PS124-1: Identify basic concepts and principles of psychology.

CHAPTER 5: Gender and Sexuality

In 1972, as the young chair of our psychology department, I was proud to make the announcement: We had concluded our search for a new colleague. We had found just who we were looking for—a bright, warm, enthusiastic woman about to receive her Ph.D. in developmental psychology. The vote was unanimous. Alas, our elderly chancellor rejected our recommendation. “As a mother of a preschooler,” he said, “she should be home with her child, not working full time.” No amount of pleading or arguing (for example, that it might be possible to parent a child while employed) could change his mind. So, with a heavy heart, I drove to her city to explain, face to face, my embarrassment in being able to offer her only a temporary position.

In this case, all’s well that ends well. She accepted a temporary position and quickly became a beloved, tenured colleague who went on to found our college’s women’s studies program. But today, she and I marvel at the swift transformation in our culture’s thinking about gender. In a thin slice of history, our ideas about the “proper” behavior for women and men have undergone an extreme makeover. Both women and men are now recognized as “fully capable of effectively carrying out organizational roles at all levels” (Wood & Eagly, 2002). Women’s employment in formerly male occupations and men’s employment in formerly female occupations have increased. And as this was happening, our views of what is “masculine” and what is “feminine” have also changed, as have our ideas about what we seek in a mate (Twenge, 1997).

In this chapter, we’ll look at some of the ways nature and nurture interact to form us as males and females. We’ll see what researchers tell us about how much males and females are alike, and how and why they differ. Along the way, we’ll take a close look at human sexuality. As part of that close look, we’ll see how evolutionary psychologists explain our sexuality.

Let’s start by considering what gender is and how it develops.

Gender Development

As we will see in Chapter 9, we humans share an irresistible urge to organize our worlds into simple categories. Among the ways we classify people—as tall or short, fat or slim, smart or dull—one stands out: Before or at your birth, everyone wanted to know, “Boy or girl?” From that time on, your sex (your biological status, defined by your chromosomes and anatomy) helped define your gender, the socially constructed roles and characteristics by which your culture defines male and female.

gender the socially constructed roles and characteristics by which a culture defines male and female.

Pink and blue baby outfits offer another example of how cultural norms vary and change. “The generally accepted rule is pink for the boy and blue for the girl,” declared the publication Earnshaw’s Infants’ Department in June 1918 (Maglaty, 2011). “The reason is that pink being a more decided and stronger color is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girls.”

How Are We Alike? How Do We Differ?

5-1: What are some gender similarities and differences in aggression, social power, and social connectedness?

Having faced similar adaptive challenges, we are in most ways alike. Tell me whether you are male or female and you give me virtually no clues to your vocabulary, intelligence, and happiness, or to the mechanisms by which you see, hear, learn, and remember. Your “opposite” sex is, in reality, your very similar sex. At conception, you received 23 chromosomes from your mother and 23 from your father. Of those 46 chromosomes, 45 are unisex—the same for males and females. (Later in this chapter we’ll return to that forty-sixth chromosome.)

But males and females do differ, and differences command attention—stimulating more than 18,000 studies (Ellis et al., 2008). Some much-talked-about gender differences are actually quite modest, as Janet Shibley Hyde (2005) illustrated by graphically representing male and female self-esteem scores across many studies. Other differences are more striking. Compared with the average man, the average woman enters puberty 2 years sooner, and her life span is 5 years longer. She carries 70 percent more fat, has 40 percent less muscle, and is 5 inches shorter. She expresses emotions more freely, can smell fainter odors, and is offered help more often. She can become sexually re-aroused soon after orgasm. She is also doubly vulnerable to depression and anxiety, and her risk of developing an eating disorder is 10 times greater than the average man’s. Yet, he is some 4 times more likely to commit suicide or exhibit alcohol use disorder. He is also more likely to be diagnosed with autism spectrum disorder, color-blindness, attention-deficit/hyperactivity disorder as a child, and antisocial personality disorder as an adult. Choose your gender and pick your vulnerability.

Gender differences appear throughout this book. For now, let’s consider some gender differences in aggression, social power, and social connectedness.

Gender and Aggression

In surveys, men admit to more aggression than women do. This aggression gender gap pertains to harmful physical aggression, rather than indirect or verbal relational aggression such as ostracism or spreading rumors. As John Archer (2004, 2006, 2009) has noted, based on statistical digests of dozen of studies, women may be slightly more likely to commit acts of relational aggression, such as passing along malicious gossip. The gap appears in everyday life at various ages and in various cultures, especially cultures with gender inequality (Archer, 2009).

aggression physical or verbal behavior intended to hurt someone.

Men’s tendency to behave more aggressively can be seen in experiments where they deliver what they believe are more painful electric shocks (Card et al., 2008). Violent crime rates illustrate the gender difference even more strikingly. The male-to-female arrest ratio for murder, for example, is 9 to 1 in the United States and 8 to 1 in Canada (FBI, 2009; Statistics Canada, 2010). Throughout the world, fighting, warring, and hunting are primarily men’s activities (Wood & Eagly, 2002, 2007). Men also express more support for war. The Iraq war, for example, was consistently supported more by American men than by American women (Newport et al., 2007).

Deadly relational aggression

Gender and Social Power

Close your eyes and imagine two adults standing side by side. The one on the left is dominant, forceful, and independent. The one on the right is submissive, nurturing, and socially connected.

Did you see the person on the left as a man, and the one on the right as a woman? If so, you are not alone.

Around the world, from Nigeria to New Zealand, people perceive such power differences between men and women (Williams & Best, 1990). Indeed, in most societies men do place more importance on power and achievement and are socially dominant (Schwartz & Rubel-Lifschitz, 2009). When groups form, whether as juries or companies, leadership tends to go to males (Colarelli et al., 2006). When salaries are paid, those in traditionally male occupations receive more. And when political leaders are elected, they usually are men, who held 80 percent of the seats in the world’s governing parliaments in 2011 (IPU, 2011). If perceived to be hungry for political power (thus violating gender norms), women more than men suffer voter backlash (Okimoto & Brescoll, 2010). Men’s power hunger is more expected and accepted.

Women’s 2011 representation in national parliaments ranged from 11 percent in the Arab States to 42 percent in Scandinavia (IPU, 2011).

As leaders, men tend to be more directive, even autocratic. Women tend to be more democratic, more welcoming of subordinates’ input in decision making (Eagly & Carli, 2007; van Engen & Willemsen, 2004). When people interact, men are more likely to utter opinions, women to express support (Aries, 1987; Wood, 1987). In everyday behavior, men tend to act as powerful people often do: They are more likely to talk assertively, interrupt, initiate touches, and stare. And they smile and apologize less (Leaper & Ayres, 2007; Major et al., 1990; Schumann & Ross, 2010). Such behaviors help sustain social power inequities.

Gender and Social Connectedness

Females are more interdependent than males, and this difference surfaces early. In children’s play, boys typically form large groups. Their games tend to be active and competitive, with little intimate discussion (Rose & Rudolph, 2006). Studies have found that girls usually play in smaller groups, often with one friend. Their play is less competitive and more imitative of social relationships (Maccoby, 1990; Roberts, 1991).

As adults, women take more pleasure in talking face to face, and they more often use conversation to explore relationships. Men enjoy doing activities side by side and tend to use conversation to communicate solutions (Tannen, 1990; Wright, 1989). The communication difference is apparent in student e-mails: In one New Zealand study, people could correctly guess the author’s gender two-thirds of the time (Thomson & Murachver, 2001).

Gender differences also appear in phone-based communication. In the United States, the average teen girl sends and receives 80 texts daily; the average boy 30 (Lenhart, 2010). In France, women have made 63 percent of phone calls and, when talking to a woman, stayed connected longer (7.2 minutes) than have men when talking to other men (4.6 minutes) (Smoreda & Licoppe, 2000).

Every man for himself, or tend and befriend?

Women worldwide have oriented their interests and vocations more to people and less to things (Eagly, 2009; Lippa, 2005, 2006, 2008). One analysis of more than a half-million people’s responses to various interest inventories revealed that “men prefer working with things and women prefer working with people” (Su et al., 2009). On entering college, American men are seven times more likely than women to express interest in computer science, and they contribute 87 percent of Wikipedia articles (Cohen, 2011; Pryor et al., 2011). In the workplace, women have been less driven by money and status and more often opted for reduced work hours (Pinker, 2008). In the home, they have been five times more likely than men to claim primary responsibility for taking care of children (Time, 2009).

