Simon LeVay 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 ten previous books, including the New York Times best-seller, When Science Goes Wrong.

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
Learning theories-gender learning
Is it a choice?
The biological alternative
Chapter 3: The Outline of a Theory 30
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 54
Origin of gendered traits
Sexual orientation and cognitive traits: visuospatial abilities 58
Verbal fluency
Memory tasks
Handedness
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
Pheromone studies
Sheep
Overview
Inhibition and sexual orientation
Chapter 9: The body
Body size and shape
Trunk and limb length
Penis size
Symmetry and developmental instability
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?
Is the older-brother effect adaptive?
Chapter 11: 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?
Diversity among gay people
Changes in the prevalence and nature of homosexuality
Sexual orientation and gender: the social fallout
Glossary
Bibliography

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.
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. LeVay also makes it clear that these lines of research have a lot of potential because--far from seeking to discover "what went wrong" in the lives of gay people, attempting to develop "cures" for homosexuality, or returning to traditional explanations that center on parent-child relationships, various forms of "training," or early sexual experiences--our modern scientists are increasingly seeing sexual variety as something to be valued, celebrated, and welcomed into society.