Sexual orientation is a biological fact established in the womb, not a social habit learned in childhood. On Huberman Lab, Dr. Marc Breedlove argues that the circuits for romantic attraction are wired by prenatal testosterone levels.

The most predictive factor for male sexual orientation is the number of older brothers. Each older brother increases the probability a man is gay by approximately 33%. A firstborn son has about a 2% chance, but the odds climb steadily with every subsequent male pregnancy. Breedlove calls this the fraternal birth order effect.

Marc Breedlove, Huberman Lab:

- 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.

This effect is a purely biological process, not a product of upbringing. Each male pregnancy leaves an imprint in the mother's body that influences the development of the next male fetus.

Physical biomarkers confirm the prenatal hormonal influence. Andrew Huberman highlights the 2D:4D finger length ratio - the length of the index finger relative to the ring finger. Higher prenatal testosterone typically leads to a more masculine ratio (shorter index finger). Research shows lesbians often have more masculinized finger ratios than heterosexual women.

A similar pattern appears in otoacoustic emissions, the faint sounds produced by the inner ear. Straight women produce more of these sounds than men do, but lesbians produce fewer, following a male-typical pattern.

Breedlove once believed social learning explained orientation, much like language acquisition. The cumulative evidence from fingers, ears, and birth order changed his mind. Biology writes the script long before a child learns to speak.