Let's Talk About (Biological) Sex, Baby

Biological sex is all over the news lately. Whether it's via President Trump's executive order https://www.whitehouse.gov/presidential-actions/2025/01/defending-women-from-gender-ideology-extremism-and-restoring-biological-truth-to-the-federal-government/affecting passport policy https://www.npr.org/2025/02/21/nx-s1-5300880/trump-passport-policy-trans-gender-intersex-nonbinary, moving trans inmates between prisons https://www.npr.org/2025/02/21/nx-s1-5305282/trans-inmates-federal-prison-policy-transfers or shifting the requirements for women in sports https://www.npr.org/2025/02/13/nx-s1-5290056/differences-between-transgender-cisgender-athletes-science — appeals to "the biological reality of sex" are constant. In truth, biological sex, like a lot of scientific categories https://www.pnas.org/doi/10.1073/pnas.1317504111, 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] mailto:[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 http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:22Regina Barber?

  Hey, Short Wavers. Regina Barber here with producer Hannah Chin. Hey, Han.

• 0:26Speaker 02

  Hey, Gina.

• 0:27Regina Barber?

  So today, you're bringing us a story about sex.

• 0:30Speaker 02

  Yeah, not like sex, drugs, and rock and roll.

• 0:33Regina Barber?

  Oh, OK.

• 0:33Speaker 02

  I'm here with a deep dive on biological sex.

• 0:37Regina Barber?

  Oh.

• 0:38Speaker 02

  Which has been mentioned in the news a lot recently.

• 0:41Speaker 03

  In the House, lawmakers are set to move forward with a bill defining biological sex.

• 0:45Speaker 04

  The bill said the bill is about creating a definition for sex separate from gender. The team prohibits college athletes from competing on a team opposite to their biological sex.

• 0:54Speaker 03

  Their biological sex. Biological sex.

• 0:57Speaker 02

  Quote, "biological sex." And just to be clear here, Gina, we're talking about sex here, not gender. That's a whole nother can of worms. But I think a lot of times, these conversations are missing something important because there's this sense that gender is socially defined and changing, as opposed to sex, which is scientifically defined and has always been binary and clear.

• 1:16Regina Barber?

  But it's not.

• 1:17Speaker 02

  No, it's not. When you get down to the biological reality, sex is way more complicated than that.

• 1:23Speaker 05

  The answer to the question, are there always two sexes, is no.

• 1:27Speaker 02

  This is Anne Fausto Sterling. She's a sexologist trained in developmental biology and emeritus professor of gender studies and biology at Brown University. And she says the animal kingdom, for example, has all kinds of sexes.

• 1:40Speaker 05

  Different organisms have different sexual systems. And sometimes there's two, sometimes there's more than two.

• 1:46Speaker 02

  If you're curious, Gina, white-throated sparrows and clam shrimp are examples of animals with more than two sexes. And some kinds of snails and worms have just one sex. They can produce both eggs and sperm.

• 1:57Speaker 05

  Sometimes the same individual animal changes sexes during the course of the life cycle. And this has been perfectly well-understood by biologists for a long time.

• 2:09Regina Barber?

  I think I remember, like, clownfish can change sexes over their lifetime.

• 2:13Speaker 02

  Yeah, Anne told me that's really common with fish.

• 2:16Speaker 05

  They'll live in a big school of fish with a dominant male and a group of females. And if the dominant male dies, one of the females literally transforms into a male. So technically, that's still two sexes, but the sexes aren't fixed for life.

• 2:34Speaker 02

  Researchers have found more than 450 fish species that can and do change sex. So yes, clownfish, but also gobies and wrasse. It's a pretty long list.

• 2:45Regina Barber?

  And there's also animals like the New Mexico whiptail lizard. Like, we've covered that on Short Wave. And that's a species that has like no males and the females just like lay eggs. They're like viable and they don't need fertilization at all.

• 2:57Speaker 02

  Totally. But this doesn't necessarily translate to humans, right? Like, lizards can do a lot of things that we can't and vice versa. We're not fungi, we're not fish, we're different.

• 3:07Regina Barber?

  So today on the show, the science of biological sex in humans.

• 3:12Speaker 02

  What is it? What's it determined by? And when policymakers refer to sex as this unchanging biological constant, is that reality?

• 3:21Regina Barber?

