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

The conversation opens with the older-brother effect: Breedlove describes a repeated population-level finding that a male’s probability of later identifying as gay rises with the number of older biological brothers. He illustrates the scale carefully: a boy with no older brothers is described as having roughly a 2% probability, one older brother raises that estimate to about 2.6%, and two raises it again to about 3.5%; a dozen older brothers would be needed to approach even odds. The figures describe group averages, not an individual prediction.

Breedlove rejects the idea that sexual orientation is a voluntary lifestyle choice. He recalls experiencing an intense crush on Marilyn Monroe before puberty, and Huberman recalls a similar early experience. Breedlove uses these memories to distinguish an early-emerging attraction from later social labels, while also acknowledging that early hormone effects in animals do not by themselves establish a single cause of human behavior. He emphasizes that humans remain highly responsive to social learning, yet social influence need not be consciously chosen or recognized.

A key line of evidence came from studies of otoacoustic emissions, the faint sounds produced by the inner ear. Breedlove explains that girls produce more of these emissions on average and that the difference is present at birth. Dennis McFadden compared straight men and gay men, and straight women and lesbians; lesbians reportedly had fewer emissions than straight women. Breedlove says the result led him to consider whether greater prenatal testosterone exposure could be associated with later attraction to women, while presenting the finding as an inference rather than a complete explanation.

The discussion then turns to the 2D:4D ratio, calculated by dividing the length of the index finger by the length of the ring finger. Breedlove describes reports that the ratio is lower in males than females and that the difference is already present in children before puberty. Since he considers a prepubertal bodily sex difference likely to reflect prenatal testosterone, the ratio became another possible correlate of prenatal hormone exposure and sexual orientation. The speakers repeatedly frame these as differences between groups, not a diagnostic test for any person.

The broader argument is deliberately probabilistic. Breedlove describes hormones, genes, development, and social experience as interacting influences rather than a single determinant. The discussion treats prenatal testosterone as a possible contributor to later attraction while separating that question from the claim that people choose their orientation. The examples establish why early biological correlates can matter scientifically, but they do not justify assigning an orientation to an individual from a finger ratio, an ear measurement, family history, or any other single feature.