Women’s emphasis on caring helps explain another interesting finding: Although 69 percent of people have said they have a close relationship with their father, 90 percent said they feel close to their mother (Hugick, 1989). When wanting understanding and someone with whom to share worries and hurts, both men and women usually turn to women, and both have reported their friendships with women to be more intimate, enjoyable, and nurturing (Rubin, 1985; Sapadin, 1988). And when coping with their own stress, women more than men turn to others for support—they tend and befriend (Tamres et al., 2002; Taylor, 2002).

Gender differences in social connectedness, power, and other traits peak in late adolescence and early adulthood—the very years most commonly studied (also the years of dating and mating). As teenagers, girls become progressively less assertive and more flirtatious; boys become more domineering and unexpressive. Following the birth of a first child, parents (women especially) become more traditional in their gender-related attitudes and behavior (Ferriman et al., 2009; Katz-Wise et al., 2010). But studies have shown that by age 50, parenthood-related gender differences subside. Men become more empathic and less domineering, and women—especially those working outside the home—become more assertive and self-confident (Kasen et al., 2006; Maccoby, 1998).

“In the long years liker must they grow; The man be more of woman, she of man.”

Alfred Lord Tennyson, The Princess, 1847

What explains our diversity? How much does biology bend the genders? To what extent are we shaped by our cultures? A bio psycho social view suggests both are important, thanks to the interplay among our biological dispositions, our developmental experiences, and our current situations (Eagly, 2009).

The Nature of Gender: Our Biology

5-2: How is our biological sex determined, and how do sex hormones influence prenatal and adolescent development?

In domains where we face similar challenges—regulating heat with sweat, preferring foods that nourish, growing calluses where the skin meets friction—men and women are similar. Even when describing the ideal mate, both prize traits such as “kind,” “honest,” and “intelligent.” But in mating-related domains, evolutionary psychologists contend, guys act like guys whether they are elephants or elephant seals, rural peasants or corporate presidents (Geary, 2010). Our biology may influence our gender differences in two ways: genetically, by our differing sex chromosomes, and physiologically, from our differing concentrations of sex hormones.

Prenatal Sexual Development

As noted earlier, males and females are variations on a single form—of the 46 chromosomes, 45 are unisex. So great is this similarity that until seven weeks after conception, you were anatomically indistinguishable from someone of the other sex. Then your genes activated your biological sex. Male or female, your sex was determined by your father’s contribution to your twenty-third pair of chromosomes, the two sex chromosomes. You received an X chromosome from your mother. From your father, you received the one chromosome that is not unisex—either another X chromosome, making you a girl, or a Y chromosome, making you a boy.

X chromosome the sex chromosome found in both men and women. Females have two X chromosomes; males have one. An X chromosome from each parent produces a female child.

Y chromosome the sex chromosome found only in males. When paired with an X chromosome from the mother, it produces a male child.

The Y chromosome includes a single gene which, about seven weeks after conception, throws a master switch triggering the testes to develop and to produce the principal male hormone, testosterone. This hormone starts the development of external male sex organs. Females also have testosterone, but less of it.

testosterone the most important of the male sex hormones. Both males and females have it, but the additional testosterone in males stimulates the growth of the male sex organs in the fetus and the development of the male sex characteristics during puberty.

Another key period for sexual differentiation falls during the fourth and fifth prenatal months. During this period, sex hormones bathe the fetal brain and influence its wiring. Different patterns for males and females develop under the influence of the male’s greater testosterone and the female’s ovarian hormones (Hines, 2004; Udry, 2000).

Adolescent Sexual Development

Pronounced physical differences emerge during adolescence, when boys and girls enter puberty and mature sexually. A surge of hormones triggers a two-year period of rapid physical development, usually beginning at about age 11 in girls and at about age 13 in boys. A year or two before that, however, boys and girls often feel the first stirrings of sexual attraction (McClintock & Herdt, 1996).

puberty the period of sexual maturation, during which a person becomes capable of reproducing.

About the time of puberty, boys’ growth propels them to greater height than their female counterparts. During this growth spurt, the primary sex characteristics—the reproductive organs and external genitalia—develop dramatically. So do secondary sex characteristics, the nonreproductive traits such as breasts and hips in girls, facial hair and deepened voice in boys, and pubic and underarm hair in both sexes.

Throughout childhood, boys and girls are similar in height. At puberty, girls surge ahead briefly, but then boys overtake them at about age 14. (Data from Tanner, 1978.) Recent studies suggest that sexual development and growth spurts are beginning somewhat earlier than was the case a half-century ago (Herman-Giddens et al., 2001).

At about age 11 in girls and age 13 in boys, a surge of hormones triggers a variety of physical changes.

primary sex characteristics the body structures (ovaries, testes, and external genitalia) that make sexual reproduction possible.

secondary sex characteristics nonreproductive sexual traits, such as female breasts and hips, male voice quality, and body hair.

In various countries, girls are developing breasts earlier (sometimes before age 10) and reaching puberty earlier than in the past. This phenomenon is variously attributed to increased body fat, increased hormone-mimicking chemicals, and increased stress related to family disruption (Biro et al., 2010).

Puberty’s landmarks are the first ejaculation in boys (spermarche), usually by about age 14, and the first menstrual period in girls (menarche—meh-NAR-key), usually within a year of age 12½ (Anderson et al., 2003). Menarche appears to occur a few months earlier, on average, for girls who have experienced stresses related to father absence, sexual abuse, or insecure attachments (Belsky et al., 2010; Vigil et al., 2005; Zabin et al., 2005). Girls who have been prepared for menarche usually experience it as a positive life transition. Studies have shown that nearly all adult women recall their first menstrual period and remember experiencing a mixture of feelings—pride, excitement, embarrassment, and apprehension (Greif & Ulman, 1982; Woods et al., 1983). Most men have similarly recalled their first ejaculation, which usually occurs as a nocturnal emission (Fuller & Downs, 1990).

menarche [meh-NAR-key] the first menstrual period.

Variations on Sexual Development

Sometimes nature blurs the biological line between males and females. Atypical hormone exposure or sensitivity may cause atypical fetal development. Intersex individuals are born with intermediate or unusual combinations of male and female physical features. Genetic males, for example, may be born with normal male hormones and testes but without a penis or with a very small one.

Gender in the spotlight

Until recently, pediatricians and other medical experts often recommended surgery to create a female identity for these children. One study reviewed 14 cases of boys who had undergone early sex-reassignment surgery and had been raised as girls. Of those cases, 6 had later declared themselves as males, 5 were living as females, and 3 had an unclear gender identity (Reiner & Gearhart, 2004).

Although not born with an intersex condition, a little boy who lost his penis during a botched circumcision became a famous case illustrating the problems involved in sex-reassignment surgery. His parents followed a psychiatrist’s advice to raise him as a girl rather than as a damaged boy. Alas, “Brenda” Reimer was not like most other girls. “She” didn’t like dolls. She tore her dresses with rough-and-tumble play. At puberty she wanted no part of kissing boys. Finally, Brenda’s parents explained what had happened, whereupon this young person immediately rejected the assigned female identity. He cut his hair and chose a male name, David. He eventually married a woman and became a stepfather. And, sadly, he later committed suicide (Colapinto, 2000).

The bottom line: “Sex matters,” concluded the National Academy of Sciences (2001). In combination with the environment, sex-related genes and physiology “result in behavioral and cognitive differences between males and females.” Nature and nurture work together.

The Nurture of Gender: Our Culture

5-3: How do gender roles and gender typing influence gender development?

For most people, their biological sex and their gender are tightly interwined. What biology initiates, culture accentuates.

Gender Roles

You may recall from Chapter 1 that culture is everything shared by a group and transmitted across generations. We can see culture’s shaping power in gender roles—the social expectations that guide men’s and women’s behavior. (In psychology, as in the theater, a role refers to a cluster of prescribed actions, the behaviors we expect of those who occupy a particular social position.)

gender role a set of expected behaviors for males or for females.

role a set of expectations (norms) about a social position, defining how those in the position ought to behave.

Gender roles vary over time and place. In North America, men were traditionally expected to initiate dates, drive the car, and pick up the check. Women were expected to decorate the home, buy and care for the children’s clothes, and select the wedding gifts. Up through the 1990s, Mom (about 90 percent of the time in two-parent U.S. families) stayed home with a sick child, arranged for the baby sitter, and called the doctor (Maccoby, 1995). Even in recent years, compared with employed women, employed men in the United States have spent about an hour and a half more on the job and about one hour less on daily household activities and caregiving (Amato et al., 2007; Bureau of Labor Statistics, 2004; Fisher et al., 2006). Ditto Australia, where, compared with men, women have devoted 54 percent more time to unpaid household work and 71 percent more time to child care (Trewin, 2001).

