What causes a child to grow up gay or straight or bisexual? Traditional explanations, such as those put forward by Sigmund Freud, have centered on parent-child relationships, on various forms of "training," or on early sexual experiences. In this book, neuroscientist Simon LeVay summarizes a wealth of evidence that points in a radically different direction: A person's sexual orientation results primarily from an interaction between genes, sex hormones, and the developing brain. The same biological processes that influence a broad array of gendered traits are also the guiding factors behind a person's emerging sexuality. LeVay helped initiate this field of research with a much-publicized 1991 study in which he reported on a difference in brain structure between gay and straight men. Since that time, an entire scientific discipline has sprung up around the quest for a biological explanation for sexual orientation. In this book, LeVay takes the reader on a whirlwind tour of laboratories that specialize in genetics, endocrinology, neuroscience, cognitive psychology, evolutionary psychology, and family demographics. Although many details remain unresolved, the general conclusion is inescapable: a person's sexual orientation is a central aspect of his or her identity--one that arises in large part from biological processes operating before birth. And far from seeking to discover "what went wrong" in the lives of gay people, or attempting to develop "cures" for homosexuality, today's scientists see sexual orientation as an aspect of human diversity-something that is worth studying simply because it is part of what makes us human.