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

We open with a jolt: the more older brothers a boy has, the higher his odds of being gay. Welcome to the show—today we’re sticking to biology and statistics, not politics—and we’re diving into prenatal hormones, finger-length ratios, brain circuits, and yes, even gay rams.

That older-brother pattern has shown up again and again. If a newborn boy has no older brothers, his chance of being gay later is around two percent; each older brother nudges that up, roughly linearly, though you’d need a lot of brothers to reach coin-flip odds.

Great to see you after decades—blond highlights and all—and let’s jump straight to the deep end: that finger-length ratio work and what it really says about hormones and orientation.

First, orientation isn’t a choice; most of us felt our first crush before puberty—I sure did—and we didn’t decide it. I once thought social learning could explain most of it, but a study on tiny sounds from the inner ear shifted me: lesbians had fewer of these emissions than straight women, which fits greater prenatal androgen exposure.

For listeners glancing at their hands, here’s the gist: men tend to have a shorter index finger relative to the ring finger, especially on the right hand, while women’s are closer in length; we’re talking averages, not a parlor test.

We tested this at Bay Area street fairs—anonymous surveys, a dollar scratcher for thanks, and photocopied hands—and found no meaningful digit-ratio difference between gay and straight men, but lesbians showed a slightly more male-typical pattern. That points to group-level prenatal androgen effects without allowing you to guess any one person’s history.

Why no clear digit-ratio difference in men? Gay men are very male-typical on big sex differences like interest in multiple and younger partners and visual erotica, so the key may be how the brain responds to testosterone, not how much it got. In mice, the finger-ratio sex difference vanishes when androgen receptors are knocked out, which nails a mechanism.

What about the brain itself—any reliable differences linked to orientation?

Simon LeVay reported a tiny hypothalamic nucleus smaller in gay men than in straight men, with replication later; AIDS status didn’t explain it. We still can’t say if that difference causes orientation or reflects adult plasticity, because brain structures can change with hormones and life.

Hormones and behavior run in a loop: competition winners show higher testosterone after, losers lower; even election outcomes budge levels. In animals, female odors spike male testosterone, and the hypothalamus, while less flexible than cortex, stays more plastic across life than many assumed.

Rats taught me the limits of animal models for orientation: a seasoned male will try to mount any rat, and a neonatally castrated male given female hormones will display classic receptive posture. That’s motor pattern control, not partner sex preference; humans care far more about who the partner is.

Sheep are a rare exception: a small fraction of rams consistently refuse females and mate only with males, even when fertile ewes are right there for hours. They also show preoptic-area differences in steroid signaling—strikingly, the same brain region that differs in humans.

Here’s a hypothesis to ground what we’re seeing: in men, a neural pathway may actively suppress desire for male partners—an aversive signal—while women show more flexibility with less of that brake. It would explain sex differences in openness without invoking stereotypes.

That fits the data: culture can amplify or soften these tendencies, but biology likely sets the baseline, especially for men.

Let’s touch intersex conditions that illuminate development: congenital adrenal hyperplasia and androgen insensitivity.

In CAH, XX babies get extra androgens from enlarged adrenals, leading to more male-typical genital development; medical care replaces missing adrenal steroids, and elective genital surgeries are now often delayed until the individual can consent. As adults, CAH women report same-sex attraction more than average yet are mostly straight, and the proportion reporting it tends to rise with age—consistent with prenatal androgens nudging orientation without determining it.

In complete androgen insensitivity, XY babies can’t respond to testosterone, so they develop as girls, look very feminine, and typically grow up attracted to men. It’s a fascinating case, but it can’t tell us whether prenatal androgens would have mattered, because there’s no working receptor and strong female socialization.

Back to the older-brother effect: it only shows up with brothers from the same mother and even when siblings were raised apart, while stepbrothers don’t count. The best explanation is a maternal immune response—antibodies that build after carrying sons and cross into later male fetuses, with evidence pointing to antibodies against a Y-linked synapse-related protein called neuroligin 4Y—subtly shifting brain development and increasing the odds of a gay orientation without predicting any one person.