Other societies have different expectations. In nomadic societies of food-gathering people, there is little division of labor by sex. Boys and girls receive much the same upbringing. In agricultural societies, where women work in the nearby fields and men roam while herding livestock, children have typically been socialized into more distinct gender roles (Segall et al., 1990; Van Leeuwen, 1978).

Among industrialized countries, gender roles and attitudes vary widely. Australia and the Scandinavian countries offer the greatest gender equity, Middle Eastern and North African countries the least (Social Watch, 2006). And consider: Would you agree that “when jobs are scarce, men should have more rights to a job?” In the United States, Britain, and Spain, about one in eight adults agree. In Nigeria, Pakistan, and India, about four in five do (Pew, 2010). We are one species, but my how we differ.

The gendered tsunami

To see how gender role attitudes vary over time, consider women’s voting rights. At the opening of the twentieth century, only one country—New Zealand—granted women the right to vote (Briscoe, 1997). By the late 1960s and early 1970s, women had become a force in the voting booth and the workplace in many countries. Nearly 50 percent of employed Americans are now women, as are 54 percent of college graduates, up from 36 percent in just four decades (Fry & Cohn, 2010). In today’s postindustrial economy, the jobs expected to grow the most in the years ahead are the ones women have gravitated toward—those that require not size and strength but social intelligence, open communication, and the ability to sit still and focus (Rosin, 2010). These are big gender changes in but a thin slice of history.

Gender roles can smooth social relations, avoiding irritating discussions about whose job it is to get the car fixed and who should buy the birthday presents. But these quick and easy assumptions come at a cost: If we deviate from conventions, we may feel anxious.

How Do We Learn to Be Male or Female?

Gender identity is a person’s sense of being male or female. Social learning theory assumes that children acquire this identity by observing and imitating others’ gender-linked behaviors and by being rewarded or punished for acting in certain ways themselves (“Nicole, you’re such a good mommy to your dolls”; “Big boys don’t cry, Alex.”). Some critics have objected, saying that parental modeling and rewarding of male-female differences aren’t enough to explain gender typing, the way some children seem more attuned than others to traditional male or female roles (Lytton & Romney, 1991). In fact, even in families that discourage traditional gender typing, children organize themselves into “boy worlds” and “girl worlds,” each guided by rules for what boys and girls do.

gender identity our sense of being male or female.

social learning theory the theory that we learn social behavior by observing and imitating and by being rewarded or punished.

gender typing the acquisition of a traditional masculine or feminine role.

Cognition (thinking) also matters. In your own childhood you formed schemas, or concepts that helped you make sense of your world. One of these was your gender schema, your framework for organizing boy-girl characteristics (Bem, 1987, 1993). This gender schema then became a lens through which you viewed your experiences.

Gender schemas form early in life, and social learning helps form them. Before age 1, you began to discriminate male and female voices and faces (Martin et al., 2002). After age 2, language forced you to begin organizing your world on the basis of gender. English, for example, uses the pronouns he and she; other languages classify objects as masculine (“le train”) or feminine (“la table”).

The social learning of gender

Young children are “gender detectives” (Martin & Ruble, 2004). Once they grasp that two sorts of people exist—and that they are of one sort—they search for clues about gender, and they find them in language, dress, toys, and songs. Girls, they may decide, are the ones with long hair. Having divided the human world in half, 3-year-olds will then like their own kind better and seek them out for play. And having compared themselves with their concept of gender, they will adjust their behavior accordingly. (“I am male—thus, masculine, strong, aggressive,” or “I am female—therefore, feminine, sweet, and helpful.”) These rigid boy-girl stereotypes peak at about age 5 or 6. If the new neighbor is a boy, a 6-year-old girl may assume he just cannot share her interests. For young children, gender looms large.

For some people, comparing themselves with their culture’s concepts of gender produces feelings of confusion and discord. Transgender people’s gender identity (their sense of being male or female) or gender expression (their communication of gender identity through behavior or appearance) differs from that typical of their birth sex (APA, 2010). A person may feel like a man in a woman’s body, or a woman in a man’s body. These include transsexual people, who live, or wish to live, as members of the gender opposite to their birth sex, often aided by medical treatment that supports gender reassignment. Note that gender identity is distinct from sexual orientation (the direction of one’s sexual attraction). Transgender people may be heterosexual, homosexual, bisexual, or asexual.

transgender an umbrella term describing people whose gender identity or expression differs from that associated with their birth sex.

“The more I was treated as a woman, the more woman I became.”

Jan Morris, male-to-female transsexual

Some transgender persons express their gender identity by dressing as a person of the other biological sex typically would. Most cross-dressers are biological males, the majority of whom feel an attraction to females (APA, 2010).

Human Sexuality

As you’ve probably noticed, we can hardly talk about gender without talking about our sexuality. For all but the tiny fraction of us considered asexual, dating and mating become top priority from puberty on. Physiological and psychological influences affect our sexual feelings and behaviors.

In a British survey of 18,876 people, 1 percent were seemingly asexual, having “never felt sexually attracted to anyone at all” (Bogaert, 2004, 2006b).

The Physiology of Sex

Sex is not like hunger, because it is not an actual need. (Without it, we may feel like dying, but we will not.) Yet sex is part of life. Had this not been so for all your ancestors, you would not be reading this book. Sexual motivation is nature’s clever way of making people procreate, thus enabling our species’ survival. When two people feel an attraction, they hardly stop to think of themselves as guided by their genes. As the pleasure we take in eating is nature’s method of getting our body nourishment, so the desires and pleasures of sex are our genes’ way of preserving and spreading themselves. Life is sexually transmitted.

“It is a near-universal experience, the invisible clause on one’s birth certificate stipulating that one will, upon reaching maturity, feel the urge to engage in activities often associated with the issuance of more birth certificates.”

Science writer Natalie Angier, 2007

Hormones and Sexual Behavior

5-4: How do hormones influence human sexual motivation?

Among the forces driving sexual behavior are the sex hormones. The main male sex hormone, as we saw earlier, is testosterone. The main female sex hormones are the estrogens, such as estradiol. Sex hormones influence us at many points in the life span:

estrogens sex hormones, such as estradiol, secreted in greater amounts by females than by males and contributing to female sex characteristics. In nonhuman female mammals, estrogen levels peak during ovulation, promoting sexual receptivity.

•  During the prenatal period, they direct our development as males or females.

•  During puberty, a sex hormone surge ushers us into adolescence.

•  After puberty and well into the late adult years, sex hormones activate sexual behavior.

In most mammals, nature neatly synchronizes sex with fertility. Females become sexually receptive (“in heat”) when their estrogens peak at ovulation. In experiments, researchers can cause female animals to become receptive by injecting them with estrogens. Male hormone levels are more constant, and hormone injection does not so easily manipulate the sexual behavior of male animals (Feder, 1984). Nevertheless, male rats that have had their testes (which manufacture testosterone) surgically removed will gradually lose much of their interest in receptive females. They gradually regain it if injected with testosterone.

Hormones do influence human sexual behavior, but in a looser way. Among women with mates, sexual desire rises slightly at ovulation (Pillsworth et al., 2004). One study invited partnered women not at risk for pregnancy to keep a diary of their sexual activity. (These women were either using intrauterine devices or had undergone surgery to prevent pregnancy.) On the days around ovulation, intercourse was 24 percent more frequent (Wilcox et al., 2004).

Women more than other mammalian females are responsive to their testosterone level (van Anders & Dunn, 2009). You may recall that women have testosterone, though at lower levels than are found in men. If a woman’s natural testosterone level drops, as happens with removal of the ovaries or adrenal glands, her sexual interest may wane. But as controlled experiments with hundreds of surgically or naturally menopausal women have demonstrated, testosterone-replacement therapy can often restore diminished sexual activity, arousal, and desire (Braunstein et al., 2005; Buster et al., 2005; Petersen & Hyde, 2011).

In human males with abnormally low testosterone levels, testosterone-replacement therapy often increases sexual desire and also energy and vitality (Yates, 2000). But normal fluctuations in testosterone levels, from man to man and hour to hour, have little effect on sexual drive (Byrne, 1982). Indeed, male hormones sometimes vary in response to sexual stimulation. In one study, Australian skateboarders’ testosterone surged in the presence of an attractive female, contributing to riskier moves and more crash landings (Ronay & von Hippel, 2010). Thus, sexual arousal can be a cause as well as a consequence of increased testosterone levels. At the other end of the mating spectrum, studies in both North America and China have found that married fathers tend to have lower testosterone levels than do bachelors and married men without children (Gray et al., 2006).

