One Reality, Two Sexes — and Many Endless Debates …

A Conversation with Colin Wright

Evolutionary biologist Colin Wright discusses the science of biological sex and why a straightforward biological fact has become one of the most contentious issues of our time—including among skeptics.

Introduction

In November 2025, Wright published “Why There Are Exactly Two Sexes” in Archives of Sexual Behavior (Vol. 54, pp. 3941–3945), a peer-reviewed commentary that has become a touchstone in debates over biological sex. The paper systematically addresses the various models that have been proposed to challenge the sex binary—from chromosomal definitions to sex spectrum theories—and explains why biologists define sex in terms of gametes: sperm and ova.

We spoke with Wright about the science underlying the sex binary, the professional consequences of defending it, and what skeptics can learn from disagreements within their own community.

Q: As skeptics, we care about pseudoscience, but also about exaggerated or plainly false scientific claims. In your view, which findings are misrepresented the most, be it in academia or in public?

A: The reality of the binary nature of biological sex, and the claim that it does not exist.

From New Atheism to the Sex Debate

Q: You came up through the skeptic and new atheist movements. How did you end up focusing on this particular topic?

A: I was heavily involved in pushing back against intelligent design and creationism. Those attacks tended to come from outside the academy—people learning things in churches or popular outlets that weren’t scientifically justifiable. I was in the academy pushing back against public misconceptions that originated outside of it.

With the sex biology issue, the misconceptions started outside the academy, but then quickly moved inside. These ideologies have entered many academic departments, so the leading proponents now tend to be within the academy, and the defenders are as well. A moral component has been attached to this question—it’s not just a scientific question anymore.

When I started talking about it, I expected a scientific argument—‘this is the evidence, you’re wrong because of this.’ But it totally avoided scientific disagreement. They went straight to ‘you’re a terrible, evil person parroting far-right fascist, white supremacist talking points.’ The academy and scientists are now the loudest promoters of these misconceptions, and the academy has a legitimizing effect. If journals and scientists say it, people assume it must be true.

The Archives of Sexual Behavior Paper

Q: What motivated you to write “Why There Are Exactly Two Sexes” for Archives of Sexual Behavior?

A: I’ve been writing about this for about seven years. In 2023, I had a peer-reviewed book chapter on the two sexes with Emma Hilton. But the main criticism—partly valid—was that much of what I write appears in popular outlets like the Wall Street Journal, Newsweek, and Quillette. These aren’t peer-reviewed journals, so that activists would dismiss my arguments as not scientifically rigorous.

They ignore the fact that people who submit articles pushing back against the sex spectrum stuff often get rejected outright, because journals view these perspectives as socially taboo and morally questionable. So when there was a call for commentaries from Archives of Sexual Behavior on this topic, I thought it was a good opportunity to write something succinct—to get all my arguments into an actual scientific journal that both scientists and the public can cite.

What’s bizarre is that there really weren’t any papers that just spelled out in simple terms what biological sexes are—what it means to be male or female. It’s like gravity: we all observe this force and take for granted that it exists. The same was true for sexes. All biologists understood it once we discovered what sperm and ova were, but few actually wrote it down in explicit terms. Activists exploited that gap. I decided to address this one question head-on because it’s the one being exploited most.

How Biologists Define Sex

Q: For readers who aren’t biologists: how do scientists define biological sex, and why do they say there are exactly two?

A: We need to think about how humanity viewed sex before we even knew what gametes were. We lacked the tools to observe them, yet it wasn’t difficult to recognize that there were two human sexes. We’re sexually dimorphic—men and women look different, have different reproductive anatomy, different functions. Women can get pregnant, gestate, and give birth; men cannot. We had a functional concept of sex, but we didn’t understand the fundamental basis.

We discovered sperm under a microscope in the 1600s and ova in the early 1800s. That’s when we began to understand that what we call males and females are two fundamentally different reproductive strategies: an individual either produces many small sex cells called sperm, or larger sex cells called ova. And this wasn’t unique to humans—it showed up in countless animals and even plants.

We call species that reproduce by fusing a large gamete and a small gamete ‘anisogamous.’ We termed these strategies the ‘male’ strategy and ‘female’ strategy. What does it mean to be male? It means being an individual whose reproductive system has the biological function to produce many small gametes. Females have the biological function to produce fewer, larger gametes.

This became the universal concept of male and female. It’s what unites all species that look and behave nothing like humans. What makes me male, and a snail male, and a fish male, and an asparagus plant male, is that we’re all deploying this reproductive strategy involving small gametes. We have otherwise nothing in common—we look entirely different, our hormones are different.

There are only two sexes because there are only two types of gametes. There’s no intermediate one. Therefore, there can only be two sexes that an individual can have.

Addressing the Counter-Arguments

Q: Your paper identifies five main models used to argue against the sex binary. Could you briefly outline them?