  You're listening to Short Wave, the science podcast from NPR.

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• 4:54Regina Barber?

  Okay, Hannah, so we were just talking about biological sex and how there's like a lot of variation in other animals. But what about humans? Like, what's the determining factor for like sex in us?

• 5:05Speaker 02

  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.

• 5:17Regina Barber?

  Wait, 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 is assigned to male.

• 5:46Speaker 02

  Right, that's true for most humans. Not all, I'll get to that later, but most. And Hannah Claire says that nowadays, when doctors predict fetal sex, usually they're looking at the chromosomes.

• 5:57Speaker 07

  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.

• 6:12Speaker 02

  Hannah's a genetic counseling researcher with experience in OB-GYN 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.

• 6:31Speaker 07

  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.

• 6:52Regina Barber?

  Well, that's, that's really cool.

• 6:54Speaker 02

  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.

• 7:19Regina Barber?

  Right. And you, you mentioned a second metric being chemical, right? Like, what do chemicals tell us about sex?

• 7:24Speaker 02

  Yeah, so a big part of sex and how it develops has to do with hormones.

• 7:29Regina Barber?

  Right.

• 7:29Speaker 02

  And those chemical hormones, they fluctuate through your whole life. Like, as a little kid, you had a different hormone profile than when you went through puberty, or then when you start going through menopause.

• 7:39Regina Barber?

  Wow, yep, yep. So when does this, like, first chemical change actually happen? Puberty?

• 7:45Speaker 02

  Way earlier. All humans have hormones like testosterone, estrogen, progesterone, et cetera. They just have them in different quantities and different cycles.

• 7:54Regina Barber?

  Wow.

• 7:54Speaker 02

  And those hormones really fluctuate through life. So a fetus gets the first hit of these hormones in the womb. That triggers things like genital growth and certain types of brain development. Then there's another hormone surge in babies after birth within the first six months. It's one that endocrinologists call mini puberty.

• 8:12Regina Barber?

  Wow, I did not know any of this. It's like baby puberty, okay.

• 8:15Speaker 02

  Yeah. And after that, in early childhood, the hormones kind of take a break. One pediatrician I talked to said, and I quote, "That the testes are fast asleep." So the testes are active and inactive at specific periods during childhood.

• 8:32Speaker 02

  And adolescents. I mean, these glands are not making things all the time. They kind of go up in that sort of very similar to ovaries. This is Faisal Ahmed. He's a pediatric endocrinologist at the University of Glasgow. And he says that once adolescent puberty hits, there's usually an increase in hormones.

• 8:47Regina Barber?

  Yeah.

• 8:48Speaker 02

  Those chemicals can also be delayed or boosted, for example, during gender affirmative hormone therapy. And they're usually what trigger the development of a bunch of other characteristics that we use to determine sex. And this brings us to the last criteria, physical.

• 9:01Regina Barber?

  OK, and I'm guessing that's like genitals.

• 9:04Speaker 02

  Well, yes, this can refer to internal genitalia, like ovaries, or external genitalia, like penis and testes. Or we could also look at secondary sexual characteristics, things that usually don't develop until puberty, like breasts or facial hair. Or even things that are determined in part by hormones and are often used to differentiate sex, like your voice, or your height, or the distribution of fat and muscle on your body.

• 9:30Regina Barber?

  Wow, I didn't even think about those last things. Like, you're totally right.

• 9:33Speaker 02

  Yeah. And those physical traits are really the main observable characteristics, the ones that don't require lab work. So usually when people who are not doctors or scientists are talking about biological sex, this is what they really mean. But these physical characteristics don't really fall on a strict binary. I mean, we have tall women and short men. We have women with flatter chests and men without facial hair.

• 9:56Regina Barber?

  Yeah.

• 9:57Speaker 02

  People's appearances can really vary, but I digress.

• 10:00Regina Barber?

  OK, so physical traits, hormones, chromosomes, we have all these different ways to determine sex. And I'm guessing like that most of the time, they align, but not all the time.

• 10:11Speaker 02

  Exactly. All of these things have the potential to differ from one another or to be ambiguous or unclear. Like, someone's chromosomes might be XY, but they don't have a penis. Or they do have a penis, but they also have internal ovaries. And these differences generally fall under the umbrella of something called intersex conditions. Intersex is an umbrella term for biological conditions where a child is born with, like, physical characteristics.

