Is Sex Definitely (Not) Binary? It's Complicated

On March 22nd, 2022, Senator Marsha Blackburn and then Supreme Court candidate, now Justice Katanji Brown Jackson, had an infamous exchange:

Blackburn: “Can you provide a definition for the word woman?”

Brown Jackson: “I can’t…”

Blackburn: “You can’t?”

Brown Jackson: “Not in this context. I’m not a biologist.”

Blackburn: “The meaning of the word woman is so unclear and controversial that you can’t give me a definition?”

A culture war over gender identity is raging throughout the nation. Dozens of states have enacted hundreds of pieces of legislation restricting the health care options parents can choose, what words teachers can use, what books can be in the library, who can play what school sports, who can use which restrooms, and more. At the heart of this war is the question: Is sex a binary set of two clear categories, immutably determined and clear before birth, or is sex more complicated than that, a non-binary continuum that is not always clear before birth or even years into a person’s life? Both sides of the debate believe science is on their side. I’ve taught a course at Harvard for several years on this topic, and my students are often shocked to discover that not only both views are wrong, but both are right. How this is true, I hope, will change how you see this issue.

The one way that sex is binary

There is one (and only one) way in which sex is binary, and this is what the biological definition of male and female is based on. Any species where an offspring is created by combining the genes of two parents reproduces sexually; these species have sex. But just because a species has sex doesn’t mean that species has distinct sexes. Many species of fungus, such as the yeast we use to make bread and beer, reproduce sexually (at least sometimes) but can do so with any other member of their species. But the rest of us sexual reproducers, making up the majority of multicellular life, have both sex and sexes: Some members of the species produce big sex cells (e.g., eggs), and others produce small sex cells (e.g., sperm). Because those sex cells aren’t mutually compatible (egg-egg fusions and sperm-sperm fusions are not viable) it is now meaningful to say a species has two categories for sexual reproduction. The category that makes small sex cells is labeled male, and the category that makes large sex cells is labeled female. This is the biological definition of sex, and the only way that sex is truly binary.

You might ask: Why are so many species like this? Why is the pattern of big-little, egg-sperm so common? The answer is simple: Two parents means a conflict of interests.¹ This is easy to understand intuitively with an example from human parenting: Both parents may want their child to be read a bedtime story every night, but at the same time they can fight about who should do that reading. Both parents have other uses of their time and it is these private, non-overlapping interests that create the conflict. This same conflict exists even at the microscopic level: How much material will each parent put into their sex cell? Putting in more resources makes the offspring more likely to succeed. Think about all of the offspring, I mean seeds, that we eat. The carbohydrate-rich endosperm that makes the almonds we eat so nutritious was put there not for our consumption but as a source of energy for the seed to use when starting off its own life. If less were put in, the almond would be less likely to survive to become a tree. But at some point that seed has enough and more wouldn't be helpful.

Now imagine a chance mutation that causes the bearer to create slightly smaller but more sex cells, making it possible to have more offspring (make more almonds). The bearers of genes like this pay a greater cost in mating with each other than non-bearers do when mating with them (in our example, because they would be making almonds less likely to grow into almond trees). This creates a selection pressure or a "preference" by smaller cell producers to mate with larger cell producers, nudging the offspring back towards that optimal level of total investment. But, this new more frequent matching of smaller sex cells and larger sex cells puts a complementary selection pressure on those larger sex cell producers. Having more at risk by putting more of their eggs in one basket, or rather, putting more of their investment in one sex cell, natural selection would act on these larger sex cell producers to invest even more, essentially to keep a rainy-day fund on the chance they match with a smaller sex cell producer. These two forces playing out over time send a species down the path where eventually sex cells of the same size are no longer mutually compatible. The species now has two sexes: one that makes big sex cells (females) and one that makes small cells (males). This is the process that leads to distinct, binary sex categories and is the common evolutionary history of all species that have both sex and sexes, humans included.

That's it. That's the biological definition of sex, sex categories, and where they came from. It isn't about what chromosomes you have, how much of which hormones you have, what your body parts look like, or how you act. If you want to talk about the scientific, biological definition of sex, the only way you can say there are two sex categories is to use this definition.

If I asked you what biological sex is based on, my guess is you wouldn't say something like: producing sperm or eggs through typical development. What would you say?

In every other way, sex is nonbinary

Looking at our genes and our bodies that they build, we can’t find anything that is meaningfully binary, or anything that divides people into two clear categories of male and female. Instead, we only find traits that are bimodal—a smooth distribution that has two peaks around the averages for typical egg producers and sperm producers. Consider our chromosomes: We often think "XX" means "female" and "XY" means "male," but there are babies born with the "XX" pair of chromosomes that have a penis, testicles, etc., and there are babies born with the "XY" pair of chromosomes that have ovaries, uterus, vagina, etc., (and can even become pregnant).^2,3 We often think all of our cells are either "XX cells" or "XY cells," but many people (likely most of us) have a mixture of "XX cells" and "XY cells" in their body to a small extent (microchimerism),⁴ or more (chimerism).

In the sports world many have tried to use "sex" hormones, but there are plenty of men with lower testosterone than plenty of women, and plenty of women with lower estrogen than plenty of men. Some point to reproductive anatomy, but there are lots of ways for the continuous variation in the size and shape of body parts, like the clitoris/penis (that start from the same tissue during pregnancy), to overlap in ways that make the categories messy.³

The biology of sex just isn't as clear as most people think. The traits we tend to think are responsible for making sex binary categories actually don't. The only way that sex categories are binary is the one way that, excepting pregnancy, has the least relevance to our everyday lives.