A: First, there’s the conflation of mating types with sexes. Some fungi and slime molds reproduce sexually using gametes of the same size—we call these isogamous species. They have chemical compatibility types between gametes, sometimes thousands of them. Articles about ‘the slime mold with 30,000 sexes’ are based on a fundamental misunderstanding. Sexes refer only to males and females, which are defined by different-sized gametes. Species with same-sized gametes don’t have males and females—they have mating types.

Second, there’s the chromosomal or karyotype model. You’ll hear people say, ‘if you’re XX you’re female, if you’re XY you’re male.’ But this conflates how sex is determined in humans with what sex is. Many crocodilians and turtles don’t have sex chromosomes at all—their sex is determined by egg incubation temperature. People with Klinefelter syndrome (XXY) aren’t a third sex; they’re biologically male. These are chromosomal variations within the two sexes.

Third, there’s the sex spectrum model, which holds that sex is a continuous variable based on genital morphology. Some proponents think males and females aren’t real entities but exist only in a statistical sense—you can be varying degrees of male or female, but not definitively male or female. This ignores gametes entirely and has circular problems: how do you know what genital shape is ‘male’ unless you already know what males are, rooted in gametes?

Fourth, there’s the polythetic categories model—like family resemblance, in which members share overlapping characteristics, with no single feature necessary for membership. They try to apply this to sex, saying it’s a combination of chromosomes, hormones, height, and voice pitch, and many other sex-related traits. But how do you define which chromosomes or hormone profiles are ‘male’ without presupposing what males are, rooted in gametes?

Fifth—and most influential—is the multi-level model, which says we can’t talk about bodies having a sex. Instead, you’d say someone is ‘genetically male’ or ‘hormonally female’ or has a ‘male height.’ But again, how are they determining which chromosomes are male without presupposing that males and females exist apart from chromosomes, inevitably rooted in gametes?

Q: Do proponents of these different models argue among themselves?

A: That’s a good question, and one I’ve asked myself many times. There’s a strange silence and truce between them. I’m not arguing with the sex spectrum people while they argue with the karyotype people and the mating type people about the real definition. It’s all five models versus the binary model.

Even more baffling: there aren’t distinct camps. The same individuals hold all these views simultaneously, even though many are mutually exclusive. Different models are often invoked interchangeably, depending on what seems most persuasive in the moment. The conclusion then becomes, ‘See? Sex isn’t binary.’

It’s rooted in a Queer Theory approach where binaries are thought to signal artificiality and oppression. The emphasis often seems to be less on how many sexes there are than on rejecting the idea that there are only two. Three is fine, ten is fine, an infinite spectrum is fine—it just can’t be two, because two is bigoted. That’s really how the approach works.

The State of the Scientific “Debate”

Q: Is there a genuine controversy among biologists about the definition of sex?

A: If you’re looking at people studying the evolution of biological sex and sexual reproduction, there is overwhelming consensus. However, they’re insulated from cultural debates—just nerds studying computer models of how selection favored two different gamete types. It’s one of the most robust areas in evolutionary biology.

There are parallels to creationism. If you ask creationists about evolution, they’ll say it’s ‘hotly debated’ because they want to portray it that way. With sex, activist and online discourse began framing the issue as controversial. The controversy was spoken into existence, even though it’s baseless and incoherent.

No empirical findings have overturned the core biological framework. No Nobel Prizes have been given for discovering the supposed sex spectrum. Again, it was just spoken into existence.

Professional Consequences

Q: Why aren’t more scientists speaking up?

A: Prominent biologists are speaking up. Richard Dawkins hasn’t written a scientific paper on it, but he’s written explicitly about biological sex and why the gamete definition is the universal and only coherent one. He’s one of the most preeminent biologists of our time, and he totally agrees with me. Jerry Coyne, one of the best evolutionary biologists in the world, shares my work and agrees with what I’m saying, because I’m not saying anything extreme. I’m really just articulating basic biology.

But we’d get more scientists willing to write about this if people who do—like me—weren’t immediately slandered as white supremacists and transphobes. Constantly defending yourself from such accusations is not a great way to build a career. That’s why it’s almost impossible to get pre-tenure faculty to speak up about this.

Writing about it has significantly harmed my academic career. Not everyone can start a Substack and make money as a public commentator. I was fortunate to get into this topic early, before anyone was really talking about it. Many people simply can’t afford to jump into this debate. It would be terrible for their careers.

What Would Change Your Mind?

Q: What evidence would you need to change your view that there are only two sexes?

A: That’s a crucial question. In the skeptic community, you always need to have something that could convince you you’re wrong. If you don’t, you’re just a zealot, not doing science.

For me, it’s really easy: we define sexes by the type of gamete an individual is biologically capable of producing. You’d need to present a third novel gamete type—in addition to or intermediate between sperm and ova—that an individual’s reproductive system could have the function to produce. That’s the only thing that could make there be more than two sexes.

Reactions to Your Sexual Archives Commentary

Q: Your Archives of Sexual Behavior article has drawn critique from other commentators (Mahr, 2026). One central criticism is that your claim that there are only two sexes is not simply an empirical conclusion, but effectively an a priori assumption built into your framework.

