Gay, Straight, and the Reason Why

What causes a child to grow up gay or straight? In this book, 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 helped create this field in 1991 with a much-publicized study in Science, where he reported on a difference in the brain structure between gay and straight men. Since then, an entire scientific discipline has sprung up around the quest for a biological explanation of sexual orientation. In this book, LeVay provides a clear explanation of where the science stands today, taking 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. LeVay also reports on the prevalence of homosexual behavior among wild animals, ranging from Graylag geese to the Bonobo chimpanzee. In this revised edition LeVay broadens his horizons. He adds a new chapter on bisexuality, reviews some uncommon forms of sexuality such as asexuality and pedophilia, and considers whether there could be a biological basis for subtypes of gay people such as "butch" and "femme" lesbians. Although many details remain unresolved, the general conclusion is quite clear: A person's sexual orientation arises in large part from biological processes that are already underway before birth.

Simon LeVay, PhD, is a British-born neuroscientist who has served on the faculties of Harvard Medical School and the Salk Institute for Biological Studies. He has written eleven previous books, including the New York Times best-seller, When Science Goes Wrong (2008) and the textbook Discovering Human Sexuality (2015).

- Introduction - Chapter 1: What Is Sexual Orientation? - Criteria for sexual orientation - Sexual orientation in men and women - Stability of sexual orientation - Prevalence of different orientations - Are there categories? - Sexual orientation across cultures - Chapter 2: Why We Need Biology - Psychoanalytic theories - Learning theories-influence of early sexual experiences - Childhood abuse - Learning theories-gender learning - Is it a choice? - The biological alternative - Chapter 3: The Outline of a Theory - Male and female brains - Male and female behaviors - Development of sex differences in animals - Sexual partner preference in animals - Origins of variation within each sex - Relevance to human sexual orientation - Sexual orientation in nature - Chapter 4: Childhood - Development of gendered childhood traits - Childhood traits associated with adult sexual orientation: retrospective studies - Prospective studies - Contrasting models - Chapter 5: Characteristics of gay and straight adults - Gendered traits in adulthood - Origin of gendered traits - Sexual orientation and cognitive traits: visuospatial abilities - Verbal fluency - Memory tasks - Handedness - Olfaction - Intelligence - Personality traits: Masculinity-femininity - Occupational preferences - Other personality traits - Sexuality - Overview - Chapter 6: The role of sex hormones - Hormone levels in gay and straight adults - Why focus on prenatal sex hormones? - Hormone levels during development - Congenital adrenal hyperplasia - Finger length studies - The inner ear - Central auditory system - Action of sex hormones on the developing brain - Possible causes of variability in prenatal androgen levels - Chapter 7: The role of genes - Sibling studies - Is the family clustering caused by genes? - Twin studies - Molecular genetics-candidate-gene studies - Genome scans - Genes and sexuality in fruit flies - Genes, homosexuality, and evolution - Kin selection - The "fertile female" hypothesis - Beneficial effects on same-sex relatives - Chapter 8: The brain - A brief tour of the brain - The hypothalamus and sexual orientation - Other brain regions - Brain activity - Chemosignals - Stress response - Functional lateralization - Sheep - Inhibition and sexual orientation - Overview - Chapter 9: The body - Body size and shape - Trunk and limb length - Penis size - Sturcture of the face - Hair whorl direction - Gaydar - Overview - Chapter 10: The older-brother effect - How well established is the older-brother effect? - How strong is the older-brother effect? - The older-brother effect and handedness - What causes the older-brother effect? - Chapter 11: Beyond gay and straight - Bisexuality in men - Bisexuality in women - Asexuality - Age preference and pedophilia - Butch-femme and top-bottom - Transgender/transexuality - Chapter 12: Conclusions - Sexual orientation is linked to other gendered traits - A common origin for gender-shifted traits? - The role of genes - Does the older-brother effect work through prenatal hormones? - Is there a random biological influence? - How does sexual orientation become categorical? - Unconscious processes in sexual attraction and arousal - Changes in the prevalence and nature of homosexuality - What is sexual orientation? - Sexual orientation and gender: the social fallout - Glossary - Bibliography