Large hormonal surges or declines do affect men and women’s desire. These shifts tend to occur at two predictable points in the life span, and sometimes at an unpredictable third point:

• 1. As we have seen, a surge in sex hormones triggers the development of sex characteristics during puberty. Interest in dating and sexual stimulation usually increases at this time. If the hormonal surge is precluded—as it was during the 1600s and 1700s for prepubertal boys who were castrated to preserve their soprano voices for Italian opera—sex characteristics and sexual desire do not develop normally (Peschel & Peschel, 1987).

• 2. In later life, estrogen levels fall, and women experience menopause. As sex hormone levels decline, the frequency of sexual fantasies and intercourse declines as well (Leitenberg & Henning, 1995).

• 3. For some, surgery or drugs may cause hormonal shifts. When adult men were castrated, sex drive typically fell as testosterone levels declined sharply (Hucker & Bain, 1990). Male sex offenders who were taking Depo-Provera, a drug that reduces testosterone levels to that of a prepubertal boy, have similarly lost much of their sexual urge (Bilefsky, 2009; Money et al., 1983).

To summarize: We might compare human sex hormones, especially testosterone, to the fuel in a car. Without fuel, a car will not run. But if the fuel level is minimally adequate, adding more fuel to the gas tank won’t change how the car runs. The analogy is imperfect, because hormones and sexual motivation interact. However, it correctly suggests that biology is a necessary but not sufficient explanation of human sexual behavior. The hormonal fuel is essential, but so are the psychological stimuli that turn on the engine, keep it running, and shift it into high gear.

The Sexual Response Cycle

5-5: What is the human sexual response cycle, and how do sexual dysfunctions and paraphilias differ?

In the 1960s, gynecologist-obstetrician William Masters and his collaborator Virginia Johnson (1966) made headlines by recording the physiological responses of volunteers who came to their lab to masturbate or have intercourse. With the help of 382 female and 312 male volunteers—a somewhat atypical sample, consisting only of people able and willing to display arousal and orgasm while scientists observed—Masters and Johnson monitored or filmed more than 10,000 sexual “cycles.” Their description of the sexual response cycle identified four stages:

• 1. Excitement: The genital areas become engorged with blood, causing a woman’s clitoris and a man’s penis to swell. A woman’s vagina expands and secretes lubricant; her breasts and nipples may enlarge.

• 2. Plateau: Excitement peaks as breathing, pulse, and blood pressure rates continue to increase. A man’s penis becomes fully engorged and some fluid—frequently containing enough live sperm to enable conception—may appear at its tip. A woman’s vaginal secretion continues to increase, and her clitoris retracts. Orgasm feels imminent.

• 3. Orgasm: Muscle contractions appear all over the body and are accompanied by further increases in breathing, pulse, and blood pressure rates. A woman’s arousal and orgasm facilitate conception: They help propel semen from the penis, position the uterus to receive sperm, and draw the sperm further inward, increasing retention of deposited sperm (Furlow & Thornhill, 1996). The pleasurable feeling of sexual release apparently is much the same for both sexes. One panel of experts could not reliably distinguish between descriptions of orgasm written by men and those written by women (Vance & Wagner, 1976). In another study, PET scans showed that the same subcortical brain regions were active in men and women during orgasm (Holstege et al., 2003a,b). And when people passionately in love undergo fMRI scans while viewing photos of their beloved or of a stranger, men’s and women’s brain responses to their partner are pretty similar (Fisher et al., 2002).

• 4. Resolution: The body gradually returns to its unaroused state as the genital blood vessels release their accumulated blood. This happens relatively quickly if orgasm has occurred, relatively slowly otherwise. (It’s like the nasal tickle that goes away rapidly if you have sneezed, slowly otherwise.) Men then enter a refractory period that lasts from a few minutes to a day or more, during which they are incapable of another orgasm. A woman’s much shorter refractory period may enable her to have another orgasm if restimulated during or soon after resolution.

refractory period a resting period after orgasm, during which a man cannot achieve another orgasm.

sexual response cycle the four stages of sexual responding described by Masters and Johnson—excitement, plateau, orgasm, and resolution.

A nonsmoking 50-year-old male has about a 1-in-a-million chance of a heart attack during any hour. This increases to merely 2-in-a-million in the two hours during and following sex (with no increase for those who exercise regularly). Compared with risks associated with heavy exertion or anger, this risk seems not worth losing sleep (or sex) over (Jackson, 2009; Muller et al., 1996).

Sexual Dysfunctions and Paraphilias

Masters and Johnson sought not only to describe the human sexual response cycle but also to understand and treat the inability to complete it. Sexual dysfunctions are problems that consistently impair sexual arousal or functioning. Some involve sexual motivation, especially lack of sexual energy and arousability. For men, others include erectile disorder (inability to have or maintain an erection) and premature ejaculation. For women, the problem may be pain or female orgasmic disorder (distress over infrequently or never experiencing orgasm). In separate surveys of some 3000 Boston women and 32,000 other American women, about 4 in 10 reported a sexual problem, such as orgasmic disorder or low desire, but only about 1 in 8 reported that this caused personal distress (Lutfey et al., 2009; Shifren et al., 2008). Most women who have experienced sexual distress have related it to their emotional relationship with the partner during sex (Bancroft et al., 2003).

sexual dysfunction a problem that consistently impairs sexual arousal or functioning.

Therapy can help men and women with sexual dysfunctions. In behaviorally oriented therapy, for example, men learn ways to control their urge to ejaculate, and women are trained to bring themselves to orgasm. Starting with the introduction of Viagra in 1998, erectile disorder has been routinely treated by taking a pill. Equally effective drug treatments for female sexual interest/arousal disorder are not yet available.

Sexual dysfunction involves problems with arousal or sexual functioning. People with paraphilias do experience sexual desire, but they direct it in unusual ways. The American Psychiatric Association (2013) only classifies such behavior as disordered if

•  a person experiences distress from their unusual sexual interest or

•  it entails harm or risk of harm to others.

paraphilias experiencing sexual arousal from fantasies, behaviors, or urges involving non-human objects, the suffering of self or others, and/or nonconsenting persons.

The serial killer Jeffrey Dahmer had necrophilia, a sexual attraction to corpses. People with the paraphilic disorder pedophilia experience sexual arousal toward children who haven’t entered puberty. Those with exhibitionism derive pleasure from exposing themselves sexually to others, without consent.

Sexually Transmitted Infections

5-6: How can sexually transmitted infections be prevented?

Rates of sexually transmitted infections (STIs; also called STDs for sexually transmitted diseases) are rising, and two-thirds of the new infections have occurred in people under 25 (CASA, 2004). Teenage girls, because of their not yet fully mature biological development and lower levels of protective antibodies, are especially vulnerable (Dehne & Riedner, 2005; Guttmacher, 1994). A Centers for Disease Control study of sexually experienced 14-to 19-year-old U.S. females found 39.5 percent had STIs (Forhan et al., 2008).

To comprehend the mathematics of infection transmission, imagine this scenario. Over the course of a year, Pat has sex with 9 people, each of whom over the same period has sex with 9 other people, who in turn have sex with 9 others. How many “phantom” sex partners (past partners of partners) will Pat have? The actual number—511—is more than five times the estimate given by the average student (Brannon & Brock, 1993).

Condoms offer only limited protection against certain skin-to-skin STIs, such as herpes, but they do reduce other risks (Medical Institute, 1994; NIH, 2001). The effects were clear when Thailand promoted 100 percent condom use by commercial sex workers. Over a four-year period, as condom use soared from 14 to 94 percent, the annual number of bacterial STIs plummeted from 410,406 to 27,362 (WHO, 2000).

Across the available studies, condoms also have been 80 percent effective in preventing transmission of HIV (human immunodeficiency virus—the virus that causes AIDS) from an infected partner (Weller & Davis-Beaty, 2002; WHO, 2003). Although AIDS can be transmitted by other means, such as needle sharing during drug use, its sexual transmission is most common. Women’s AIDS rates are increasing fastest, partly because the virus is passed from man to woman much more often than from woman to man. A man’s semen can carry more of the virus than can a woman’s vaginal and cervical secretions. The HIV-infected semen can also linger for days in a woman’s vagina and cervix, increasing the time of exposure (Allen & Setlow, 1991; WHO, 2004).

