Dr. Marc Breedlove — A professor of neuroscience at Michigan State University and a long-standing pioneer in how hormones shape the developing brain. He authored leading textbooks on hormones and behavior and published the landmark 2000 study linking finger-length ratios to sexual orientation.
The gist
Andrew Huberman and Dr. Marc Breedlove discuss the biology of sexual orientation through the lens of hormones, genetics, and statistics rather than politics. They explore how prenatal testosterone exposure correlates with finger-length (2D:4D) ratios and sexual orientation, how brain regions like the preoptic area differ between gay and straight individuals (including 'gay rams'), and the fraternal birth-order effect in which each older brother raises a male's odds of being gay. Breedlove explains intersex conditions (CAH and androgen insensitivity syndrome), the difference between attraction and aversion in partner choice, and why group statistics carry no predictive power for any one individual. The episode closes with Breedlove's personal story of going from a working-class Ozarks upbringing to Yale and a career in neuroscience.
Big reveals
- Each older brother (from the same mother) raises a male's odds of being gay by about a third; you'd need roughly a dozen for a 50/50 chance.
- An oddball oto-acoustic-emissions study first convinced Breedlove that prenatal testosterone might influence sexual orientation.
- Breedlove argues gay men got the same prenatal testosterone as straight men, but their brains responded differently.
- Simon LeVay's study found the preoptic-area nucleus (INAH-3) is smaller in gay men, similar in size to women's.
- 'Gay rams' will mount only other males and never a female even over 12 hours, suggesting a built-in aversion, not just preference.
- Huberman proposes an aversive 'anti-same-sex' circuit in males that may be weaker or absent in females.
- Older brothers raised apart still raise the odds of being gay, while step-brothers don't, ruling out social causes.
- The maternal immunization hypothesis: a mother's immune system makes antibodies (to neuroligin-4Y) against male antigens, perturbing later sons' brain development.
Things worth remembering
- The 2D:4D ratio (index finger / ring finger length) differs by sex, is present in children, and is more pronounced on the right hand.
- Lesbians on average have more masculine digit ratios than straight women, replicated across many labs and meta-analyses.
- The sex difference in height is about two standard deviations; the digit-ratio sex difference is only about half a standard deviation.
- Winning a competition or having your candidate win an election can raise testosterone; losing lowers it.
- A company harvests and sells wool clothing from rescued 'gay rams' instead of sending them to slaughter and it sold out.
- About 105 boys are born for every 100 girls; gay men in surveys had ~140 older brothers per 100 older sisters.
- Blanchard estimates about 1 in 7 gay men are gay due to the older-brother (maternal) effect.
- Carrier status for congenital adrenal hyperplasia (one mutant copy) is roughly 1 in 12 and usually symptomless.
- Julian Davidson's double-blind studies showed men almost always knew when they were getting testosterone because they felt so much better.
- Even monkeys show toy preferences: male monkeys gravitate to wheeled toys, females to dolls (Melissa Hines).
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Untitled book on the biology of sexual orientation
Marc Breedlove
“you're writing a book about the biology of sexual orientation. ... when can we expect that book to hit the shelves?” — Andrew Huberman 02:08:02Find it on Amazon
Textbooks on hormones and behavior / developmental neurobiology
Marc Breedlove
“Mark has um authored some of the most important textbooks on hormones and behavior, developmental neurobiology. He's a true scholar of the whole field.” — Andrew Huberman 00:45:35Find it on Amazon