How Hormones Shape Sexual Orientation & Behavior | Dr. Marc Breedlove

**Andrew Huberman** (0:00)
The larger the number of older brothers that a male has, the higher the probability that he is gay.

**Marc Breedlove** (0:07)
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.

**Andrew Huberman** (0:43)
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. Marc Breedlove. Dr. Marc 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. 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 long-standing 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. Marc Breedlove. Dr. Marc Breedlove, welcome.

**Marc Breedlove** (3:26)
Thank you, I'm delighted to be here. Very exciting.

**Andrew Huberman** (3:29)
Been 25 years since we stood in the same physical space.

**Marc Breedlove** (3:32)
I know, how can that be possible? I feel like, you know, just saw you a few days ago, but then, well, you look great, you look the same, so we can talk longevity protocols in the end, but. I'm trying to have a blonde look in my hair, apparently, so.

**Andrew Huberman** (3:46)
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. All right. 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.