The strongest predictor of whether a man will be gay is the number of older brothers he has. On Huberman Lab, Dr. Marc Breedlove detailed the fraternal birth order effect: each older brother increases the next son’s likelihood by roughly 33%, climbing from a 2% baseline. Breedlove calls this one of the most solid findings in sexuality research.

The effect has nothing to do with family dynamics or upbringing. It’s a purely biological consequence, where each male pregnancy leaves a trace in the mother that influences the hormone environment for the next male fetus.

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.

Physical biomarkers confirm the role of prenatal hormones. Andrew Huberman pointed to the 2D:4D finger length ratio - the comparison of index to ring finger. Higher testosterone in the womb leads to a more masculine ratio (shorter index finger). This pattern shows up in lesbians, who tend to have more masculinized finger ratios than heterosexual women.

Another marker comes from the inner ear. Straight women produce more spontaneous otoacoustic emissions than men, but lesbians produce fewer, again aligning with a male-typical pattern. Breedlove said this physical evidence changed his mind; he once thought orientation was learned like language. Now, he sees biology writing the script long before birth.