What causes a child to grow up gay, straight, or bisexual? 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.

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

"Although the book's chief appeal probably will be to professionals dealing with these issues, other interested readers will find it an informative and generally approachable read." --Publishers Weekly "Simon LeVay, a former Harvard neuroscientist, has written, Gay, Straight, and the Reason Why: The Science of Sexual Orientation, a comprehensive, engaging and occasionally quite funny look at the current state of the research on the topic." --Schuyler Velasco, Salon "This book will serve both as a resource for researchers looking for what is yet unknown and what questions need further research and as a fascinating read for the educated layperson, who will be intrigued by some of the factors that may relate to homosexuality... Recommended." --Choice "LeVay also does a nice job illustrating that the argument for sexual orientation as determined is not a singular cause-and-effect argument...Yet, it is important for scholars and clinicians alike across various paradigms to be aware of the research reviewed in Gay, Straight, and the Reason Why." --PsycCritiques "Simon LeVay's book, Gay, Straight, and the Reason Why, offers an excellent review of scientific research on the causes and correlates of sexual orientation. It provides a clear and comprehensive summary of recent studies of sexual orientation- a review that should be useful to lay people and journalists as well as to professionals in the field. LeVay has a knack for describing complicated scientific topics- brain anatomy, behavior genetics, endocrinology, cognitive psychology- in straight-forward and easy-to-understand ways. He provides the reader with a good sense of where research on sexual orientation stands today, and where further research is needed." -- Richard Lippa, Sex Roles "The theory that sexual orientation has a biological basis receives support in neuroscientist Simon LeVay's book. Relating evidence from genetics, neuroscience and developmental biology, he suggests that prenatal interactions between hormones and the developing brain influence adult sexuality." -- Nature

Selling point: Neuroscientist Simon LeVay presents the best current summary of the science of sexual orientation. Selling point: Marshalls a wealth of evidence that points toward the radical new explanation that sexual orientation results primarily from an interaction between genes, sex hormones, and the developing brain. Selling point: Illustrates how the same biological processes that influence a broad array of gendered traits are also the guiding factors behind a person's emerging sexuality. Selling point: Clear, straightforward prose that is accessible to everyone who has questions about what causes a person to grow up gay, straight, or bisexual.

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

Selling point: Neuroscientist Simon LeVay presents the best current summary of the science of sexual orientation. Selling point: Marshalls a wealth of evidence that points toward the radical new explanation that sexual orientation results primarily from an interaction between genes, sex hormones, and the developing brain. Selling point: Illustrates how the same biological processes that influence a broad array of gendered traits are also the guiding factors behind a person's emerging sexuality. Selling point: Clear, straightforward prose that is accessible to everyone who has questions about what causes a person to grow up gay, straight, or bisexual.