Why biologists don't believe "there are only two genders"

A quick introduction to biological sex

[cw: transphobia]

We saw two weeks ago that the bible doesn’t actually contain the anti-trans-people position that conservative voices often describe as “the biblical stance.” For more on this, my friend Shaney Swift gave me permission to share a post she wrote describing her journey, as a Christian, to fully affirming trans people and their identities.

This post is about the other objection I hear raised in Evangelical circles: doesn’t basic biology tell us that there are only two genders? Aren’t we being asked to affirm something that goes counter to basic science?

I don’t think this is always (or even usually) in bad faith — we’re often taught about biological sex in school as if it’s a perfect binary where everybody is either a man or a woman with no ambiguity, and if I hadn’t gone to MIT and made friends with biologists I might not have learned that this isn’t even close to how actual scientists think about biological sex. On the contrary, biologists are quite a bit more affirming of trans identities than the general population.

If this is surprising to you, then great! You are the target audience for this post. Come learn something about biology and hopefully be a bit more welcoming to the trans people in your life.

The scientific perspective: the president of the International Genetics Federation comments on an anti-trans video

A note on terminology: people who think a lot about gender usually distinguish gender (a society’s social roles around male/female/other gender identities) from “biological sex,” which is what this post is going to be about. Some societies recognize two genders, while others recognize more — going more into detail on this gets into sociology that I do not know well enough to write about. This post is just going to be about biological sex.

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If you’re going to say that “biological sex” is a perfect binary where everyone is either male or female, you need to have a strict definition of “biological sex.” This is quite a bit harder than you might at first guess.

You might start by looking at people you’re pretty sure are mare or female and writing down a definition that distinguishes them. There are some obvious bad ways to do this — if you say that “women” are people with long, flowing hair and “men” are people with beards, you’ll quickly get stuck when you come across people like this:

“But that’s a dumb example, Colin! Nobody would use something as ambiguous as ‘having long hair’ to define biological sex. That’s why we do it scientifically.”

Great. Let’s talk about the two “scientific” ways people have told me they define biological sex:

Genitalia: “if you have a penis, you’re a man. If you have a vagina, you’re a woman.”

Sex Chromosomes: “if you have XX chromosomes, you’re a woman. If you have XY chromosomes, you’re a man.”

The first thing to note here is that both of these suffer the exact same problems as the “long hair / beard” dichotomy. Some people are born with both a penis and a vagina, and some are born with neither. Some people only have a single X chromosome. Others can be XXX, XXY, XYY, and even XXXXY.

To make things even more complicated, these two definitions can disagree. You can have XY chromosomes and an otherwise “female” body. You can be XX and otherwise male. You can even have different parts of your body have different sex chromosomes — and in most cases, you wouldn’t even know it!

(This last bit is not just theoretical, and might apply to you without you even knowing it: I’ve heard professors advise against assigning students to look at their own sex chromosomes, because they’ve had too many students face surprises about their “biological sex” that make them uncomfortable in ways a required classroom assignment probably shouldn’t.)

So we have two ways to define biological sex, both of which are ambiguous and which disagree with one another. We can try to resolve this by adding more biology, but things won’t get more binary:

Brain Characteristics: Overall, men and women have slightly different brain structures, which makes sense because different brains need to interface with bodies that come with different parts, different hormones, different possibilities of pregnancy, etc.

This means you can look at part of somebody’s brain structure and put them into a “male” bucket or the “female” bucket (in real life there’s some overlap). This gives us dozens, if not hundreds, of new scientific markers to distinguish men from women.

So, for example, you might look at something like “brain activation upon exposure to androstadienone” or “distribution of gray matter within the brain”. You can use these to construct a gender binary, but it wouldn’t be the one conservatives describe as “biological sex.”

Why? Because on these and other measures, trans people’s brains don’t correspond precisely with the sex they were assigned at birth — they also contain a lot of features corresponding to the gender they identify as. In other words, here we have a perfectly workable definition of biological sex that agrees that (non-nonbinary) trans people are the sex they identify as, rather than the sex they were assigned at birth.

So we don’t have a single biological switch that’s set to either male or female. We have literally thousands of correlated switches — for many people, they’re almost all set to “male” or almost all set to “female” and we have a clear idea of biological sex. But for other people the switches disagree — you may have a thousand of them set to the “male” option and nine hundred set to the “female” option, and still more set to options that don’t correspond to either sex!

This doesn’t mean you should go around asking your trans friends intimate questions about their anatomy and brain chemistry and genes and whatnot to figure out what all their switches are set to. For one thing, we don’t know nearly enough about biology to find “all” or even “most” of the switches.

But more importantly, knowing how somebody’s brain processes androstadienone does not really help you to understand or love them better. Much more important than the underlying biological processes is the way that these processes shape a person’s way of experiencing the world. And this is precisely what your friends and neighbors are trying to tell you when they explain their gender identities to you.

They are telling you that even though you thought they seemed male, most of their switches are actually set to female. Or that they don’t really experience life as male or female because their switches are set to either a complicated mixture or to neither. Or that it changes over time and sometimes they feel more male and other times more female because life and biology are both complicated and their experience of gender is a lot more complicated than yours.

If you listen to them, you might learn a lot and have a meaningful moment with a friend.

If you say “I’m sorry, but I’ve decided your switches are set to male and I will only interact with you that way”, you’re being cruel to somebody in defense of a generally inaccurate model of biology.

That isn’t very sensible, and definitely isn’t a model of loving thy neighbor.

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So where does this leave us? What does biology say about the gender binary?

We know from experience that there are “male” and “female” bins that most people fit reasonably well into. But biology tells us this isn’t perfect — there are a lot of people (at least a couple million) to whom the male-female binary doesn’t really apply.

This doesn’t mean we have to throw gender away — lots of people identify with the biological sex they were assigned at birth (I’m definitely a man!) and gender can be a generally useful way of understanding the world.

But “generally useful” isn’t the same as “hard-and-fast rule to apply to every single person.” As we talked about last time, the “land vs sea” binary is pretty useful when I’m trying to figure out whether I need a boat to go somewhere. But it doesn’t mean we go around telling marshlands they have to “really” be one or the other — we recognize that our categories aren’t a good fit for wetlands and swamps and we celebrate them for what they are.

So, given that:

• “Biological sex” isn’t strictly binary

• There are lots of equally valid ways to define biological sex, some of which assign trans people the genders they identify as rather than the sexes they were assigned at birth.

• Millions of people literally don’t have a biological sex under any definition.

• Throughout history, it hasn’t been at all unusual for societies to recognize more than two genders

• The bible doesn’t teach that gender is strictly binary

And, probably most importantly:

• There is no question among mental health professionals that conservative stances on trans identities have had really, really bad effects on trans people, including children

I think it’s clear that Christians (and everyone else!) should adopt a much more affirming and loving stance towards our trans friends and neighbors.