How Hormones Shape Sexual Orientation & Behavior | Dr. Marc Breedlove
In a Nutshell
Prenatal testosterone exposure shapes sexual orientation, evidenced by masculine finger length ratios (2D:4D) and otoacoustic emissions in lesbians, equivalent ratios in gay men, and conditions like CAH increasing lesbian likelihood in women. The fraternal birth order effect raises gay probability in males with more older brothers (2% baseline to 50/50 after ~12) via maternal antibodies to Y-chromosome proteins, not social factors. Animal models (gay rams) and brain differences (smaller INAH-3 in gay men) confirm biological mechanisms, with group trends but no individual predictions; adult brain plasticity to androgens exists but orientation forms early.
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The larger the number of older brothers that a male has, the higher the probability that he is gay. This has been seen over and over and is one of the rock solid findings in human sexuality. For a baby boy born today with no older brothers, odds of being gay when he grows up are about 2%. With one older brother, odds go up by a third to 2.6%. With two older brothers, up a third again to 3.5%. It takes about a dozen older brothers to have a 50/50 chance.
Welcome to the Huberman Lab podcast, where we discuss science and science-based tools for everyday life. Andrew Huberman is a professor of neurobiology and ophthalmology at Stanford School of Medicine. Guest is Dr. Marc Breedlove, professor of neuroscience at Michigan State University, expert in how hormones shape the developing brain, particularly influencing sexual orientation. Prenatal testosterone exposure impacts finger length ratios and plays a role in sexual orientation, with correlations confirmed many times in humans and animals.
Every time a woman is pregnant with a male, a biological trace biases the likelihood that her next male offspring will be heterosexual or homosexual. These are solid biological findings with understood mechanisms in animals and humans. Prenatal hormones shape attraction to one sex and aversion to the other, forming specific circuits. Hormones also impact rough-and-tumble play and social play in kids, and the interplay of nature and nurture in male-female differences and sexual orientation.
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