Chapter 1: What is the relationship between older brothers and sexual orientation?
The larger the number of older brothers that a male has, the higher the probability that he is gay.
It's been seen over and over. I mean, it's really one of the rock-solid findings in human sexuality. So the way to emphasize the difference is if a baby boy is born today, if he has no older brothers, his odds of being gay when he grows up is about 2%, right? Pretty low. But if he had one older brother, his odds go up by a third. Okay, 2.6.
And if he has two older brothers, they go up a third again. All right, now we're at 3.5. It turns out you got to have like a dozen older brothers just to have a 50-50 chance.
Welcome to the Huberman Lab Podcast, where we discuss science and science-based tools for everyday life. I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. My guest today is Dr. Mark Breedlove.
Dr. Mark Breedlove is a professor of neuroscience at Michigan State University, and he is an expert in how hormones shape the developing brain, in particular, how they influence sexual orientation.
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Chapter 2: How does prenatal testosterone influence finger length ratios?
As you'll learn today, the amount of testosterone that a fetus is exposed to while in the mother has a profound impact, not only on the ratio of finger lengths, yes, you heard that right, but it also plays a meaningful role in sexual orientation. And in fact, there's a correlation there between finger length ratios and sexual orientation.
Now, as wild as that may seem, that result has now been confirmed many times over in humans and in animals. And today you'll understand why. You'll also learn that every time a woman is pregnant with a male, there's a biological trace of that, which biases the likelihood that her next male offspring will be either heterosexual or homosexual.
Now, I know this sounds really out there, but these are extremely solid biological findings for which the mechanisms are now understood for both animals and humans. It turns out that the hormones we are exposed to while we are in the womb shape not only the preference for whether somebody is attracted to males or females, but also an aversion to the opposite.
meaning there appears to be the formation of circuits for being attracted to one sex and not attracted to the other. Today, you'll also learn how hormones impact the amount of rough and tumble or social play that kids engage in the interplay between nature and nurture in shaping male versus female differences and sexual orientation.
Dr. Breedlove is one of the longstanding pioneers in this field of how hormones shape brain development and psychology. We approach these questions through the lens of biology and statistics. So today's is not a political discussion. Instead, it's a discussion about what is known and what is still not known about this profound aspect of our species. Oh, and we also talk about gay Rams.
Yes, that's a real thing. And it has important implications for everything we've mentioned thus far. By the end of today's episode, you'll surely think differently about the relationship between hormones and brain development, nature and nurture and romantic partner choice. Before we begin, I'd like to emphasize that this podcast is separate from my teaching and research roles at Stanford.
It is, however, part of my desire and effort to bring zero cost to consumer information about science and science-related tools to the general public. In keeping with that theme, today's episode does include sponsors. And now for my discussion with Dr. Mark Breedlove. Dr. Mark Breedlove, welcome.
Thank you. I'm delighted to be here. Very exciting.
It's been 25 years since we stood in the same physical space.
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Chapter 3: What are the implications of finger length ratios on sexual orientation?
You look the same, so we can talk longevity protocols at the end, but... I'm trying to have a blonde look in my hair, apparently, so...
Well, I've wanted to have you on this podcast since I launched it because you work on one of the most interesting things in the world, which is how and why people become who they are and how hormones play a role there, how genes play a role there. If you're willing, I'd like to jump from the high dive to the deep end first, right on.
Let's talk about this finger length ratios, sexual orientation study that you published and somehow I landed on that paper. That's not why I want to talk about it. I want to talk about it because it's an incredibly interesting set of findings. Other people have done the same-ish experiments, and there's a whole context there about how hormones influence sexual orientation independent of behavior.
We need to step back a little bit for the context. One thing that your listeners might not know is, you know, in the year 2000, there was still a lot of people who regarded ā same-sex orientation as a choice, a lifestyle choice. That was the political combination of words that meant you could disapprove of people because they were attracted to the same sex. Of course, I'm at Berkeley.
I didn't have any truck with such notions at all. And I've always been convinced that sexual orientation is not a choice. And there's an exercise they do in class where I ask ā so I'm going to put you to it. So remember the first time you had a crush, right? It might have been someone on TV. It might have been someone at a playground, et cetera.
So think about it and I want you to tell me about how old you were at the time in a moment. My guess is it was before puberty. Yeah, I was six.
Yeah. And I hit puberty somewhere starting around 14.
So it had nothing to do with puberty, right? It was this thing that happened. And I'll share my experience. So I'm about six or seven. I couldn't have been more than seven. And Marilyn Monroe is on TV. I'm dating myself. And... There's a close-up, you know, with that face and the mole, et cetera. And afterwards, I'm just so agitated. I don't, you know, I don't, and I know nothing about sex.