AIDS (acquired immune deficiency syndrome) a life-threatening, sexually transmitted infection caused by the human immunodeficiency virus (HIV). AIDS depletes the immune system, leaving the person vulnerable to infections.

Most U.S. AIDS cases have been people in midlife and younger—ages 25 to 44 (U.S. Centers for Disease Control and Prevention, 2011). Given AIDS’ long incubation period, this means that many of these young people were infected as teens. In 2009, the death of 1.8 million AIDS victims worldwide left behind countless grief-stricken partners and millions of orphaned children (UNAIDS, 2010). Sub-Saharan Africa is home to two-thirds of those infected with HIV, and medical treatment and care for the dying are sapping the region’s social resources.

Many people assume that oral sex falls in the category of “safe sex,” but recent studies show a significant link between oral sex and transmission of STIs, such as the human papilloma virus (HPV). Risks rise with the number of sexual partners (Gillison et al., 2012). Most HPVs can now be prevented with a vaccination administered before sexual contact.

The Psychology of Sex

5-7: How do external and imagined stimuli contribute to sexual arousal?

Biological factors powerfully influence our sexual motivation and behavior. Yet the wide variations over time, across place, and among individuals document the great influence of psychological factors as well. Thus, despite the shared biology that underlies sexual motivation, the 281 expressed reasons for having sex (at last count) ranged widely—from “to get closer to God” to “to get my boyfriend to shut up” (Buss, 2008; Meston & Buss, 2007).

External Stimuli

Men and women become aroused when they see, hear, or read erotic material (Heiman, 1975; Stockton & Murnen, 1992). In 132 experiments, men’s feelings of sexual arousal have much more closely mirrored their (more obvious) genital response than have women’s (Chivers et al., 2010).

People may find sexual arousal either pleasing or disturbing. (Those who wish to control their arousal often limit their exposure to such materials, just as those wishing to control hunger limit their exposure to tempting cues.) With repeated exposure, the emotional response to any erotic stimulus often lessens, or habituates. During the 1920s, when Western women’s rising hemlines first reached the knee, an exposed leg was a mildly erotic stimulus.

Can exposure to sexually explicit material have adverse effects? Research has indicated that it can. Depictions of women being sexually coerced—and liking it—have increased viewers’ acceptance of the false idea that women enjoy rape, and have increased male viewers’ willingness to hurt women (Malamuth & Check, 1981; Zillmann, 1989). Viewing images of sexually attractive women and men may also lead people to devalue their own partners and relationships. After male collegians viewed TV or magazine depictions of sexually attractive women, they often found an average woman, or their own girlfriend or wife, less attractive (Kenrick & Gutierres, 1980; Kenrick et al., 1989; Weaver et al., 1984). Viewing X-rated sex films has similarly tended to diminish people’s satisfaction with their own sexual partner (Zillmann, 1989). Perhaps reading or watching erotica creates expectations that few men and women can fulfill.

Imagined Stimuli

The brain, it has been said, is our most significant sex organ. The stimuli inside our heads—our imagination—can influence sexual arousal and desire. People who, because of a spinalcord injury, have no genital sensation can still feel sexual desire (Willmuth, 1987).

Wide-awake people become sexually aroused not only by memories of prior sexual activities but also by fantasies. About 95 percent of both men and women have said they have sexual fantasies. Men (whether gay or straight) fantasize about sex more often, more physically, and less romantically. They also prefer less personal and faster-paced sexual content in books and videos (Leitenberg & Henning, 1995). Fantasizing about sex does not indicate a sexual problem or dissatisfaction. If anything, sexually active people have more sexual fantasies.

Teen Pregnancy

5-8: What factors influence teenagers’ sexual behaviors and use of contraceptives?

Compared with European teens, American teens have a higher rate of STIs and also of teen pregnancy (Call et al., 2002; Sullivan/Anderson, 2009). What environmental factors contribute to teen pregnancy?

“Condoms should be used on every conceivable occasion.”

Anonymous

Minimal communication about birth control Many teenagers are uncomfortable discussing contraception with their parents, partners, and peers. Teens who talk freely with parents, and who are in an exclusive relationship with a partner with whom they communicate openly, are more likely to use contraceptives (Aspy et al., 2007; Milan & Kilmann, 1987).

Guilt related to sexual activity In another survey, 72 percent of sexually active 12- to 17-year-old American girls said they regretted having had sex (Reuters, 2000). Sexual inhibitions or ambivalence can restrain sexual activity, but if passion overwhelms intentions they may also reduce attempts at birth control (Gerrard & Luus, 1995; MacDonald & Hynie, 2008).

Alcohol use Sexually active teens are typically alcohol-using teens (Zimmer-Gembeck & Helfand, 2008), and those who use alcohol prior to sex are less likely to use condoms (Kotchick et al., 2001). By depressing the brain centers that control judgment, inhibition, and self-awareness, alcohol disarms normal restraints, a phenomenon well known to sexually coercive males.

Mass media norms of unprotected promiscuity Media help write the “social scripts” that affect our perceptions and actions. So what sexual scripts do today’s media write on our minds? An average hour of prime-time television on the three major U.S. networks has contained 15 sexual acts, words, and innuendos. The partners were usually unmarried, with no prior romantic relationship, and few communicated any concern for birth control or STIs (Brown et al., 2002; Kunkel, 2001; Sapolsky & Tabarlet, 1991). The more sexual content adolescents view (even when controlling for other predictors of early sexual activity), the more likely they are to perceive their peers as sexually active, to develop sexually permissive attitudes, and to experience early intercourse (Escobar-Chaves et al., 2005; Martino et al., 2005; Ward & Friedman, 2006). (See Close-Up: The Sexualization of Girls.)

Keeping abreast of hypersexuality

An analysis of the 60 top-selling video games found 489 characters, 86 percent of whom were males (like most of the game players). The female characters were much more likely than the male characters to be “hyper-sexualized”—partially nude or revealingly clothed, with large breasts and tiny waists (Downs & Smith, 2010).

Eidos Scripps Howard Photo Service/Newscom

The National Longitudinal Study of Adolescent Health among 12,000 teens found several factors that predicted several sexual restraint:

•  High intelligence Teens with high rather than average intelligence test scores more often delayed sex, partly because they appreciated possible negative consequences and were more focused on future achievement than on here-and-now pleasures (Halpern et al., 2000).

•  Religious engagement Actively religious teens have more often reserved sexual activity for adulthood (Lucero et al., 2008).

•  Father presence In studies that followed hundreds of New Zealand and U.S. girls from age 5 to 18, a father’s absence was linked to sexual activity before age 16 and to teen pregnancy (Ellis et al., 2003). These associations held even after adjusting for other adverse influences, such as poverty. Close family attachments—families that eat together and where parents know their teens’ activities and friends—also predicted later sexual initiation (Coley et al., 2008).

•  Participation in service learning programs Several experiments have found that teens volunteering as tutors or teachers’ aides, or participating in community projects, had lower pregnancy rates than were found among comparable teens randomly assigned to control conditions (Kirby, 2002; O’Donnell et al., 2002). Researchers are unsure why. Does service learning promote a sense of personal competence, control, and responsibility? Does it encourage more future-oriented thinking? Or does it simply reduce opportunities for unprotected sex?

* * *

We have considered some of the research on the biological and psychological aspects of human sexuality. It’s important to remember, though, that scientific research on human sexuality does not aim to define the personal meaning of sex in our own lives. We could know every available fact about sex—that the initial spasms of male and female orgasm come at 0.8-second intervals, that the female nipples expand 10 millimeters at the peak of sexual arousal, that systolic blood pressure rises some 60 points and respiration rate to 40 breaths per minute—but fail to understand the human significance of sexual intimacy.

CLOSE UP: The Sexualization of Girls

As you have surely noticed, TV, the Internet, music videos and lyrics, movies, magazines, sports media, and advertising often portray women and even girls as sexual objects. The frequent result, according to both an American Psychological Association (APA, 2007) task force and the Scottish Parliament (2010), is harm to their self-image, and unhealthy sexual development.

Sexualization occurs when girls

•  are led to value themselves in terms of their sexual appeal.

•  compare themselves to narrowly defined beauty standards.

•  • see themselves as sexual beings for others’ use.

In experiments, the APA task force reported, being made self-conscious about one’s body, such as by wearing a swimsuit, disrupts thinking when doing math computations or logical reasoning. Sexualization also contributes to eating disorders and depression, and to unrealistic expectations regarding sexuality.

