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What causes a child to grow up gay or straight? Neuroscientist Simon LeVay summarizes a wealth of scientific evidence that points to one inescapable conclusion: Sexual orientation results primarily from an interaction between genes, sex hormones, and the cells of the developing body and brain. LeVay takes the reader on a whirlwind tour of laboratories that specialize in genetics, endocrinology, neuroscience, cognitive psychology, evolutionary psychology, and family demographics. He describes, for instance, how researchers have manipulated the sex hormone levels of animals during development, causing them to mate preferentially with animals of their own gender. In this second edition, LeVay adds a chapter on bisexuality, reviews some uncommon forms of sexuality, and considers whether there could be a biological basis for subtypes of gay people such as "butch" and "femme" lesbians.