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Key points
- Scientists don't entirely agree on how to define "male" and "female."
- Intersex people challenge the idea that all humans are either exclusively female or male.
- We suggest defining "sex" in terms of biological function, consistent with evolutionary biology and with progressive social politics.
This post was co-authored with Maximiliana Jewett Rifkin.
Some kinds of things exist quite independently of our beliefs and practices. Water, clouds, and grass would exist even if humans never evolved. Other kinds of things depend in some way on what we think and do. Presidents or money exist only because we agree to treat them a certain way.
We are inclined to think one’s gender identity—whether one identifies as a woman, man, nonbinary, or some combination of those—depends on what one believes and feels about oneself. If you want to know what gender someone is, one of the best ways to find out is to ask them.
Similarly, we think one’s gender social position—whether someone has the social status of woman or man, or nonbinary—depends on others’ beliefs about oneself. If you want to respect someone’s gender, one of the best ways to do that is to politely ask them their preferences (e.g., their pronouns).
In contrast, people tend to think that one’s sex—whether you’re female or male—is dictated by your biology. A traditional idea is that, at least in humans, there are only two sexes, and you’re born with one or the other.
Today, scientists, philosophers, and other theorists are increasingly challenging the idea that humans are locked into an unchanging, two-sex system. What if there are three or more sexes? What if people can change their sex just as they change their gender?
Intersex Individuals and the Sex Binary
One way to think about how there might be more than two sexes is to consider the idea that there are intersex individuals. Intersex people are born with some combination of characteristics associated with males and females.
Intersex people are only recently climbing out of stigma and forming advocacy groups. Maybe we should recognize three sexes: male, female, and intersex. For that matter, maybe we should think of sex as a spectrum, the same way we think about skin color.
There are many different ways of being intersex. For some, it’s having genitals that are similar in some ways to a vagina and in other ways to a penis (so-called “mixed genitalia”). For others, it involves having a combination of ovaries and testes (“ovotesticular condition”).
In Search of Clear Definitions
Another way to think about how sex might be shaped by culture is to consider the fact that biologists still lack a clear working definition of “female” and “male.” That might seem shocking, but remember that biologists often have a hard time defining central terms such as “life,” “species,” or “organism.”
You might think that defining "sex" is quite simple. Being male means having a penis; being female means having a vagina. Right?
Wrong. Most male birds and fish don’t have penises. Intriguingly, in barkflies, the female has a protruding genital called a “gynosome.” She inserts it into the male to collect sperm. Also, a human male can lose a penis through accident.
Many would say that, “scientifically speaking,” your sex depends on your chromosomes. Did nature give you the XX combination of sex chromosomes or the XY combination?
But chromosomes aren’t the key to sex, either. Alligators, for example, don’t have sex chromosomes at all. Their sex depends on the temperature at which they’re incubated.
Even some humans don’t have either XX or XY. In the case of trisomy, people are born with three sex chromosomes, like XXX or XXY. It’s also possible to be born with a single sex chromosome.
There’s also complete androgen insensitivity syndrome (CAIS). This happens when someone is born with the XY chromosome, but their body can’t recognize testosterone, and they develop characteristics associated with females.
Sex and Anisogamy
There’s one definition of sex that we suspect is closest to the truth, the “anisogamy” definition. In this view, an animal’s sex doesn’t depend on its genitals or even its chromosomes, but on what kind of sex cell, or gamete, it can make.
Whether we’re talking about gorillas, goldfish, barkflies, or humans, being female has to do with making large sex cells (ova), and being male has to do with making small sex cells (sperm). In humans, for example, the volume of an ovum is about 10 million times greater than a single sperm.
This definition, though, faces an obvious problem. Many are born infertile for life. If sex depends on what kind of sex cell an animal makes, then infertile animals would be neither male nor female. That seems absurd, and, in the case of humans, insulting.
Recently we wrote a paper arguing that biologists shouldn’t define an animal’s sex in terms of what sex cell it’s able to make, but what sex cell it’s designed to make. Although we’re not the first to propose the idea, we develop it in a comprehensive way.
Suppose an infertile male animal has undescended testes for life. In our view, that animal is male because it has a part (the testes) that’s designed to produce sperm.
By “designed,” we just mean in the biological sense of being shaped by evolutionary natural selection. In the same way, you might say that the zebra’s stripes are “designed” to ward off biting flies. We don’t mean anyone literally designed them in our deep evolutionary past.
Some might worry that by trying to define sex in terms of anisogamy, we’re advancing some patriarchal or transphobic project of trying to lock everyone into two unchanging sexes. But we actually think the biological definition of sex has quite progressive implications.
For one, we don’t think sex is binary, in the sense that everyone’s either exclusively male or exclusively female. Some people are both male and female. As we mentioned, in ovotesticular condition, people are born with one ovary and one testes—and, hence, “designed” to make both ova and sperm.
We also describe how humans might literally change their sex. Unfortunately, the phrase “sex change” has problematic implications because it once referred to gender-affirming surgery to change genitalia. We don’t mean it in that sense, but rather in the sense of changing the functions of one’s bodily parts and processes, namely, the function of producing ova or sperm.
These results are important because they undermine support for the idea that biology justifies traditional policies surrounding sex, such as limiting people’s ability to make associated medical or legal changes.
Regardless of how biologists choose to define sex, we think a person’s sex determines neither their gender nor what rights they have in society. In our view, it’s vital to respect people’s first-person authority over their gender identity and to use gender terms on the basis of consent.
