It all started on Twitter. Again.

Briscoe Cain, a Republican in the Texas state legislature, tweeted this back in December:

I replied, correcting him, and my tweet went viral. You may be surprised to learn that people on the Internet frequently have strong opinions about gender. Lots of folks chose to spend their winter holiday sending me feedback. I explained things further on Twitter, but there’s a limit to the medium. So let’s break it down here, in one easy-to-read place.

There are two human sex chromosomes, X and Y. Most humans either have two Xs (XX) or one of each (XY). If you passed eighth grade, you probably already know that. So, as a science communicator, I’m not going to go out of my way to emphasize that point. If someone like Rep. Cain goes out of his way to say it, his goal is not to give a remedial biology lesson. He’s hoping to imply something more. But, his darker message does not logically follow from the image of chromosomes he shared. You can’t extrapolate the number of human genders from it, for three main reasons:

There are more than those two karyotypes (chromosome combinations) in humans. Some of us are XXY, X alone (called X0), XYY, XXX, or XXYY. These are associated with various physical traits but it’s not absolute. Unless you’ve been tested, you don’t know your own karyotype for certain. I got plenty of pushback when I said this. Folks really wanted me to remind everyone that most people are XX or XY, as if that somehow invalidates the diverse reality. About 0.2% of births are not represented in Rep. Cain’s image. Worldwide, that would be over 15 million people, similar to the population of New England. It’s not a negligible portion of humanity. Besides, these precision-obsessed trolls are oddly silent when I tweet other science facts. Some frogs can survive being frozen all winter, but no one ever complains when I say that without stressing that most frogs can’t.

Sex chromosomes are the main genetic factor determining biological sex. But like most genes, they don’t explain 100% of the variation. How do we define biological sex, anyway? Humans make two sizes of gamete, large and small. No one makes both. But many people don’t make any eggs or sperm. That doesn’t nullify their sex. And nobody, not even the most objective biomedical scientist, asks to see someone’s gametes before inferring their sex. That would be weird. Instead, we all evaluate a suite of characters that are correlated with gamete size: hairiness, body shape, genitals, chromosomes, etc. And it turns out that this set of traits doesn’t always line up the same way. XX humans can bodies that are (otherwise) male. XY humans can have bodies that are (otherwise) female. Some people are intersex, with physical features in between male and female. And of course, there is no general rule across species about which traits usually come with a side of eggs. In birds, it’s typically the females with two different sex chromosomes, while males have two of the same. The opposite of the mammalian pattern. Thus, the Y chromosome is not some overarching determinant of maleness throughout the tree of life.

Gender is a social and psychological phenomenon that is distinct from biological sex. It’s correlated with genetics: tell me your sex chromosome karyotype, and I can guess your gender with pretty decent accuracy. But there is no 1-to-1 connection. Loads of people on Twitter want to inform me that gender is a made-up idea. And yet, I bet most of these same people would get offended if I referred to them with the wrong pronoun. In contrast, I doubt they would get nearly so upset if I misstated their blood type. For most humans (and perhaps other intelligent creatures), whether you think of yourself as male or female (or something else) is a key part of your self-identity. It’s not just a bland fact about your cells. Yes, gender is in your head. But that doesn’t mean it’s not real. Love and faith are also in our heads, and yet we are happy to accept it (and grant legal rights accordingly) when someone claims to love their spouse or to be a Christian. And gender is supported by evidence. In particular, explore the copious data showing severe mental health consequences when a person is forced to reject the gender that they feel themselves to be.

Below is a figure from one of my papers on sex variation in wild strawberries. Obviously humans are not plants, but the figure shows how categorical thinking can oversimplify things. Fruit production (female fertility) is bimodal and inversely correlated with pollen production (male fertility). But if I simply reported that there are two sexes, I wouldn’t be telling the whole story. There’s a lot of variation around those two peaks.

Everything in biology shows natural variation, but our minds really want to pigeonhole it into discrete categories. Saying “men are XY and women are XX” is a convenient shorthand, and usually true. But don’t confuse it with the complete truth, or the way things “ought to be.” This isn’t just academic, of course. Unless you are a lifelong hermit, you have almost certainly met other people who are not male(XY) or female(XX), whether or not you know it. And these people are much more likely to be victims in one way or another, than the average person. Transgender individuals are oppressed by laws and directly targeted by aggressive acts of violence. Intersex babies are often operated upon, needlessly, just to make their genitals conform to a binary ideal. And folks who merely look a little different, whether they have a genetic condition like Turner syndrome or simply a fashion sense that flouts convention, get harassed by bullies.

Bigotry of all sorts (sexism, racism, homophobia) involves drawing clear demarcations between groups of people. But biological reality is messy, and the borders aren’t nearly as well-defined as many think. Sure, there are some absolutes in biology. Humans are not insects, for example. But for the kinds of things people actually argue about, the person insisting on crisp divisions is almost never correct. If someone claims that science supports their prejudice, they’re wrong.