Denying the Human Sex Binary Turns Biology into Nonsense

Making gametes just one of many characteristics defining sex prevents us from describing reality correctly and clearly.

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In a new piece for Scientific American, Princeton anthropologist Dr. Agustín Fuentes argues that the binary of male and female is too simplistic to describe the complexity of human sex. He claims that defining sex from “the type of gamete (sperm or ova) [an organism] has the function of producing” is not just “bad science,” but a political ploy to justify discrimination. Instead, he says we should think of sex as a combination of many biological and social characteristics that make it “dynamic, biological, cultural, and enmeshed in feedback cycles with our environments, ecologies, and multiple physiological and social processes.” Definitions are human inventions and can certainly change to incorporate new understanding.

Unfortunately, this definition of sex is muddled and incoherent. Making gametes just one of many characteristics defining sex may free us from a politically unpopular binary, but at the cost of our ability to describe reality correctly and clearly.

What is this supposedly simplistic, regressive gamete-based definition of sex? Gametes are the cells that combine during sexual reproduction to produce an offspring with genetic material from both parents. Each species that sexually reproduces needs to produce two kinds—small, mobile gametes (sperm) that make their way to large, immobile gametes (eggs). Most animals have evolved two basic body plans to produce these gametes and use them to give rise to offspring. Dr. Fuentes is correct that these body plans are complex and involve many aspects of anatomy, physiology, and behavior. For example, for human females, successfully producing eggs, getting them into contact with sperm, and generating viable offspring requires dedicated structures (including a vagina, ovaries, and uterus), complicated hormonal regulation (including puberty, the menstrual cycle, and the many hormonal changes during pregnancy, birth, and lactation), and complex biologically and socially influenced behaviors.

In the gamete-based view, the question “what sex is this person?” is asking “Which body plan would this person use to reproduce? The female (egg-based) or male (sperm-based) plan?” Humans do not have other sexes beyond males and females because there are no other gametes that human bodies have evolved to use for reproduction. Sometimes, the complex machinery involved in reproduction can develop wrong, and people can suffer from infertility or exhibit reproductive traits that are atypical for their sex, including ambiguous genitalia (intersex conditions). However, as pointed out by others, these are not additional sexes because these body plans do not produce a new type of gamete besides sperm or eggs. Someone who does not produce any gametes would also not be a third sex since they would be fundamentally incapable of sexual reproduction.

Dr. Fuentes challenges this view by building and readily destroying several strawmen. He claims that sex in human cannot be binary because (1) sex differences in physiology and behavior are not universal across species, (2) individuals with the same gametes can have different traits, and (3) individuals with different gametes can have overlapping traits. To the first point, the gamete-based view is what allows us to talk coherently about sexes across diverse organisms. We know that what a female echidna and a female human have in common is that they produce eggs that must be fertilized by sperm to reproduce. Is Dr. Fuentes saying that if we were to use his mishmash of “biological and social characteristics” to define sex, we would end up with bizarre situations where sperm-producing individuals are the “females” of some species?

The second point also falls quickly to scrutiny. There is no mainstream belief that the fact there are two body plans for reproduction means that every characteristic of individuals with those body plans must be the same. As Dr. Fuentes reminds us, “producing ova or sperm does not tell us everything (or even most things) biologically or socially” about people, including characteristics like “sexual attractions, interest in literature, engineering and math capabilities” or “love of… sports.” Just imagine a world where that was a mainstream belief. Going to the doctor would be chaos if doctors believed that there was only one acceptable body size, estrogen or testosterone level, or muscle or fat composition for each sex. Imagine being regularly shocked to meet gay males, male English teachers, and female engineers and Red Socks fans. Obviously, we don’t live in this world, because there is nothing incompatible about believing that individuals who produce the same gametes and use the same general body plan to produce offspring can vary widely in many characteristics.

Dr. Fuentes’ third and final strawman is also the basis of the “sex is a spectrum” argument, usually represented in a graph like this:

Despite detailed rebuttals, it’s obvious from this graph that people cannot be grouped into binary categories of male and female because these categories are not separate—some of the people who produce eggs have more male-typical traits, and vice versa.

