest that activists should stick to gender, but I think there is a reasonable third way. When most people are talking about sex, they are talking about perceptions of sex, much more related to point d) a ‘polythetic’ category. One can correctly point out that sex determination is binary as much as one wants, but I doubt it will be recieved in any useful way without broader framing. I think biologists can have it both ways here though, 1) continue to insist on sex determination, but also 2) fully acknowledge the biologically relevant aspects of perceived sex (again as in the description of the sex as polythetic category) without giving up on point 1. These variations seem most important to how we relate to others (and yes of course gender is fully entangled with them). This might allow the rhetoric to ease a bit and not encroach on science. However, some activists focused on rights might not want to give up the rhetoric, and certainly few conservatives interested in limiting rights likely want to concede anything. Maybe this means we need a third kind of terminology. 1) sex (binary), 2) gender (more variable) , and 3)perceived/apparent/social sex (more variable). Again this doesn’t mean giving up on anything, just a way of acknowledging with labels.
The problem is that you either allow trans-IDing males into women’s sport and other single-sex spaces or you don’t. You either medicalise a gender-dysphoric teen or you don’t. The choices have a kinda binary feel to them.
(By the way, are there really conservatives who want to take away the rights of trans-IDing people? Which rights?)
The main thing you are getting at, I think, is the idea of sex (or gender) as a social construct. We have not brought up social constructs very much, lately, although I find the term to be quite interesting.
What is the best response to people who use hermaphrodites (e.g., snails and nudibranchs), not to mention many (most??) flowering plants as arguments against the sex dichotomy?
No third gamete in hermaphrodites, just as in organisms that keep the sexes separate. Hermaphrodites have only the same two entirely dichotomous sexes. The two sets of sex organs are just located in the same individual. All that you have to concede is that in a hermaphroditic species, no individual can be said to be male or female. But the sexes it bears are. The male part of a flower or a fluke* has the body plan that makes only many small motile gametes and the female part has the plan that makes only few large gametes. No spectrum. No overlap of sperm being made in ovaries or eggs being made in testes/stamens.
(* Most human flukes and all tapeworms are hermaphrodites. This has survival value because if you are infected with a single individual it will be lonely but still fecund. The blood flukes of genus Schistosoma have separate sexes. When the larval schistosomulae enter through human skin from wading in snail-infested fresh water, and migrate to liver or urinary bladder they develop into sexed adults which must find the opposite one. They sort of fuse together like a hot dog in a bun where they copulate and produce eggs for the rest of their lives. You need to be infected with many larvae to have a good chance of developing a productive infection. People who’ve gotten swimmer’s itch have some idea of how densely infested water can be with schisotosomulae.)
If we were to define sex using chromosomes, I would still be comfortable declaring there are but two sexes. It is not the case, in my view, that genetic errors have to be accorded the same status as occasions where the process goes according to plan. This does not mean I treat affected individuals as less than human, but simply that I understand genetics and embryology sometimes goes wrong, and produces an unintended outcome. To do anything else leads to silliness—take phocomelia; shall we redefine the number of limbs a human has just because some people are born with less than four?
The very first question one needs to ask is, “If being trans is all about gender and not sex, then why are trans activists bothered about whether there are two sexes or a a spectrum.”
The fact that they keep doing this reveals the lie at the heart of the movement.
“Gender identity” is a red herring, thrown in for obfuscation.
Being trans is about sex – in particular, about pretending to have changed sex.
This is evident in everything that “trans people” do and demand.
But humans can’t sex and that’s the end of it.
There’s no need to pay any attention to the gibberish about “gender identity”
I often phrase it as “sex and gender self-identity”. The two may be the same or they may be different.
As well as just being straight up incorrect, what also annoys me is the condescending tone of so many of these “sex is a spectrum” people.
They’ll sneer that those who point out the sex binary with claims that this is merely “middle and high school biology” or some similar put-down. As if they have somehow become more erudite and sophisticated by convincing themselves of ideological bullshit. So frustrating. It’s worse than arguing with creationists.