Mindful of today’s sexualizing media, the APA has some suggestions for countering these messages. Parents, teachers, and others can teach girls “to value themselves for who they are rather than how they look.” They can teach boys “to value girls as friends, sisters, and girlfriends, rather than as sexual objects.” And they can help girls and boys develop “media literacy skills” that enable them to recognize and resist the message that women are sexual objects and that a thin, sexy look is all that matters.

Sharing love

Surely one significance of sexual intimacy is its expression of our profoundly social nature. One recent study asked 2035 married people when they started having sex (while controlling for education, religious engagement, and relationship length). Those whose relationship first developed to a deep commitment reported greater relationship satisfaction and stability—and better sex (Busby et al., 2010). Sex is a socially significant act. Men and women can achieve orgasm alone, yet most people find greater satisfaction—and experience a much greater surge in the prolactin hormone associated with sexual satisfaction and satiety—after intercourse and orgasm with their loved one (Brody & Tillmann, 2006). Sex at its human best is life uniting and love renewing.

In the remaining pages of this chapter, we’ll consider two special topics: sexual orientation (the direction of our sexual interests), and evolutionary psychology’s explanation of our sexual motivation.

Sexual Orientation

5-9: What has research taught us about sexual orientation?

We express the direction of our sexual interest in our sexual orientation—our enduring sexual attraction toward members of our own sex (homosexual orientation), the other sex (heterosexual orientation), or both sexes (bisexual orientation). As far as we know, all cultures in all times have been predominantly heterosexual (Bullough, 1990). Some cultures have condemned same-sex relations. (In Kenya and Nigeria, 98 percent have thought homosexuality is “never justified” [Pew, 2006].) Others have accepted same-sex unions. But in both cases, heterosexuality prevails and homosexuality endures.

sexual orientation an enduring sexual attraction toward members of either one’s own sex (homosexual orientation), the other sex (heterosexual orientation), or both sexes (bisexual orientation).

How many people are exclusively homosexual? About 10 percent, as the popular press has often assumed? Nearly 25 percent, as average Americans estimated in a 2011 Gallup survey (Morales, 2011)? Not according to more than a dozen national surveys that have explored sexual orientation in Europe and the United States, using methods protecting the respondents’ anonymity. The most accurate figure seems to be about 3 percent of men and 1 or 2 percent of women (Chandra et al., 2011; Herbenick et al., 2010a). Fewer than 1 percent of survey respondents—for example, only 12 people out of 7076 Dutch adults in one survey (Sandfort et al., 2001)—have reported being actively bisexual. A larger number of adults—13 percent of women and 5 percent of men in a U.S. National Center for Health Statistics survey—report some same-sex sexual contact during their lives (Chandra et al., 2011). And still more have had an occasional homosexual fantasy.

What does it feel like to be the “odd man (or woman) out” in a heterosexual culture? If you are heterosexual, one way to understand is to imagine how you would feel if you were socially isolated for openly admitting or displaying your feelings toward someone of the other sex. How would you react if you overheard people making crude jokes about heterosexual people, or if most movies, TV shows, and advertisements portrayed (or implied) homosexuality? And how would you answer if your family members were pleading with you to change your heterosexual life style and to enter into a homosexual marriage?

Driven to suicide

Facing such reactions, homosexual people often struggle with their sexual orientation. They may at first try to ignore or deny their desires, hoping they will go away. But they don’t. Some may try to change, through psychotherapy, willpower, or prayer. But the feelings typically persist, as do those of heterosexual people—who are similarly incapable of becoming homosexual (Haldeman, 1994, 2002; Myers & Scanzoni, 2005).

Most of today’s psychologists therefore view sexual orientation as neither willfully chosen nor willfully changed. “Efforts to change sexual orientation are unlikely to be successful and involve some risk of harm,” declared a 2009 American Psychological Association report. In 1973, the American Psychiatric Association dropped homosexuality from its list of “mental illnesses.” In 1993, the World Health Organization did the same, as did Japan’s and China’s psychiatric associations in 1995 and 2001. Some have noted that rates of depression and attempted suicide are higher among gays and lesbians. Many psychologists believe, however, that these symptoms may result from experiences with bullying, harassment, and discrimination (Sandfort et al., 2001; Warner et al., 2004). “Homosexuality, in and of itself, is not associated with mental disorders or emotional or social problems,” declared the American Psychological Association (2007).

Thus, sexual orientation in some ways is like handedness: Most people are one way, some the other. A very few are ambidextrous. Regardless, the way one is endures.

This conclusion is most strongly established for men. Compared with men’s sexual orientation, women’s tends to be less strongly felt and may be more variable (Chivers, 2005; Diamond, 2008; Peplau & Garnets, 2000). Men’s lesser erotic plasticity (sexual variability) is apparent in many ways (Baumeister, 2000). Adult women’s sexual drive and interests are more flexible and varying than are adult men’s. Women, more than men, for example, prefer to alternate periods of high sexual activity with periods of almost none. They are also somewhat more likely than men to feel and act on bisexual attractions (Mosher et al., 2005).

Environment and Sexual Orientation

So, our sexual orientation is something we do not choose and (especially for males) seemingly cannot change. Where, then, do these preferences come from? Let’s look first at possible environmental influences on sexual orientation. To see if you can anticipate the conclusions that have emerged from hundreds of studies, try answering Yes or No to these questions:

Note that the scientific question is not “What causes homosexuality?” (or “What causes heterosexuality?”) but “What causes differing sexual orientations?” In pursuit of answers, psychological science compares the backgrounds and physiology of people whose sexual orientations differ.

• 1. Is homosexuality linked with problems in a child’s relationships with parents, such as with a domineering mother and an ineffectual father, or a possessive mother and a hostile father?

• 2. Does homosexuality involve a fear or hatred of people of the other sex, leading individuals to direct their desires toward members of their own sex?

• 3. Is sexual orientation linked with levels of sex hormones currently in the blood?

• 4. As children, were most homosexuals molested, seduced, or otherwise sexually victimized by an adult homosexual?

Personal values affect sexual orientation less than they affect other forms of sexual behavior

The answer to all these questions has been No (Storms, 1983). In a search for possible environmental influences on sexual orientation, Kinsey Institute investigators interviewed nearly 1000 homosexuals and 500 heterosexuals. They assessed nearly every imaginable psychological cause of homosexuality—parental relationships, childhood sexual experiences, peer relationships, and dating experiences (Bell et al., 1981; Hammersmith, 1982). Their findings: Homosexuals were no more likely than heterosexuals to have been smothered by maternal love or neglected by their father. And consider this: If “distant fathers” were more likely to produce homosexual sons, then shouldn’t boys growing up in father-absent homes more often be gay? (They are not.) And shouldn’t the rising number of such homes have led to a noticeable increase in the gay population? (It has not.) Most children raised by gay or lesbian parents grow up straight and well-adjusted (Gartrell & Bos, 2010).

A bottom line has emerged from a half-century’s theory and research: If there are environmental factors that influence sexual orientation, we do not yet know what they are.

Biology and Sexual Orientation

The lack of evidence for environmental causes of homosexuality has motivated researchers to explore possible biological influences. They have considered

•  evidence of homosexuality in other species,

•  gay-straight brain differences,

•  genetics, and

•  prenatal hormones.

Same-Sex Attraction in Other Species

In Boston’s Public Gardens, caretakers have solved the mystery of why a much-loved swan couple’s eggs never hatch. Both swans are female. In New York City’s Central Park Zoo, penguins Silo and Roy spent several years as devoted same-sex partners. At least occasional same-sex relations have been observed in several hundred species (Bagemihl, 1999). Grizzlies, gorillas, monkeys, flamingos, and owls are all on the long list. Among rams, for example, some 7 to 10 percent (to sheep-breeding ranchers, the “duds”) display same-sex attraction by shunning ewes and seeking to mount other males (Perkins & Fitzgerald, 1997). Some degree of homosexual behavior seems a natural part of the animal world.

Gay-Straight Brain Differences

Researcher Simon LeVay (1991) studied sections of the hypothalamus (a brain structure linked to emotion) taken from deceased heterosexual and homosexual people. As a gay man, LeVay wanted to do “something connected with my gay identity.” To avoid biasing the results, he did a blind study, without knowing which donors were gay or straight. After nine months of peering through his microscope at a hypothalamus cell cluster that seemed to come in different sizes, he consulted the donor records. The cell cluster was reliably larger in heterosexual men than in women and homosexual men. “I was almost in a state of shock,” LeVay said (1994). “I took a walk by myself on the cliffs over the ocean. I sat for half an hour just thinking what this might mean.”

