Hey, it's A. Martinez. A lot of short daily news podcasts focus on one story, but sometimes you need un poquito más. For Up First on NPR, we bring you the three top world headlines every single day in under 15 minutes because no one story can capture all that's happening in este mundo tan grande on any given morning. So listen to the Up First podcast.
You're listening to Shortwave from NPR. Hey, Shortwavers. Regina Barber here with producer Hannah Chin. Hey, Hannah. Hey, Gina. So today you're bringing us a story about sex. Yeah, not like sex, drugs, and rock and roll. I'm here with a deep dive on biological sex, which has been mentioned in the news a lot recently. In the House, lawmakers are set to move forward with a bill defining...
Biological sex. Glenn said the bill was about creating a definition for sex separate from gender. Team prohibits college athletes from competing on a team opposite to their biological sex. Biological sex. Biological sex. biological sex. And just to be clear here, Gina, we're talking about sex here, not gender. That's a whole other 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. But it's not. No, it's not. When you get down to the biological reality, sex is way more complicated than that. The answer to the question, are there always two sexes, is no.
This is Anne Faust of 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. Different organisms have different sexual systems, and sometimes there's two, sometimes there's more than two. 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. 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. I think I remember like clownfish can change sexes over their lifetime. Yeah, Anne told me that's really common with fish. 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. 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. And there's also animals like the New Mexico whiptail lizard.
We've covered that on Shortwave. And that's a species that has no males and the females just lay eggs that are viable and they don't need fertilization at all. 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. So, today on the show, the science of biological sex in humans. What is it?
What's it determined by? And when policymakers refer to sex as this unchanging biological constant, is that reality? You're listening to Shortwave, the science podcast from NPR. On ThruLine from NPR. The consequences for the country would have been enormous. It would have been a crisis. The man who saw a dangerous omission in the U.S. Constitution and took it upon himself to fix it.
These days, there's so much news, it can be hard to keep up with what it all means for you, your family, and your community. The Consider This podcast from NPR features our award-winning journalism. Six days a week, we bring you a deep dive on a news story and provide the context and analysis that helps you make sense of the news. We get behind the headlines. We get to the truth.
Listen to the Consider This podcast from NPR. Okay, Han, 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? 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 break down each of them, right? So the first one you said is chromosomal, right? And I remember learning about this in 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. We'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? 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 the second metric for determining sex comes in. Right. And you mentioned a second metric being chemical, right? Like, what do chemicals tell us about sex?
Yeah, so a big part of sex and how it develops has to do with hormones. Right. 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 than when you start going through menopause. Wow. Yep. Yep. So when does this like first chemical change actually happen? Puberty? Way earlier, all humans have hormones like testosterone, estrogen, progesterone, etc.
They just have them in different quantities and different cycles. Wow. 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. Wow, I did not know any of this. It's like...
Baby puberty. Okay. 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 and adolescence. I mean, these glands are not making things all the time. They kind of go up and down. So 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. 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. Okay, and I'm guessing that's like genitals. 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. Wow.
I didn't even think about those last things. Like, you're totally right. 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. People's appearances can really vary. But I digress. Okay. 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.
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 or genetic characteristics that don't fall into.
are society's neat definitions of what is male or female. 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. Wait, like how common are they? 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. So chances are if you're listening to this episode and you're not intersex, you've probably at least encountered someone who is. 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. 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. 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. 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 dyspareia, pain, chronic scarring, basically medical PTSD for hundreds and hundreds of people. That's really horrible. 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. So sometimes people feel that intersex is a diagnosis, but it's not really. It's really like saying somebody has short stature.
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.
Like 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 let's try to make sure that everybody's male and female okay but i think the more you understand this area you will realize there is more to
humans than boy and girl are 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. Thank you, Han, for bringing us this story. Anytime, Gina.
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 shortwave at npr.org. News tape at the top of the episode was from the following outlets. KSNT, KPAX, KCCI, WDAL. and CBS News. This episode was produced by Burley McCoy and edited by our showrunner Rebecca Ramirez. Tyler Jones checked the facts.
Kweisi Lee was the audio engineer. Beth Donovan is our senior director. And Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. And I'm Hannah Chen. Thank you for listening to Shortwave from NPR. Hi, it's Terry Gross, host of Fresh Air. I just talked to comic Bill Burr. He's known for his anger-fueled humor, which he connects to his upbringing.
Let's talk a little bit about your childhood. Oh, Jesus. People are driving to work here. Let's try to give them something uplifting. He was hilarious and introspective in the interview, and it was a wild ride. You can hear a special extended version of this interview on the Fresh Air podcast from NPR and WHYY. On the ThruLine podcast, the myth linking autism and vaccines was decades in the making.
and was a major moment for vaccine hesitancy in America, tapping into fears involving the pharmaceutical industry and the federal government. No matter how many studies you do showing that this is not a problem, it's very hard to unring the bell. Listen to ThruLine from NPR wherever you get your podcasts.