Prenatal testosterone shapes sexual orientation through measurable biological changes like finger-length ratios, while the number of older brothers a man has increases his likelihood of being gay—findings that reveal how nature and nurture interact to form romantic partner choice and challenge long-held stereotypes about sexual development. Lesbians show more masculine finger-length ratios (2D:4D) than heterosexual women on average, suggesting exposure to higher prenatal testosterone, a pattern replicated across multiple independent labs. Each older biological brother increases a male's probability of being gay by approximately one-third per sibling, yet only one in seven gay men are gay due to this maternal immunization effect.