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.

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).

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