Biological sex is all over the news lately. Whether it's via President Trump's executive order affecting passport policy, moving trans inmates between prisons or shifting the requirements for women in sports — appeals to "the biological reality of sex" are constant. In truth, biological sex, like a lot of scientific categories, is nuanced. It's defined by multiple criteria – including chromosomal, chemical and physical factors – that can, and do, change over a person's lifetime. And it's a reality that's definitely not limited to male and female.Want the episode to cover more nuances of human biology? Let us know by emailing [email protected]! We're also always open to other story ideas you have.Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.Learn more about sponsor message choices: podcastchoices.com/adchoicesNPR Privacy Policy
Chapter 1: What is biological sex?
So in humans, sex is determined based on a variety of factors. But for the purposes of this episode, we're going to focus on three of the main ones, chromosomal, chemical and physical.
I think we need to slow down and like break down each of them. Right. Like so the first one you said is chromosomal. Right. And I remember learning about this in like high school bio. All the genetic information in our bodies are packaged in 46 chromosomes and they're coupled up to make 23 pairs. The first 22 pairs tend to look similar, like, in all humans.
But the last one is usually either an XX or an XY pair. And XX is usually assigned to female. XY is assigned to male. Right.
That's true for most humans. Not all. I'll get to that later. But most. And Hannah Clare says that nowadays, when doctors predict fetal sex, usually they're looking at the chromosomes.
So when folks say that they know the sex of their pregnancy, sometimes they're referring to ultrasound. But more often, and especially after 2010, they're referring to this test called cell-free DNA testing.
Hannah's a genetic counseling researcher with experience in OBGYN clinics. We're not using her full name here or noting her employer because she's concerned that speaking publicly could hurt her ability to fund her research. But she says this test is super common. Clinicians don't have to wait for the ultrasound to look at the fetus. They just do a little blood test.
As a pregnancy is growing, the placenta sheds DNA into the bloodstream of the pregnant person. And so what labs will do is take that blood, sort out that fetal fraction, and analyze that to look at the chromosomes.
Well, that's really cool. I know, right? So this test tells us the chromosomes that a baby has, but the Y chromosome isn't like an on-off switch for sex. There are sex-influencing genes present in the other 22 pairs of chromosomes too, and there's a lot of variation that's still possible within those genes.
So for a number of reasons, after birth, the baby can develop in a way that's different from what the tests predicted. And that's where this second metric for determining sex comes in.
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Chapter 2: Why is biological sex complex?
Right. Because like height is one of those physical characteristics you mentioned earlier.
Yeah. Faisal says that if you're short, there could be a bunch of reasons why. Like it could be that your parents are short or it could be a nutrition problem or a genetic condition. And depending on how short you are and the society that you live in, it might or might not pose a problem. Right. When I was talking to Faisal, he drew this comparison of urinal heights in Japan versus in Europe.
But if you go to the Netherlands, they're much higher up. So society is creating this thing which makes people not fit in.
And that's the thing that's key, Gina. Even though a lot of these metrics for determining sex are based in science, the way we interpret them is rooted in society. All of the scientists that I talked to agreed. Biological sex is definitely not as simple as two separate categories. And we lose a lot of nuance and knowledge when we pretend that it is. Here's Anne Fausto-Sterling again.
She's the biologist that we heard from at the very beginning of the episode.
You can think of a model in which there is there's only two. and they completely don't overlap. You always know which is which, no matter what measure you're using, whether you're looking at the genitals or the chromosomes or the gonads or the hormones. And the fact is that that model doesn't exist in nature at all.
Human biology and all its possible variations are overwhelmingly complex. One might even say too complex to fit into a 15-minute episode.
Now, if society says, that the only options we have is boy or a girl, and anything other than boy or girl is not normal, then doctors or scientists will go along with that and they will say, okay, let's try to make sure that everybody's male and female. But I think the more you understand this area, you will realize there is more to humans than boy and girl or males and females.
So while biological sex is scientific, the way that medical experts and scientists determine it is more complicated than I thought. Although saying so has become increasingly politicized. We heard that at the start of the episode, right? And how much media coverage this topic has gotten recently. And I saw the effects of it in my reporting.
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