I remember I had a hard time going to sleep. So it was like something about this was really agitating me. And I didn't choose to have that reaction. And my guess is that whatever sex you had your first crush on, that's the one you're going to be attracted to the rest of your life.
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Chapter 4: How do hormones affect brain development and behavior?
For those just listening, yeah.
Right. And the length of the ring finger. And you can do a simple ratio. Divide the length of the second digit by the fourth digit, so-called 2D-4D ratio. And a guy named John Manning was reporting that there's a sex difference there, that it's... that that ratio tends to be smaller in men than in women, and that it's present in children. It's like, well, wait a minute.
A sex difference in the body that's present before puberty, I know enough about sexual differentiation of the body, it's almost certainly due to prenatal testosterone.
Forgive me for interrupting you, but people are probably looking at their hands right now. Oh, yes. And I just want to point out that these are averages. Yes. But it's ā I think the 2D, 4D thing for people that aren't familiar, even though you explained it quite clearly, can be a little confusing. Basically, in men, the finger lengths are more different than they are in women.
And there are some differences in ā in that statement according to sexual orientation that we'll get into. But when you say, in other words, the typical heterosexual male pattern is that the pointer finger, right, is shorter than the ring finger. Right, right. Whereas in women, they tend to be more similar. Again, these are averages. And
not to give it away, but this is because people are looking at their fingers right now.
Let me just say, don't panic. We're going to walk you through this.
You're going to be fine. And the difference between men and women is more pronounced on the right hand, as I recall.
That's true too. Did I earn my authorship on the paper? Well, that and the fact that you persuaded so many people to answer our weird questions is going to see rocks their hands.
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Chapter 5: What societal shifts impact perceptions of sexual orientation?
That's my impression too. I think it's pretty clear.
Yeah. And then there's a societal shift and then it sort of becomes like, you know, like if you, like I spend some time on X, formerly known as Twitter, right? And there's some gay political accounts and you just kind of notice like it's just people are, like, comfortable with it. Men and women seem to be comfortable with that, right? I mean, of course, you don't see a lot of attacks now.
That's very different than what you would have observed, for instance, in, like, the late 80s, early 90s, right? Anyway, I think these are important biological... phenomenon, this notion of an aversive pathway. You could imagine where societal standards or community standards or household standards might amplify or reduce the sort of expression of these things.
I'm sure they do. Yeah. And I'm sure cultures can amplify or reduce that component. The question is, to what extent? I mean, and I don't think we know.
Chapter 6: How do biological factors influence sexual orientation?
One thing that we haven't talked about, and it is a small percentage of people, but it's something that people think about, is this notion of sort of neither here nor there, kind of mixed sex, right? Is there a biological correlate of that? A graduate student my year when you were my professor, Nikki Osipka, Nicola Osipka, who was already famous for training dogs for the Beastmaster show. Yes.
she's amazing, and had very well-behaved dogs that she would bring everywhere with her, studied a species of mole in Tilden Park that could transdifferentiate its testes into ovaries and back again. And I thought, well, that's like alien, weird levels of stuff.
But she would occasionally go over to UCSF when babies were born that were sort of, back then they called them, no one uses this language now, pseudo hermaphrodite.
Yes.
What is the deal? with exposure to prenatal androgens and neither clearly here nor there a genitalia.
Yeah. So in most of those cases, we're talking about congenital adrenal hyperplasia, also known as CAH. And the congenital means it's present at birth. And the adrenal hyperplasia is referring to the fact that the adrenal glands are slightly larger. And the reason they're slightly larger in this case is because these are individuals where
the fetus itself is not able to make some of the adrenal steroids that are important for staying healthy. And so the brain detecting, hey, where are the adrenal steroids that we need here, drives the pituitary to tell the adrenal gland, hey, we need more steroids. The adrenal gland gets the message, it hypertrophies, but the machinery isn't there to make those steroids.
And so instead, the adrenal gland makes testosterone and other androgens. And actually this can happen in either XX or XY individuals. And in XY individuals, people might not notice. But in XX individuals, what that means is that prenatally, her genitalia are being exposed to more testosterone than is typical.
And so under the influence of this extra testosterone, the clitoris may grow to be bigger than the typical clitoris. In some cases, in extreme cases, the phallus looks like a penis. And the skin around that area that would normally form the labia
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Chapter 7: What role does prenatal testosterone play in sexual attraction?