It should not surprise us that brains differ with sexual orientation. Remember, everything psychological is simultaneously biological. But when did the brain difference begin? At conception? During childhood or adolescence? Did experience produce the difference? Or was it genes or prenatal hormones (or genes via prenatal hormones)?

LeVay does not view this cell cluster as an “on-off button” for sexual orientation. Rather, he believes it is an important part of a brain pathway that is active during sexual behavior. He agrees that sexual behavior patterns could influence the brain’s anatomy. (Neural pathways in our brain do grow stronger with use.) In fish, birds, rats, and humans, brain structures vary with experience—including sexual experience (Breedlove, 1997). But LeVay believes it more likely that brain anatomy influences sexual orientation. His hunch seems confirmed by the discovery of a similar difference found between the 7 to 10 percent of male sheep that display same-sex attraction and the 90+ percent attracted to females (Larkin et al., 2002; Roselli et al., 2002, 2004). Moreover, such differences seem to develop soon after birth, perhaps even before birth (Rahman & Wilson, 2003).

“Gay men simply don’t have the brain cells to be attracted to women.”

Simon LeVay, The Sexual Brain, 1993

Since LeVay’s discovery, other researchers have reported additional gay-straight brain activity differences. One is an area of the hypothalamus that governs sexual arousal (Savic et al., 2005). When straight women were given a whiff of a scent derived from men’s sweat (which contains traces of male hormones), this area became active. Gay men’s brains responded similarly to the men’s scent. Straight men’s brains did not. They showed the arousal response only to a female hormone sample. In a similar study, lesbians’ responses differed from those of straight women (Kranz & Ishai, 2006; Martins et al., 2005).

Genetic Influences

Three lines of evidence suggest a genetic influence on sexual orientation.

Family Studies

“Homosexuality does appear to run in families,” noted Brian Mustanski and Michael Bailey (2003). Researchers have speculated about possible reasons why “gay genes” might persist in the human gene pool, given that same-sex couples cannot naturally reproduce. One possible answer is kin selection. Recall from Chapter 2 the evolutionary psychology reminder that many of our genes also reside in our biological relatives. Perhaps, then, gay people’s genes live on through their supporting the survival and reproductive success of their nieces, nephews, and other relatives (who also carry many of the same genes). Gay men make generous uncles, suggests one study of Samoans (Vasey & VanderLaan, 2010).

“Studies indicate that male homosexuality is more likely to be transmitted from the mother’s side of the family.”

Robert Plomin, John DeFries, Gerald McClearn, and Michael Rutter, Behavioral Genetics, 1997

An alternative “fertile females” theory suggests that maternal genetics may also be at work (Bocklandt et al., 2006). Homosexual men tend to have more homosexual relatives on their mother’s side than on their father’s (Camperio-Ciani et al., 2004, 2009; Zietsch et al., 2008). And the relatives on the mother’s side also produce more offspring than do the maternal relatives of heterosexual men. Perhaps the genes that dispose women to be strongly attracted to men, and therefore to have more children, also dispose some men to be attracted to men (LeVay, 2011).

Twin Studies

Twin studies indicate that genes influence sexual orientation. Identical twins (who have identical genes) are somewhat more likely than fraternal twins (whose genes are not identical) to share a homosexual orientation (Alanko et al., 2010; Lángström et al., 2008, 2010). However, because sexual orientation differs in many identical twin pairs (especially female twins), other factors must also play a role.

Fruit Fly Studies

Laboratory experiments on fruit flies have altered a single gene and changed the flies’ sexual orientation and behavior (Dickson, 2005). During courtship, females acted like males (pursuing other females) and males acted like females (Demir & Dickson, 2005). With humans, it’s likely that multiple genes, possibly in interaction with other influences, shape sexual orientation. In search of such genetic markers, one study financed by the U.S. National Institutes of Health is analyzing the genes of more than 1000 gay brothers.

Prenatal Influences

Twins share not only genes, but also a prenatal environment. Two sets of findings indicate that the prenatal environment matters.

“Modern scientific research indicates that sexual orientation is…partly determined by genetics, but more specifically by hormonal activity in the womb.”

Glenn Wilson and Qazi Rahman, Born Gay: The Psychobiology of Sex Orientation, 2005

First, in humans, a critical period for brain development seems to fall between the middle of the second and fifth months after conception (Ellis & Ames, 1987; Gladue, 1990; Meyer-Bahlburg, 1995). Exposure to the hormone levels typically experienced by female fetuses during this period may predispose a person (female or male) to be attracted to males in later life. When pregnant sheep were injected with testosterone during a similar critical period, their female offspring later showed homosexual behavior (Money, 1987).

Second, the mother’s immune system may play a role in the development of sexual orientation. Men who have older brothers are somewhat more likely to be gay—about one-third more likely for each additional older brother (Blanchard, 1997, 2008; Bogaert, 2003). If the odds of homosexuality are roughly 2 percent among first sons, they would rise to nearly 3 percent among second sons, 4 percent for third sons, and so on for each additional older brother. The reason for this curious effect—called the older-brother or fraternal birth-order effect—is unclear. But the explanation does seem biological. The effect does not occur among adopted brothers (Bogaert, 2006). Researchers suspect the mother’s immune system may have a defensive response to substances produced by male fetuses. After each pregnancy with a male fetus, the maternal antibodies may become stronger and may prevent the fetal brain from developing in a typical male pattern.

Gay-Straight Trait Differences

On several traits, gays and lesbians appear to fall midway between straight females and males (TABLE 5.1; see also LeVay, 2011; Rahman & Koerting, 2008). Gay men tend to be shorter and lighter than straight men—a difference that appears even at birth. Women in same-sex marriages were mostly heavier than average at birth (Bogaert, 2010; Frisch & Zdravkovic, 2010). Data from 20 studies have also revealed handedness differences: Homosexual participants were 39 percent more likely to not be right-handed (Blanchard, 2008; Lalumière et al., 2000).

Table 5.1: Biological Correlates of Sexual Orientation

Gay-straight spatial abilities also differ. On mental rotation tasks, straight men tend to outscore straight women but the scores of gays and lesbians fall between those of straight men and women (Rahman et al., 2003). But straight women and gays both outperform straight men at remembering objects’ spatial locations in tasks like those found in memory games (Hassan & Rahman, 2007).

“There is no sound scientific evidence that sexual orientation can be changed.”

UK Royal College of Psychiatrists, 2009

The consistency of the brain, genetic, and prenatal findings has swung the pendulum toward a biological explanation of sexual orientation (LeVay, 2011; Rahman & Koerting, 2008). Although “much remains to be discovered,” concludes Simon LeVay (2011, p. xvii), “the same processes that are involved in the biological development of our bodies and brains as male or female are also involved in the development of sexual orientation.”

An Evolutionary Explanation of Human Sexuality

5-10: How might an evolutionary psychologist explain gender differences in sexuality and mating preferences?

Having faced many similar challenges throughout history, men and women have adapted in similar ways. Whether male or female, we eat the same foods, avoid the same predators, and perceive, learn, and remember similarly. It is only in those domains where we have faced differing adaptive challenges—most obviously in behaviors related to reproduction—that we differ, say evolutionary psychologists.

Gender Differences in Sexuality

And differ we do. Consider women’s and men’s sex drives. Who desires more frequent sex? Thinks more about sex? Masturbates more often? Sacrifices more to gain sex? Initiates more sex—worldwide? The answers—Men, men, men, men, and men (Baumeister et al., 2001). No surprise, then, that in one BBC survey of more than 200,000 people in 53 nations, men everywhere more strongly agreed that “I have a strong sex drive” and “It doesn’t take much to get me sexually excited” (Lippa, 2008).

And there are other gender differences in sexuality (Hyde, 2005; Petersen & Hyde, 2010; Regan & Atkins, 2007). In a survey of 289,452 entering U.S. college students, 58 percent of men but only 34 percent of women agreed that “if two people really like each other, it’s all right for them to have sex even if they’ve known each other for a very short time” (Pryor et al., 2005). “I can imagine myself being comfortable and enjoying ‘casual’ sex with different partners,” agreed 48 percent of men and 12 percent of women in a survey of 4901 Australians (Bailey et al., 2000). Thus, university men in one study preferred casual hook-ups, while women preferred planned dating (Bradshaw et al., 2010). Casual, impulsive sex has been most frequent among males with traditional masculine attitudes (Pleck et al., 1993).

