Human Homosexuality, Transsexuality and Evolution: A Critical Appraisal

Joan Roughgarden

Homosexual behavior occurs naturally in many species of mammals. Among primates, homosexuality is an evolutionary innovation originating when the anthropoid lineage split from the prosimian linage, becoming prominent in socially complex old world primates. Many species possess multiple genders: multiple morphs within each sex. Homosexual behavior and transgender expression occur across all present-day human cultures, throughout recorded history, including the Bible, and in Paleolithic drawings and artifacts. A transgender "third sex" has official status in India and its neighbors. Homosexuality became a Western category of personal identity in the mid-1800s. Despite 30 years of research, a gay gene has not been located nor have the brains of homosexual men been shown to resemble the brains of women. The brains of transgender people resemble the brains of the gender they identify with, not with the brains corresponding to their biological sex; this reflects the timing of sex hormones at different stages during fetal growth. Gender identity may exist in the brain as a "cognitive lens" that controls whom to focus on as a developmental "tutor." Transgender identity is the acceptance of a tutor from the opposite sex. Gender identity is developed at birth and sexual orientation develops later during the first few years of infancy. Homosexuality and transgender are too common to be considered genetic diseases. The fertility cost of male homosexuality is as much as 50% in Western culture and often much less elsewhere. The fertility cost of homosexual behavior is not convincingly offset by a fecundity advantage to the mothers of homosexual children. Homosexual behavior is not convincingly selectively neutral. Transgender expression might have evolved through kin selection in some cultures with a third sex. Homosexual behavior is selectively advantageous by producing alliances and partnerships and by promoting cooperation through the reciprocal sharing of pleasure. The homosexual/heterosexual polymorphism may result from frequency-dependent selection between alternative same-sex alliance strategies, with homophobia emerging as a byproduct of competition between these strategies.