That flips a lot of folk theories—older sisters or a missing dad don’t raise the odds; it’s older brothers from the same mom that matter.

Exactly—older or younger sisters don’t change the likelihood, and neither do younger brothers, which points squarely to something the mother’s body remembers about previous sons.

Quick note on carriers: single-mutation CAH is common and often silent, and may even track with endurance-stress careers; classic heterozygote advantage logic applies.

Right—there are multiple enzymatic steps, carriers usually have no symptoms, and in some contexts variants can help, much like sickle-cell trait protects against malaria.

Before we close this chapter, your wild finch idea turned into a bird lesson: mosaic male–female brains do occur in birds, where local brain hormones can masculinize one side and not the other, while in mammals gonadal hormones dominate, so that kind of split doesn’t happen.

Those rare half-male, half-female birds show larger song-control nuclei on the male side, which underscores that birds can self-differentiate brain regions with local steroid production—very different from mammalian development.

In prep for today, I poked around bodybuilding forums I wish I’d skipped, where people abusing high-dose synthetic androgens describe distinct shifts in mood and libido—trenbolone especially gets tied to aggression and a burst of desire that can flip targets during a hypersexual stretch. I’m not equating that with medically guided hormone therapy, but the pattern suggests the adult hypothalamus still reacts powerfully to androgens.

Back in the day at Stanford, Julian Davidson ran double-blind trials in men who’d lost their testes, and testosterone reliably lifted energy, libido, and overall well-being so clearly that blinding barely held. Even older men responded, which tells us the adult brain is still very sensitive to testosterone.

Testosterone does tend to drop with age, but the scatter is huge—some folks in their seventies look like people in their twenties. I once met a CEO with levels in the low three hundreds who felt great across vigor, libido, and mood, which reminds me the number isn’t the whole story and more could even backfire via extra conversion to estrogen.

Agreed—no need to fix what already feels right. Men vary widely, and the decline is gentle compared to menopause, so age alone doesn’t let me predict anyone’s levels.

There’s that Dagestan meme pitching hard combat training as a way to ‘make’ boys masculine, which echoes the pediatric advice I heard as a kid that theater might ‘turn’ a boy gay. Given brain plasticity and the two-way link between behavior and hormones, how much can activities shape orientation versus just behavior?

The young brain is more malleable, but decades of searching never found strong social predictors for sexual orientation. Culture and family clearly mold many gendered behaviors, yet one sturdy difference—rough-and-tumble play—shows up across species, and mixed groups land in the middle because girls dial boys down and boys dial girls up.

Let’s shift to your path—you took an unusual route into science; where did you start?

I grew up working-class in the Ozarks, loved books, and got lucky—like finding a college-board guide that promised to meet full need at Yale and a teacher who linked me to my alumni interviewer. I arrived feeling out of place among prep-school roommates and easygoing views on marijuana while back home a kid got life for possession, so I buried myself in learning, overloaded on classes and labs, and discovered comparative psychology and then neuroscience, which hooked me.

I’ve always admired how you chase curiosity without playing the career game; luck tends to find people who are ready. Did you notice early sex differences with your own kids?

They were striking—my daughter craved dresses and clomped around in her mom’s shoes even though her scientist mom rarely wore them, while one son could turn anything into a toy gun and was drawn to wheels. I’m no hard determinist, but nature shows up early, and even in monkeys males lean toward wheeled toys, as Melissa Hines reported.

You’re also writing a book on the biology of sexual orientation; when might it land?

I’m six chapters into an eleven-chapter plan and aiming to finish a first draft by fall, then it goes through review. And yes, you owe me a purchased copy.

Deal—I’ll read it and share thoughts; thanks for today and let’s do this again.

This was a joy—thanks.

Thanks for listening; links to Dr. Breedlove’s work are in the show notes, and the best no-cost support is subscribing on YouTube and following on Spotify and Apple, where you can leave up to a five-star review and comments. Supporting the sponsors helps the show, and if you have questions or topic ideas, drop them in the YouTube comments because I do read them. Thanks again for joining us—and thanks, as always, for your interest in science.