Sexual orientation is set before birth. Dr. Marc Breedlove argues prenatal testosterone exposure builds the brain circuits for attraction.

The most predictable factor in male sexuality is the number of older brothers. Each one increases the odds of being gay by about 33%. Breedlove calls this the fraternal birth order effect, a purely biological bias left in the mother's body after each male pregnancy.

Physical signs confirm this hormonal blueprint. The 2D:4D finger length ratio - comparing index and ring finger - shows a person's exposure. Men typically have shorter index fingers, a sign of high testosterone.

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.

Lesbians often have more masculinized finger ratios than straight women. This pattern matches inner ear sounds. Straight women produce more than men, but lesbians produce fewer, aligning with a male-like pattern.

Breedlove once thought attraction was learned, like language. The physical evidence - from fingers and ears - changed his mind. Biology writes the script long before a child speaks.