How Hormones Shape Sexual Orientation & Behavior | Dr. Marc Breedlove
Dr. Marc Breedlove discusses how hormones, particularly prenatal testosterone, shape sexual orientation and behavior. He presents compelling research on digit ratio correlations with sexual orientation, the fraternal birth order effect, and biological mechanisms underlying sexual preference development.
Summary
Dr. Marc Breedlove, a neuroscience professor at Michigan State University, explores the biological foundations of sexual orientation and behavior. The conversation begins with his groundbreaking finger length ratio study, which found that lesbians on average have more masculine 2D:4D ratios than straight women, suggesting greater prenatal testosterone exposure. This finding, along with similar patterns in otoacoustic emissions research, challenged the prevailing view in 2000 that sexual orientation was purely a lifestyle choice.
A major focus is the fraternal birth order effect - a robust finding showing that each older brother increases a male's probability of being gay by about one-third. This effect appears to be biological rather than social, as it occurs even when brothers are raised apart but doesn't affect step-brothers. The leading explanation is the maternal immunization hypothesis, where mothers develop antibodies against male-specific antigens with each son, potentially affecting subsequent male offspring's brain development.
Breedlove discusses brain differences, particularly Simon LeVay's research on the sexually dimorphic nucleus of the preoptic area, which is smaller in gay men compared to straight men. He also covers intersex conditions like congenital adrenal hyperplasia and androgen insensitivity syndrome, which provide natural experiments in hormonal influences on development.
The discussion includes fascinating examples from animal research, including 'gay rams' that show exclusive same-sex mounting behavior and have different testosterone processing in their brains. Breedlove emphasizes that while these biological correlates are statistically significant across groups, they have no predictive value for individuals. He also notes that there appear to be both appetitive (attraction to one sex) and aversive (rejection of the other sex) components to sexual orientation, with potential sex differences in these pathways.
About this episode
Dr. Marc Breedlove, PhD, is a professor of neuroscience at Michigan State University and an expert on how hormones shape brain development and sexual orientation. We discuss how prenatal testosterone impacts whether someone is romantically attracted to men or women later in life, and what correlates of sexual orientation — such as finger-length ratios — tell us about the role of hormones in brain and psychological development. We also discuss why the number of older brothers a male has biases sexual orientation. Throughout, we explain how nature and nurture interact to shape male-female differences, behavior, and romantic partner choice. Show notes: https://go.hubermanlab.com/iJe27oR Thank you to our sponsors AG1: https://drinkag1.com/huberman David: https://davidprotein.com/huberman Rorra: https://rorra.com/huberman Function: https://functionhealth.com/huberman Huberman Lab Instagram: https://www.instagram.com/hubermanlab Threads: https://www.threads.net/@hubermanlab X: https://x.com/hubermanlab Facebook: https://www.facebook.com/hubermanlab TikTok: https://www.tiktok.com/@hubermanlab LinkedIn: https://www.linkedin.com/in/andrew-huberman Website: https://www.hubermanlab.com Newsletter: https://www.hubermanlab.com/newsletter Dr. Marc Breedlove Website: https://marcbreedlove.com Curriculum vitae: https://docs.google.com/document/d/1mj4jJNGNUTtmiGkY9gMxRftfr4MovUD959Qi1Y-3z8Y/edit?tab=t.0 Books: https://marcbreedlove.com/books Publications: https://pubmed.ncbi.nlm.nih.gov/?term=breedlove+sm&sort=date TED talk: https://www.youtube.com/watch?v=3HX-A89XI3I Whom You Love lectures: https://www.youtube.com/user/WhomYouLove2012 YouTube: https://www.youtube.com/@Dr.Breedlove/playlists LinkedIn: https://www.linkedin.com/in/marcbreedlove Timestamps 00:00:00 Marc Breedlove 00:03:24 Hormones & Sexual Orientation 00:07:37 Prenatal Testosterone, Finger Ratio, Men & Women Differences 00:14:08 Sponsors: David & Rorra 00:16:46 Finger Ratios, Prenatal Testosterone, Gay & Straight Men/Women 00:23:57 Mice & Sex Differences, Androgens 00:26:54 Brain Differences & Sexual Orientation 00:33:52 Group vs Individual Differences, Height Analogy; Bisexuality 00:36:57 Brain Development, Hormones & Behavior; Brain Plasticity 00:42:52 Sponsor: AG1 00:44:16 Sexual Behavior, Libido 00:51:37 Gay Rams, Brain Differences 00:58:00 Aversion Pathway, Men vs Women, Same-Sex Partner 01:06:58 Congenital Adrenal Hyperplasia (CAH), Intersex Phenotypes 01:13:55 Androgen Insensitivity Syndrome (AIS) 01:18:14 Sponsor: Function 01:19:25 Gay Men & Older Brothers, Maternal Immunization Hypothesis 01:32:55 CAH Carriers, Advantage, Stress Tolerance 01:35:41 Birds & Sexual Differentiation, Gynandromorphs 01:41:32 Anabolic Steroids, Hypersexuality; Adult Brain Plasticity 01:45:31 Age & Testosterone Decline; Sexual Orientation & Activities 01:53:14 Marc’s Academic Journey, Ozarks, Luck 02:02:35 Exploration; Kids & Sex Differences 02:08:47 Zero-Cost Support, YouTube, Spotify & Apple Follow, Reviews & Feedback, Sponsors, Protocols Book, Social Media, Neural Network Newsletter #HubermanLab #Science #Hormones Disclaimer & Disclosures: https://www.hubermanlab.com/disclaimer
Key Insights
- Each older brother increases a male's probability of being gay by about one-third, from 2% with no older brothers to 2.6% with one older brother
- The fraternal birth order effect is biological rather than social, as it occurs even when brothers are raised apart but doesn't affect step-brothers
- Lesbians have more masculine digit ratios (2D:4D) than straight women on average, suggesting greater prenatal testosterone exposure
- The maternal immunization hypothesis suggests mothers develop antibodies against male-specific antigens with each son, potentially affecting subsequent male offspring's brain development
- Simon LeVay found that the sexually dimorphic nucleus of the preoptic area is smaller in gay men compared to straight men
- Gay rams show exclusive same-sex mounting behavior and have different testosterone processing in their preoptic area compared to straight rams
- Sexual orientation appears to involve both appetitive (attraction to one sex) and aversive (rejection of the other sex) components
- Women show more plasticity in sexual orientation than men throughout their lives
- The human brain continues growing at fetal rates until 6-10 years of age, making early development crucial for hormone effects
- In congenital adrenal hyperplasia, women with the condition are more likely to be lesbians, and this percentage increases as they age
- Androgen insensitivity syndrome individuals are XY but appear completely female and are typically attracted to men
- There's no evidence that social factors like absent fathers or theater participation significantly influence sexual orientation
- Testosterone in adult men definitively affects libido, energy, and overall well-being, as shown in double-blind placebo-controlled studies
- Statistical group differences in sexual orientation research have no predictive value for individuals, as most people with any given biological marker are straight
- Rough and tumble play differences between boys and girls persist across cultures and species, likely representing one of the most robust biological sex differences
Topics
Transcript
[0:00] 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 it's really one of the u rock solid findings in human sexuality. So, the way to emphasize the difference is if a baby boy is born today, um if 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 [0:30] 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…
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