A: My reply is that the my claim that there are only two sexes is not an a priori assumption. It’s an a posteriori inference based on the observable fact that sexual reproduction in anisogamous species involves only two gamete types — small gametes (sperm) and large gametes (ova). In the article, I explicitly frame the disagreement this way: the labels we give to individual organisms that produce either large or small gametes—female and male, respectively—is a human convention, but the things being labeled are recurring natural phenomena that we did not invent and cannot change.

So what I’m doing is not “assume two sexes, then define sex by gametes.” The order is the reverse: we observe a stable reproductive division (sperm-producers vs ova-producers) across anisogamous taxa, and we use that division to define the two reproductive classes in a way that is cross-taxon coherent and explanatorily useful.

Q: Another central criticism is challenging the idea that the gamete-centered definition is objective or universal, calling it an “illusion of view-from-nowhere”. How do you reply to that?

A: I grant the trivial point that science is a human activity with a long history. But that does not imply that biology cannot describe mind-independent features of the world. In the article, I make this point by analogy: humans discover facts (like the structure of water or the heliocentric organization of the solar system) and then represent them through language and models, but the truth of those facts doesn’t depend on culture or how individual scientists are socially situated.

If we were visited by an advanced alien society from another galaxy, they would have absolutely nothing in common with us culturally or even biologically. But they would still be able to make the basic observations that the Sun sits at the center of our solarsystem, that water is composed of two hydrogen atoms and one oxygen atom, and that certain species reproduce by fusing two distinct types of gametes.

The gamete-based classification is “objective” because it tracks a real functional distinction in nature—what reproductive role an organism’s reproductive system has with respect to sperm vs ova. That is precisely what makes “male” and “female” comparable across taxa rather than collapsing into shifting trait-bundles that vary from species to species. No other basis for the sexes is biologically coherent.

Q: Yet another criticism raised is that a binary framing of sex risks invalidating the lived experiences of intersex and gender-diverse individuals. What is your response to that concern, and how do you see the relationship between a biological classification claim and preferences or recommendations for specific policies?

A: First, I’d separate questions that are often blended. One question is descriptive: what “sex” is as a biological classification in anisogamous species. Another question is normative: what legal rights, accommodations, or social practices we should adopt in light of conflicts of interest, fairness concerns, compassion, and so on. The article is explicit that these are distinct domains: policy preferences should never be used to dictate biology, even if biology can and should, when necessary and appropriate, inform policy discussions.

Q: The critique also refers to the claim that around 2% of people are intersex. How do you assess that number, and how is it relevant to the claim that there are only two biological sexes?

A: I assess the “~2% intersex” figure as misleading because it depends on an overly expansive definition of “intersex” that counts many conditions that are not sex-ambiguous in any clinically meaningful sense—i.e., it treats “intersex” as any deviation from a “Platonic ideal” of dimorphism across chromosomes, genitalia, gonads, or hormones. That definitional move inflates prevalence by design.

In the article, I point to Leonard Sax’s response: if “intersex” is to remain clinically meaningful, it should be restricted to cases where chromosomal sex is inconsistent with phenotypic sex, or where the phenotype appears sexually ambiguous. This results in an estimate on the order of ~0.018% rather than ~2%.

But—and this is crucial—whichever estimate one prefers or is more accurate, it is not actually relevant to the question of how many sexes there are, because rare developmental conditions do not constitute additional sexes. They do not create new reproductive classes with the function of producing novel gamete types beyond sperm and ova. At most, they create atypical or ambiguous development relative to one of the two existing reproductive classes.

How to Engage

Q: How do you deal with people who seem impervious to evidence on this issue?

A: Very few can have their mind changed, but I’ve encountered some who were genuinely confused—they bought into activist rhetoric but weren’t themselves ideological. I’ve been able to explain things, and they’ll say, ‘I see what you’re saying. I’ll modify how I speak about this.’ That’s great.

But most people won’t change, because it has become political to them. So I’m not fixated on trying to change their minds. Instead, I try to model the legitimate scientific approach: stick to facts, make arguments, don’t call people names, don’t make ad hominem remarks. Be the calmest person in the room. Address the facts directly but politely.

If you do all this and you’re met with vitriol and name-calling, people watching can see there’s some weird dynamic—this guy is clearly not crazy, he’s intelligent and calm. That juxtaposition does a lot of harm to their cause. People can see who the ideologue is in the scenario. I think that’s valuable.

Author References

Photo: Buttonslives, Colin Wright, CC BY-SA 4.0

Colin M. Wright

Colin M. Wright, Ph.D., is an evolutionary biologist and a fellow at the Manhattan Institute for Policy Research. He received his doctorate from the University of California, Santa Barbara, where he studied social spider behavior and colony dynamics. His 2025 paper “Why There Are Exactly Two Sexes” (Archives of Sexual Behavior, 54, 3941–3945) has become a touchstone in debates over biological sex. He writes the Substack newsletter Reality’s Last Stand and has published in The Wall Street Journal, Quillette, Newsweek, and other outlets.