Except, oops, that’s a plot of dog and horse body weights.

The fact that small horses and large dogs have similar weights means that “horse” and “dog” are a false binary, and all of these animals fall along a dog-horse spectrum. Don’t scoop me before I get this submitted to Nature.

Alright, I was a bit rude just there to Dr. Fuentes. Let’s be fair and ask: what would be so wrong with taking up his definition of sex as a conglomerate of physical and behavioral traits, of which gamete type is just one? For his definition to work, we would first need to agree on what physical and behavioral traits we must use to quantify sex—otherwise, we will get different answers about where a person falls on the spectrum. This gets tricky fast for behavioral traits. For example, can we determine sex by asking people about their interest in literature in general, or would we need to distinguish an interest in rugged war stories by Hemmingway from romantic comedies of manners by Austen?

Let’s say we eventually did decide on a comprehensive panel of traits to measure human sex. We then quickly see that his definition is not so much unscientific as it as a kind of anti-science that makes biology less capable of making sense of the world around us. For example, people who produce eggs typically have hormone levels that change over time as part of their menstrual cycle. In the gamete-based view, this person is always female, because they will always use eggs to reproduce. But if hormone levels are instead some of the physical traits that define one’s sex, this definition quickly devolves into nonsense where people become “maler” and “femaler” every month while their underlying reproductive body plan remains the same.

Even measuring sex for an individual with ambiguous traits—which should be a strong case for Dr. Fuentes’ definition—quickly breaks down into gibberish. Consider a person who produces eggs but has the rare disorder called congenital adrenal hyperplasia (CAH). CAH causes the adrenal glands to overproduce androgens, the hormones that drive the development of male-typical characteristics. This person would have ovaries and a uterus but also likely have ambiguous external genitalia, an irregular menstrual cycle, and significant facial and body hair, falling well within the overlapping zone for physical traits. But what is their sex? In the gamete-based view they are unambiguously female, since this person would only ever reproduce through producing eggs and never by producing sperm or another yet-to-be-discovered gamete. This person could struggle to get pregnant due to CAH, but this doesn’t change the fact that their only viable reproductive strategy is female (through eggs). Instead, Dr. Fuentes’ view of sex would say that this person has traits that place them in the middle of the range across humans, making them approximately 50/50 male- and female-typical.

The problem is that this information is essentially useless. Imagine reading that as a doctor and trying to figure out whether your patient is a female facing serious medical problems or a healthy short male who likes gossiping and Pride and Prejudice more than sports. The doctor would need the gamete-based view of sex to make sense of what they’re seeing, since these traits—facial hair, high androgens—only emerge as symptoms in the context of a female sex.

How has a researcher at such a respected institution convinced himself of something so confused and incoherent? Since Dr. Fuentes has taken the liberty of speculating about others’ political motivations, I will speculate about his.

I believe Dr. Fuentes and other political progressives prefer this definition of sex because it makes it impossible to legally protect single-sex spaces. Take the case of Adam Graham (Isla Bryson), a male convicted of two rapes who began identifying as a woman and was sent to a female prison until public outcry reversed this decision. The gamete-based view is clear that Graham was born a male and no amount of hormone therapy, interest in Jane Eyre, or disinterest in football can make him less so. There is therefore no biological reason to think that he would be less of a risk to female prisoners than any other violent male. But in Dr. Fuentes’ view, who’s to say that Graham couldn’t come up with enough physical and behavioral changes to move himself into the female-typical end of the spectrum? And if sex is just a position along a spectrum, how can anyone say that Graham couldn’t become “female enough” for female prison?

Maybe I’m putting words in Dr. Fuentes’ mouth, but I believe these are the kinds of extreme political positions that he and other activists are trying to re-write biology to support. 

Dr. Fuentes, vote for anyone or anything you like. But leave biology out of your political project. Some of us would still like to use it to make sense of the natural world.

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Catherine Hawkins (a pseudonym) has a PhD in ecology and evolutionary biology, and is currently a professor of plant biology at an R1 university in the United States. 

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