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.