Human Sexuality FOURTH EDITION SIMON LEVAY • JANICE BALDWIN Sinauer Associates, Inc. • Publishers Sunderland, Massachusetts U.S.A © Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher. LEVAY4E_FM.indd III 10/20/11 2:53 PM Brief Contents CHAPTER ONE Sexuality: Pathways to Understanding 3 CHAPTER TWO Sex and Evolution 29 CHAPTER THREE Women’s Bodies 59 CHAPTER FOUR Men’s Bodies 87 CHAPTER FIVE Sex Hormones and the Menstrual Cycle 119 CHAPTER SIX Sexual Development 153 CHAPTER SEVEN Gender 191 CHAPTER EIGHT Attraction, Arousal, and Response 219 CHAPTER NINE Sexual Behavior 251 CHAPTER TEN Sexual Relationships 283 CHAPTER ELEVEN Fertility, Pregnancy, and Childbirth 319 CHAPTER TWELVE Contraception and Abortion 357 CHAPTER THIRTEEN Sexuality across the Life Span 399 CHAPTER FOURTEEN Sexual Orientation 447 CHAPTER FIFTEEN Atypical Sexuality 483 CHAPTER SIXTEEN Sexual Disorders 517 CHAPTER SEVENTEEN Sexually Transmitted Diseases 545 CHAPTER EIGHTEEN Sexual Assault, Harassment, and Partner Violence 579 CHAPTER NINETEEN Sex as a Commodity 607 © Sinauer Associates, Inc. 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LEVAY4E_FM.indd VI 10/20/11 2:53 PM Contents CHAPTER ONE Sexuality: Pathways to Understanding 3 Why Study Human Sexuality? 3 BOX 1.2 Meet My Dads 15 Sex Research Has Developed from Sociologists focus on the connection between sex Converging Strands 4 and society 16 Observational studies began with Aristotle 4 Sociologists may take an ethnographic approach 18 Sexual ethics have religious roots 5 The economic approach weighs costs and Sex researchers emerged in the late nineteenth benefits 18 century 6 Sex research is becoming a discipline in its own right 19 BOX 1.1 Freud and Hirschfeld: Contrasting Theories on Sexual Orientation 8 Religions Offer Diverse Teachings on Psychological anthropologists investigated non- Sexuality 19 Western perspectives on sex 8 BOX 1.3 Sexual Health for the Millennium 20 Feminists campaigned for sexual reforms 10 Catholicism teaches conservative attitudes toward Biomedical research had a practical impact on people’s sexuality 20 sex lives 11 Protestant teachings are more flexible 21 Sexuality Can Be Studied with a Wide Judaism spans a spectrum 22 Variety of Methods 12 Global religions are embedded in diverse Biomedical research focuses on the underlying societies 22 mechanisms of sex 12 Sex Education Faces Significant Psychologists take diverse approaches to sexuality 13 Obstacles 23 © Sinauer Associates, Inc. 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LEVAY4E_FM.indd VII 10/20/11 2:53 PM VIII CONTENTS CHAPTER TWO Sex and Evolution 29 Diverse Methods of Reproduction Have Females often choose among males 43 Evolved 29 Sometimes males make significant investments in Rival Theories Offer Explanations for reproduction 45 Sexual Reproduction 31 BOX 2.3 When Males Get Pregnant 46 BOX 2.1 The Cellular Basis of Reproduction 32 If males invest, sexual selection may work Sexual reproduction may limit harmful mutations 33 differently 46 Sexual reproduction may generate beneficial gene Diverse Relationship Styles Have combinations 34 Evolved 48 Future research may solve the puzzle 35 Social and sexual arrangements are not necessarily the same 48 Why Are There Two Sexes? 35 Male promiscuity offers obvious evolutionary BOX 2.2 The Paradox of Sexless Species 36 benefits 49 Hermaphrodites combine male and female Why are females promiscuous? 49 reproductive functions 37 Female promiscuity leads to adaptive responses Can there be more than two sexes? 38 by males 50 Why are there equal numbers of males and Males may copulate with females by force 51 females? 38 Sometimes, Helping Relatives Reproduce Evolution Has Led to Diverse Methods of Is a Good Strategy 52 Sex Determination 39 Kin selection explains some altruistic animal Sex may be determined by chromosomes 39 behavior 53 Sex may be determined by temperature 40 Sex Has Acquired Other Functions beyond Sexual Selection Produces Anatomical and Reproduction 53 Behavioral Differences between Males BOX 2.4 Does Sexual Morality Have an and Females 41 Evolutionary Basis? The Case of Incest 54 Males and females follow different reproductive Female and male bonobos engage in non- strategies 41 reproductive sex 55 Females and males are exposed to different Bonobos use sex for conflict resolution and alliance reproductive risks 42 formation 56 Males often compete for access to females 42 CHAPTER THREE Women’s Bodies 59 A Woman’s Vulva Includes Her Mons, Cancer can affect the cervix or the endometrium 70 Labia, Vaginal Opening, and Clitoris 59 BOX 3.3 Genital Self-Examination 71 BOX 3.1 Genital Cosmetic Surgery 62 Other uterine conditions include fibroids, There is more to the clitoris than meets the eye 62 endometriosis, abnormal bleeding, and prolapse 73 Should hysterectomy be so common? 