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

KEYPOINTS

• 01

  Finger-length ratio is a window onto prenatal testosterone

  The 2D:4D ratio (index finger length divided by ring finger length) is smaller on average in males and is already present in nine-year-olds, before puberty. Breedlove, a Michigan State neuroscientist, says that points to prenatal testosterone. In mice the difference disappears when the androgen receptor is knocked out, and the growing bones of the fourth digit carry more androgen receptor than those of the second.

• 02

  Lesbians average a more masculine digit ratio; gay and straight men don't differ

  In Breedlove's original Bay Area study (he measured every digit himself, twice), lesbians had more masculine ratios than straight women, a finding confirmed across labs and now a third meta-analysis. Gay and straight men showed no difference. He reads that as suggesting prenatal testosterone nudges attraction toward women, and that gay men probably received similar testosterone but their brains responded to it differently.

• 03

  Gay men are not 'under-masculinized' where the big sex differences lie

  The largest human sex differences are in sexual attitudes — interest in multiple and younger partners, casual sex, visual pornography — not math or verbal skills. On all of these, Breedlove says, gay men are 'totally masculine', which undercuts the old stereotype of low androgen exposure. If anything, testosterone measurements pointed the other way.

• 04

  Group differences offer almost no individual prediction

  Height differs between the sexes by about two standard deviations: guess 'male' above 5'5½" and you're right only about 80% of the time. The digit-ratio difference is about half a standard deviation, a quarter of that. His joke: to guess orientation from a hand and be right about 95% of the time, always guess 'straight'. Your own ratio tells you nothing about your prenatal hormones.

• 05

  The 'fraternal birth order' effect is one of sexuality's most robust findings

  First noticed by Ray Blanchard and replicated worldwide: a boy with no older brothers has about a 2% chance of being gay. Each older brother from the same mother raises the odds by about a third (2.6%, then 3.5%), linearly, so it would take roughly a dozen to reach 50/50. Gay men had about 140 older brothers per 100 older sisters, versus the usual 105:100.

• 06

  The effect is biological, via the mother

  Anthony Bogaert found that older stepbrothers have no effect, while biological older brothers raised apart count just as much, ruling out rough play or bullying. In the maternal immunization hypothesis, the mother's immune system sees Y-linked proteins at each son's birth and makes antibodies that cross the placenta. Mothers of gay younger sons had higher antibodies to neuroligin 4Y, a synapse-building protein. Blanchard estimates about one in seven gay men owe their orientation to this effect.

• 07

  The hypothalamus keeps turning up

  Simon LeVay found a preoptic-area nucleus (INAH3), larger in men than women, that was smaller in gay men — comparable to women's. He stressed that adult tissue can't establish cause, since adult brains change: Brad Cook showed a medial amygdala sex difference vanishes within weeks of removing male testosterone in animals. Chuck Roselli studied the minority of rams that never mount ewes, and their preoptic area processes testosterone differently.

• 08

  Partner choice may involve an aversion pathway, not just attraction

  Gay rams don't merely prefer males; they never once mount a female, which Breedlove can only explain as aversion. He and Huberman suggest that people's aversion to same-sex sex (or opposite-sex sex) may have a biological component as well as social conditioning, though context matters (for example, same-sex sex in prisons). Huberman argues that public trust in science depends on discussing this openly.

• 09

  There is no single cause

  Breedlove emphasizes many developmental pathways to being gay or straight: prenatal testosterone appears to have a say for women, the mother's body for some men, and other factors remain unknown. He stresses that orientation is not a choice, and that a first crush typically arrives about six years after birth, long after these prenatal influences.