Why Sex isn't Binary

By Darren Incorvaia

I was inspired to write this by an opinion piece that came out in the Wall Street Journal earlier this year, by Colin Wright and Emma Hilton. Both are biologists. In it, they decry “transgender ideology” and claim that binary sex is biological reality. Their position is wrong and unscientific, and I’m going to tell you why by describing the evidence for why viewing sex as a binary doesn’t fit with nature. Let us begin

Wright and Hilton define sex based on gametes, or sex cells. They state:​

“In humans, as in most animals or plants, an organism’s biological sex corresponds to one of two distinct types of reproductive anatomy that develop for the production of small or large sex cells—sperm and eggs, respectively—and associated biological functions in sexual reproduction.”

It’s certainly true that there are two types of human sex cell, sperm and egg, produced by testes and ovaries respectively. But the issue at hand isn’t whether gametes occur in a binary; it’s whether sex does. And in humans, it is possible for an individual to possess both ovaries and testes, and to produce both types of sex cells (or neither). Historically these individuals have been called “true hermaphrodites,” and today they’re generally classified under the broader umbrella term of intersex. If we’re classifying humans into male or female sex categories on the basis of possessing certain gametes, two categories isn’t enough. Sex isn’t binary. Boom.

Okay that was way too easy. Maybe you’re thinking, “Well what about chromosomes? There are only two types of sex chromosome arrangements: XX or XY! If we define sex using chromosomes, isn’t it binary?” And the answer is: no. XX and XY are not the only sex chromosome arrangements humans can have: there’s also X, XXX, XXXX, XXXXX, XXY, XXXY, XXXXY, XXYY, or XYY If we’re defining sex on the basis of sex chromosome arrangements, two categories isn’t enough. Sex isn’t binary. Boom.

BUT WAIT, maybe you’re thinking or saying aloud like a weirdo. “Isn’t Y the male chromosome and X the female chromosome? Isn’t that a binary?” And oh, my poor misguided imaginary friend, the answer is again no. Obviously the most common “male” sex chromosome arrangement is XY, which means X isn’t uniquely a “female chromosome.” But there’s also the added fact that possessing a Y chromosome doesn’t always mean an individual develops traits we traditionally consider male, like testes and a penis. The gene that primarily controls development of these traits (or genes that control this gene), sry, can sometimes be non-functional or jump off the chromosome entirely (this is an oversimplification of course but you get the gist). In these cases the individual usually develops traditionally “female” traits, like ovaries and a vagina, or some combination of male and female traits. Sex isn’t binary. Boom.

If we move up the biological scale to external anatomy, the idea of a sex binary gets even more laughable. Babies are often classified as male or female based on external genitalia, but it’s long been known that individuals with “ambiguous” genitalia exist (they fit under the umbrella term intersex as well). How do Wright and Hilton deal with this conundrum to their argument? They dismiss it entirely:

“The existence of only two sexes does not mean sex is never ambiguous. But intersex individuals are extremely rare, and they are neither a third sex nor proof that sex is a ‘spectrum’ or a ‘social construct.’ Not everyone needs to be discretely assignable to one or the other sex in order for biological sex to be functionally binary.”

I have no idea what they mean by “functionally binary,” and no explanation or citation is offered. Using rarity as an excuse to dismiss data is patently unscientific. If I survey an area and find three species of frog live there, but one of them is extremely rare, would it be appropriate for me to say there are “functionally” two species in the area? No, it wouldn’t. If I do a study and find evidence that this rare species doesn’t impact a certain ecological process or something, that’s fine. To just outright ignore this species in discussions of diversity in the area is not fine: it’s lying. It’s fraud. Why is that okay when talking about diversity in human sex? (BONUS FACT: intersex individuals occur at the same frequency as natural redheads. Do we exclude redheads from discussions of diversity in human hair color?? No!!)

To preempt a potential counter-point: Jerry Coyne, a highly-cited evolutionary biologist, made a similar “sex is functionally binary” argument in one of his blog posts. He stated:

“For all practical purposes, sex is a binary, and it should be, since evolution produced (in most animals) two sexes that must mate to produce offspring. If you’re neither, or an intermediate, you don’t leave offspring and you don’t leave your genes.”

First of all, there are documented cases of intersex individuals–including true hermaphrodites–reproducing (see here, here, and here). So, to hell with that argument. But furthermore, even if intersex individuals couldn’t reproduce, that doesn’t mean they can’t pass on copies of their genes to the next generation. Any evolutionary biologist–especially one with any knowledge of eusocial insects (which all evolutionary biologists should)–knows this. It’s literally foundational to the field, considered a major extension of Darwin’s theory of natural selection (see here for an example). For an evolutionary biologist, especially one as prominent as Jerry Coyne, to ignore this is (at best) very curious or (most likely, in my opinion) driven by ideology.

So we’ve seen that whether you define sex based on chromosomes, gametes, gonads, or genitals, two categories just doesn’t cut it. It’s unscientific to pretend otherwise, and researchers in the field know this. I’m not saying that sex is a “social construct,” as Wright and Hilton would most likely accuse me of, but the sex categories of “male” and “female” certainly are. Hell, having two sexes isn’t even ubiquitous in human societies–some have included a third sex, and I’m sure there are many more ways sex has been defined/classified in human societies that I’m not aware of. Science has moved beyond a simple sex binary, and its proponents should be seen for what they are: ideologues weaponizing scientific language to justify their anti-transgender bigotry. History will judge them accordingly, and so should you.

Sex isn’t binary. Boom.

Check out the Intersex Society of North America’s website for more info!

A version of this article was originally printed here on July 8, 2020

Darren Incorvaia

Darren is a PhD Candidate at Michigan State University studying the modulation of foraging behavior in bumblebees. Since 2015 he has written a blog, The Kingdom Has Come, focusing on many aspects of science (with an emphasis on animals) in a humorous light. In his spare time, Darren enjoys hiking, running, reading, and googling pictures of animals.