74 BOX 3.2 Female Circumcision 64 The appearance of the vaginal opening is variable 65 The Oviducts Are the Site of Fertilization 74 The Vagina Is the Outermost Portion of the Female Reproductive Tract 67 BOX 3.4 Hysteria 75 The G-spot is a controversial erogenous zone 69 The Ovaries Produce Ova and Sex The Anus Can Also Be a Sex Organ 69 Hormones 76 The Uterus Serves a Double Duty 70 The Breasts Have Both Erotic and Reproductive Significance 77 © Sinauer Associates, Inc. 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LEVAY4E_FM.indd VIII 10/20/11 2:53 PM CONTENTS IX Breast cancer mortality can be reduced 78 Treatment depends on the diagnostic findings and the Many factors affect the risk of breast cancer 79 woman’s choice 82 Early detection is important 80 Most women with breast cancer return to an active sex life 83 BOX 3.5 Breast Self-Examination 81 BOX 3.6 Is There Sex after Mastectomy? 84 CHAPTER FOUR Men’s Bodies 87 The Male External Genitalia Are the Penis BOX 4.4 Disorders of the Prostate Gland 100 and Scrotum 87 The Nervous System Orchestrates Sexual The penis combines erotic, reproductive, and Arousal 103 excretory functions 88 Erection can be mediated by a spinal reflex 103 BOX 4.1 Male Circumcision 90 BOX 4.5 The Nervous System 104 BOX 4.2 How Big Should a Penis Be? 93 Erectile tissue forms a hydraulic system 107 The scrotum regulates the temperature of Muscles are also involved in erection 109 the testicles 94 The brain influences erection 110 The Testes Produce Sperm and Sex Ejaculation Requires Coordination of Hormones 95 Muscles and Glands 111 BOX 4.3 Disorders of the Testicles 97 Emission is the passage of semen into the urethra 111 Other glands contribute their secretions to Ejaculation is caused by contractions of many muscles the semen 98 in the pelvic floor 112 What is semen? 99 Nudity Is Culturally Regulated 113 CHAPTER FIVE Sex Hormones and the Menstrual Cycle 119 Sex Steroids Consist of Three Groups 119 Other pituitary hormones include prolactin and growth hormone 127 BOX 5.1 Testicles by the Ton—The Discovery of Sex Steroids 121 Protein hormones are secreted by other tissues 128 Testosterone is a sex hormone in both men Prostaglandins and Monoamines Also and women 123 Influence Sexual Functions 128 Estradiol is synthesized from testosterone in A Brain–Pituitary–Testis Feedback Loop both sexes 124 Controls Testicular Function 128 Progesterone is a female hormone 124 The regulation of testosterone levels and Sex steroids activate specific receptor molecules 124 spermatogenesis is coupled 128 Proteins and Peptide Hormones Testosterone and inhibin exert negative feedback on Are Gene Products 125 gonadotropin release 129 Testosterone has multiple functions in men 129 Oxytocin is secreted by neuroendocrine cells of the hypothalamus 125 BOX 5.2 Steroids and Sports 131 GnRH stimulates the release of pituitary sex Estradiol has wide-ranging effects in men’s bodies 132 hormones 125 Menstruation Has Biological and Social The pituitary gonadotropins are follicle-stimulating Aspects 132 hormone (FSH) and luteinizing hormone (LH) 126 BOX 5.3 Attitudes toward Menstruation 133 © Sinauer Associates, Inc. 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LEVAY4E_FM.indd IX 10/20/11 2:53 PM X CONTENTS Women use pads, tampons, or cups during Sex steroids affect systems in women besides the menstruation 134 reproductive tract 143 The Menstrual Cycle Involves the Ovaries, The menstrual cycle influences sexuality 144 Brain, Pituitary, and Uterus 136 Menstrual Problems Are Common but The cycle is of variable length 136 Treatable 145 The cycle consists of menstrual, follicular, and Menstrual pain might or might not reflect underlying luteal phases 136 pelvic disease 145 BOX 5.4 Menstrual Synchrony: Reality or Premenstrual syndrome has physical and psychological Myth? 137 aspects 146 The menstrual phase is triggered by a drop in Menstruation stops during pregnancy—and for many progesterone levels 137 other reasons 147 The follicular phase is marked by the maturation of Sex Hormone-Related Compounds Exist in ovarian follicles 138 the Environment 148 During the luteal phase the uterus is prepared for BOX 5.5 Sex Hormones in the Environment 149 pregnancy 142 CHAPTER SIX Sexual Development 153 Humans Pass through