“It’s not that gay men are oversexed; they are simply men whose male desires bounce off other male desires rather than off female desires.”

Steven Pinker, How the Mind Works, 1997

In surveys, gay men (like straight men) report more interest in uncommitted sex, more responsiveness to visual sexual stimuli, and more concern with their partner’s physical attractiveness than do lesbian women (Bailey et al., 1994; Doyle, 2005; Schmitt, 2007).

In another survey of U.S. 18- to 59-year-olds, 48 percent of the women but only 25 percent of the men cited affection as a reason for first intercourse. And how often do they think about sex? “Every day” or “Several times a day,” acknowledged 19 percent of the women and 54 percent of the men (Laumann et al., 1994). Ditto for the sexual thoughts of Canadians: “Several times a day,” agreed 11 percent of women and 46 percent of men (Fischtein et al., 2007).

Natural Selection and Mating Preferences

Evolutionary psychologists use natural selection to explain why—worldwide—women’s approach to sex is usually more relational, and men’s more recreational (Schmitt, 2005, 2007). Our natural yearnings are our genes’ way of reproducing themselves. “Humans are living fossils—collections of mechanisms produced by prior selection pressures,” said evolutionary psychologist David Buss (1995).

The explanation goes like this: Most women incubate and nurse one infant at a time. Men, however, can spread their genes by mating with many females. In our ancestral history, men most often sent their genes into the future by pairing widely, women by pairing wisely. Thus, heterosexual women prefer stick-around dads over likely cads. They are attracted to men who seem mature, dominant, bold, and affluent, with a potential for long-term mating and investment in their joint offspring (Gangestad & Simpson, 2000; Singh, 1995). In one study of hundreds of Welsh pedestrians, men rated a woman as equally attractive whether pictured at the wheel of a humble Ford Fiesta or a swanky Bentley. Women, however, found the man more attractive if he was in the luxury car (Dunn & Searle, 2010). From an evolutionary perspective, such attributes connote a man’s capacity to support and protect a family (Buss, 1996, 2009; Geary, 1998).

For heterosexual men, some desired traits, such as a woman’s youthful appearance, cross place and time (Buss, 1994). Evolutionary psychologists say that men who were drawn to healthy, fertile-appearing women—women with smooth skin and a youthful shape suggesting many childbearing years to come—stood a better chance of sending their genes into the future. And sure enough, men feel most attracted to women whose waists (thanks to their genes or their surgeons) are roughly a third narrower than their hips—a sign of future fertility (Perilloux et al., 2010).

Moreover, just as evolutionary psychology predicts, men are most attracted to women whose ages in the ancestral past (when ovulation began later than today) would have been associated with peak fertility (Kenrick et al., 2009). Thus, teen boys are most excited by a woman several years older than themselves, mid-twenties men prefer women around their own age, and older men prefer younger women. This pattern consistently appears across European singles ads, Indian marital ads, and marriage records from North and South America, Africa, and the Philippines (Singh, 1993; Singh & Randall, 2007).

There is a principle at work here, say evolutionary psychologists: Nature selects behaviors that increase the likelihood of sending one’s genes into the future. As mobile gene machines, we are designed to prefer whatever worked for our ancestors in their environments. They were predisposed to act in ways that would leave grandchildren. Had they not been, we wouldn’t be here. And as carriers of their genetic legacy, we are similarly predisposed.

Critiquing the Evolutionary Perspective

5-11: What are the key criticisms of evolutionary explanations of human sexuality, and how do evolutionary psychologists respond?

Without disputing nature’s selection of traits that enhance gene survival, critics see some problems with evolutionary psychology’s explanation of our mating preferences. One is that evolutionary psychology starts with an effect (such as the gender sexuality difference) and works backwards to propose an explanation. They invite us to imagine a different result and reason backwards. If men were uniformly loyal to their mates, might we not reason that the children of these committed, supportive fathers would more often survive to pass on their father’s genes? Might not this bond with one woman also increase their otherwise slim odds of conceiving a child, while also preventing her from mating with competing men? Might not a ritualized bond—a marriage—also spare women from chronic male harassment? Such suggestions are, in fact, evolutionary explanations for why humans tend to pair off monogamously (Gray & Anderson, 2010). One can hardly lose at hindsight explanation, which is, said paleontologist Stephen Jay Gould (1997), mere “speculation [and] guesswork in the cocktail party mode.”

Some also worry about the social consequences of evolutionary psychology’s approach. Does it suggest that genes are destiny? Does it mean that any effort to remake society is useless (Rose, 1999)? Does it undercut moral responsibility (Buller, 2005, 2009)? Could it be used to rationalize “high-status men marrying a series of young, fertile women” (Looy, 2001)?

Others argue that evolutionary explanations blur the line between genetic legacy and social-cultural tradition. Cultural expectations also shape socialization. If men are socialized to value lifelong commitment, they may sexually bond with one partner. If women are socialized to accept casual sex, they may willingly have sex with many partners.

Cultural expectations can bend the genders. Show Alice Eagly and Wendy Wood (1999; Eagly, 2009) a culture with gender inequality—where men are providers and women are homemakers—and they will show you a culture where men strongly desire youth and domestic skill in their potential mates, and where women seek status and earning potential in their mates. Show Eagly and Wood a culture with gender equality, and they will show you a culture with smaller gender differences in mate preferences.

Much of who we are is not hard-wired, agree evolutionary psychologists. “Evolution forcefully rejects a genetic determinism,” insists one research team (Confer et al., 2010). Evolutionary psychologists remind us that men and women, having faced similar adaptive problems, are far more alike than different, and that humans have a great capacity for learning and social progress. Indeed, natural selection has prepared us to flexibly adjust and respond to varied environments, to adapt and survive, whether we live in igloos or tree houses.

Culture matters

Evolutionary psychologists acknowledge struggling to explain some traits and behaviors such as same-sex attraction and suicide (Confer et al., 2010). But they ask us to remember evolutionary psychology’s testable predictions. Evolutionary psychologists predict, and have confirmed, that we tend to favor others to the extent that they share our genes or can later return our favors. They predict, and have confirmed, that human memory should be well-suited to retaining survival-relevant information (such as food locations, for which females exhibit superiority). They predict, and have confirmed, various other male and female mating strategies.

Evolutionary psychologists also remind us that the study of how we came to be need not dictate how we ought to be. Understanding our tendencies sometimes helps us overcome them.

Reflections on Gender, Sexuality, and Nature–Nurture Interaction

Our ancestral history helped form us as a species. Where there is variation, natural selection, and heredity, there will be evolution. Our genes form us. This is a great truth about human nature.

But our culture and experiences also form us. If their genes and hormones predispose males to be more physically aggressive than females, culture may magnify this gender difference by encouraging males to be macho and females to be the kinder, gentler sex. If men are encouraged toward roles that demand physical power, and women toward more nurturing roles, each may act accordingly. By exhibiting the actions expected of those who fill such roles, they will shape their own traits. Presidents in time become more presidential, servants more servile. Gender roles similarly shape us.

Today, in many cultures, gender roles are converging. Brute strength is becoming increasingly irrelevant to power and status (think Bill Gates and Hillary Clinton). From 1960 into the next century, women soared from 6 percent to 50 percent of U.S. medical students (AMA, 2010). In the mid-1960s, U.S. married women devoted seven times as many hours to housework as did their husbands; by 2003 this gap had shrunk to two times as many (Bianchi et al., 2000, 2006). Such swift changes signal that biology does not fix gender roles.

If nature and nurture jointly form us, are we “nothing but” the product of nature and nurture? Are we rigidly determined?

We are the product of nature and nurture, but we are also an open system. Genes are all pervasive but not all powerful; people may defy their genetic bent to reproduce by electing celibacy. Culture, too, is all pervasive but not all powerful; people may defy peer pressures and do the opposite of the expected. To excuse our failings by blaming our nature and nurture is what philosopher-novelist Jean-Paul Sartre called “bad faith”—attributing responsibility for one’s fate to bad genes or bad influences.

In reality, we are both the creatures and the creators of our worlds. So many things about us—including our gender identity and mating behaviors—are the products of our genes and environments. Yet the future-shaping stream of causation runs through our present choices: Our decisions today design our environments tomorrow. Mind matters. The human environment is not like the weather—something that just happens. We are its architects. Our hopes, goals, and expectations influence our future. And that is what enables cultures to vary and to change.