• 10:37Speaker 03

  Or genetic characteristics that don't fall into our society's neat definitions of what is male or female.

• 10:45Speaker 02

  This is Eileen Wong. She's a physician, specifically an adult urologist. And she says that although intersex conditions are rare, they're not as rare as you think.

• 10:54Regina Barber?

  Wait, like, how common are they?

• 10:56Speaker 02

  Well, estimates can vary, but the most common number that I've seen thrown around is that intersex conditions overall affect one to two people in every 100. So that would make it about as common as having red hair and even more common than being born an identical twin.

• 11:11Regina Barber?

  So chances are, if you're listening to this episode and you're not intersex, you've probably at least encountered someone who is.

• 11:16Speaker 02

  Exactly. And Eileen is really passionate about intersex awareness because she says her training, she went to med school at Yale, she did her residency at Stanford, it still left her really unprepared to treat intersex patients.

• 11:29Speaker 03

  Once you operate on an intersex body, that patient will need to deal with those complications for the rest of their life. You can't change them back to what nature made them as.

• 11:39Speaker 02

  Eileen told me that in the past, there was this big push to normalize intersex patients' bodies. Doctors would look at an intersex child and operate on them, usually without those children's full understanding or consent, so their bodies would conform to more typical sex assignments.

• 11:54Speaker 03

  Kids were, quote, "normalized." They were stigmatized. They were lied to. Their parents were told that they shouldn't tell their children because it would ruin them psychologically. They were subjected to surgeries, including literal clitoral amputations that caused dyspareria, pain, chronic scarring, basically medical PTSD for hundreds and hundreds of people.

• 12:15Regina Barber?

  That's really horrible.

• 12:16Speaker 02

  Yeah. And in 2018, the American Academy of Family Physicians issued a statement opposing medically unnecessary surgeries on intersex children, basically saying, "This is harmful and we shouldn't do it anymore." But Eileen says there's still a huge information gap when it comes to intersex bodies and medical treatment. Faisal specializes in this kind of treatment, and here he is again.

• 12:37Speaker 05

  So sometimes people feel that, you know, intersex is a diagnosis, but it's not really. It's really like saying...

• 12:43Speaker 03

  Somebody has short stature.

• 12:46Regina Barber?

  Right, because like height is one of those physical characteristics you mentioned earlier.

• 12:50Speaker 02

  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. Like, when I was talking to Faisal, he drew this comparison of urinal heights in Japan versus in Europe.

• 13:10Speaker 03

  But if you go to Netherlands, they're much higher up. So society is creating this thing which makes people not fit in.

• 13:18Speaker 02

  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.

• 13:45Speaker 05

  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.

• 14:05Speaker 02

  Human biology and all its possible variations are overwhelmingly complex. One might even say too complex to fit into a 15-minute episode.

• 14:15Speaker 03

  Now, if society says that the only options we have is boy or a girl, and anything other than boy or girl is, is not normal, then doctors or scientists will go along with that. And they will say, okay, let's, let's try to make sure that everybody's male and female. Okay, but I, I think.

• 14:34Speaker 02

  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. I reached out to 11 experts for this story, Gina, and less than half of them agreed to talk to me. Some people who declined, they cited fear for their jobs or their institutions. It feels like a reminder that science doesn't happen in a vacuum.

• 15:23Regina Barber?

  Thank you, Hannah, for bringing us this story.

• 15:25Speaker 02

  Anytime, Gina.

• 15:27Regina Barber?

  If you liked this episode, make sure you never miss a new one by following us on whatever podcasting platform you're listening from. And if you have a science question you'd like us to investigate, send us an email at [email protected].

• 15:39Speaker 02

  Newspaper at the top of the episode was from the following outlets: KSNT, KPAX, KCCI, WDAL, and CBS News.

• 15:48Regina Barber?

  This episode was produced by Burleigh McCoy and edited by our showrunner Rebecca Ramirez. Tyler Jones checked the facts. Kwasi Lee was the audio engineer.

• 15:56Speaker 02

  Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy.

• 16:01Regina Barber?

  I'm Regina Barber.

• 16:02Speaker 02

  And I'm Hannah Chen.

• 16:03Regina Barber?

  Thank you for listening to Short Wave from NPR.

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