So these individuals are identified at birth typically, especially in XX individuals. And there's an easy treatment, which is, oh, they can't make, you know, you do the test and you go, oh, they can't make adrenal steroids, so we'll give them some. And so for the rest of their lives, they take adrenal steroids orally and get the benefits of that. And that shuts off.
the hyperactive adrenal glands so that it shuts off the role of the output of testosterone. So this is what's known as an intersex phenotype. And yes, you're right. In the older literature, they were sometimes known as pseudohermaphrodite, with the idea being whether a hermaphrodite is an individual that can function and reproduce either as a male or female.
And so supposedly they were pseudo because they can't do that because they have only ovaries. Well, You can imagine, first of all, being called hermaphrodite, nobody liked that. And if you ask them, well, is it better if I put this pseudo at the beginning, does that make you feel like you're being less stigmatized? No. But a much more accurate description is to say that it's intersex.
They have a phallus that's somewhere between a clitoris and a penis. And the skin around there is sort of like a scrotum and sort of like labia. So in the old days, once this got recognized, it was standard procedure to tell the parents, oh, this is an emergency. We need to do cosmetic surgery. We need to do surgery to make this little girl look like all the other little girls.
You know, sometimes the surgery was, you know, there could sometimes be successful or not. Indeed, they knew how to make her look like other girls. But many of those intersex folks, when they grew up, were pretty angry. that someone had done this surgery on them that wasn't needed medically, right? They were already taking care of the problem with the exogenous adrenal steroids.
And so who asked you to deal with my, you know, to do surgery on my clitoris? In some cases, the tip of the clitoris was missing. And so these women grew up and were anorgasmic because they couldn't get stimulation that they normally would have had.
These days, there's much more, thanks to the activists like Cheryl Chase and others who started getting the pediatrician's attention, hey, you need to think about that. You're doing elective surgery on an infant who cannot possibly have informed consent.
And so these days, there's more of a wait-and-see attitude, which I think is absolutely... So wait till they're grown up and ask them then if they want to have surgery. And my guess is most will say no. I think that's been the pattern so far. So these are females who are exposed to more testosterone than other females.
So does that mean that they're going to be attracted to women when they grow up? And the answer is, well, interestingly, if you look at groups of women with CAH, they are more likely to be same-sex attracted, to be lesbians, than the population at large. But most of them are straight.
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Chapter 8: How do personal experiences shape understanding of sexual orientation?
and if she passes that X chromosome on to a daughter, then she's sort of duplicated herself. What's interesting is when that X chromosome is given to a son, in other words, that egg with an X chromosome that has a dysfunctional copy of the angio-receptor gene, if it gets fertilized by a Y-bearing sperm, now we have an XY individual and
as you've explained clearly in your basics podcast, we know what will happen in development. The Y chromosome will mean that the indifferent gonad will develop as testes. The testes will secrete two hormones that are going to guide sexual differentiation in the periphery, one of them being anti-malarian hormone.
which is going to suppress the development of the malaria ducts, and therefore no oviduct, no uterus will form. And the testes will also release testosterone, which normally would masculinize the body, but in this case, because there's no functional antireceptor to respond to it, the testosterone goes round and round, but the body doesn't respond. And so the wolfian ducts don't develop.
The periphery looks like a typical female. And these individuals, when they're born, often are not identified because the baby's born. The doctor does that very careful analysis by looking between the legs and says, congratulations, you have a girl. And they grow up to be girls, undetected. And they come to a doctor's attention when puberty happens.
And all their classmates are having their period. but she's not. And so she'll eventually go to an OBGYN who will first do an exam, a vaginal exam, and he'll notice that the vagina is relatively short because the inner part of the vagina is normally derived from the malaria ducts. Well, in this woman, in this teenage girl, the malaria ducts never develop because of anti-malarian hormone.
So there'll be no cervix. that can be seen in the exam, and if he takes blood plasma levels, he'll see that this very feminine looking girl, teenage girl, has very high levels of testosterone. And presumably testes.
And if he does a karyotype, he'll see that she has an XY karyotype, and yes, there are testes in there, typically in the abdomen, and they're releasing lots of testosterone because there's no negative feedback to tell the brain you know, hey, you can stop sending signals to the testes now. In these cases, you can ask, well, what's the sexual orientation of these women?
And the vast majority of them grow up to be straight. They are attracted to men. And they might be very, they're often very interested in having a family. Of course, they can't carry children themselves, but they can adopt and things like that. And so they're very much feminine, very straight women. But they're XY. But they're XYs.
So the question is, unfortunately in terms of understanding whether prenatal testosterone alters our sexual orientation, these individuals aren't useful to us because we don't know if they're straight women because their brains could never respond to the prenatal testosterone Or are they straight women because they were raised as girls and socialized to be attracted to men?
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