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

What It Covers

Stanford neuroscientist Andrew Huberman interviews Michigan State neuroscience professor Marc Breedlove on how prenatal testosterone exposure shapes sexual orientation in humans and animals. They cover digit ratio research, the fraternal birth order effect, congenital adrenal hyperplasia, androgen insensitivity syndrome, gay rams, and the biological evidence that both attraction and aversion to specific sexes have distinct neural substrates.

Key Questions Answered

• •Fraternal Birth Order Effect: Each additional older biological brother increases a male's probability of being gay by roughly one-third. Starting at a 2% baseline with no older brothers, one older brother raises odds to 2.6%, two to 3.5%. Approximately one in seven gay men are gay specifically because their mother previously carried sons, according to Ray Blanchard's statistical modeling. Older stepbrothers or sisters produce no equivalent shift, confirming the effect is biological, not social.
• •Maternal Immunization Hypothesis: When a mother delivers a son, her immune system encounters Y-chromosome-specific proteins for the first time and generates antibodies. With each subsequent male pregnancy, antibody levels rise and cross the placenta. Researchers identified elevated maternal antibodies to neuroligin-4Y, a synapse-formation protein encoded on the Y chromosome, in mothers whose later-born sons showed higher rates of same-sex orientation. This mechanism explains why the effect tracks biological motherhood, not shared upbringing.
• •Digit Ratio as Prenatal Testosterone Marker: The ratio of index finger length to ring finger length (2D:4D) reflects prenatal androgen exposure. Men average a lower ratio than women, driven by greater androgen receptor density in fourth-digit bone tissue. Lesbians show a more masculinized 2D:4D ratio than heterosexual women across multiple independent replications and meta-analyses. Gay men show no significant digit ratio difference from straight men, suggesting equivalent prenatal testosterone but differential brain response to it.
• •Aversive Sexual Circuitry: Sexual orientation involves not only attraction circuits but also aversion circuits. Gay rams placed with a dozen receptive ewes for twelve hours never mount a single female, behavior unexplainable by simple lack of drive. Chuck Roselli's dissections found preoptic area differences between gay and straight rams matching Simon LeVay's findings in human brains. This dual-pathway model — desire for one sex plus aversion to the other — may explain why male human heterosexuality often includes active resistance to same-sex scenarios.
• •Congenital Adrenal Hyperplasia (CAH) and Orientation: CAH causes fetal adrenal glands to overproduce testosterone due to a steroid synthesis enzyme deficiency. XX individuals with CAH are born with masculinized genitalia and show elevated rates of same-sex attraction compared to the general female population. Notably, the percentage reporting lesbian orientation increases with age in longitudinal surveys, suggesting some initially follow heterosexual social scripts before later self-identifying differently. Heterozygous CAH carriers occur at roughly 1-in-12 frequency with minimal phenotypic expression.
• •Preoptic Area and Sexual Orientation: Simon LeVay identified a hypothalamic nucleus (INAH-3) that is larger in men than women and significantly smaller in gay men, approximating female-typical size. An independent replication by William Byne, who began skeptical of the finding, confirmed the result. The chicken-and-egg causality problem remains unresolved — whether a smaller INAH-3 produces same-sex orientation or same-sex orientation reshapes INAH-3 — because the nucleus can only be measured postmortem and adult hypothalamic nuclei retain measurable plasticity.
• •Female vs. Male Orientation Plasticity: Women show greater fluidity in reported sexual orientation across the lifespan than men. Heterosexual women more readily consider same-sex scenarios, and cases of women identifying as straight then later as lesbian are documented more frequently than the reverse male pattern. One proposed mechanism is that males possess a dedicated aversion circuit suppressing same-sex attraction that females lack or express less robustly. Cultural amplification or suppression of these biological tendencies likely modulates expression without eliminating the underlying substrate.