Sex is a part of life. For all but the tiny fraction of us considered asexual, dating and mating become a high priority from puberty on. Physiological and psychological influences affect our sexual feelings and behaviors.

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 motivates. 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 ancestral 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.

11-5 How do hormones influence human sexual motivation?

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

In most mammals, nature neatly synchronizes sex with fertility. Females become sexually receptive (in other animals, “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 slowly regain it if injected with testosterone.

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Hormones do influence human sexual behavior, but in a looser way. Among women with mates, sexual desire rises slightly at ovulation, when there is a surge of estrogens and a smaller surge of testosterone, a change that men can sometimes detect in women’s behaviors and voices (Haselton & Gildersleeve, 2011). One study invited partnered women to keep a diary of their sexual activity. On the days around ovulation, intercourse was 24 percent more frequent (Wilcox et al., 2004).

Women have much less testosterone than men. And more than other mammalian females, women are responsive to their testosterone level (van Anders, 2012). 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 (Escasa et al., 2011). 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, international studies have found that married fathers tend to have lower testosterone levels than do bachelors and married men without children (Gettler et al., 2013; Gray et al., 2006).

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

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.

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11-6 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 reported observing more than 10,000 sexual “cycles.” Their description of the sexual response cycle identified four stages:

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).

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Therapy can help men and women with sexual dysfunctions (Frühauf et al., 2013). 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

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

11-7 How can sexually transmitted infections be prevented?

Worldwide, more than 1 million people acquire a sexually transmitted infection (STI; also called STD for sexually transmitted disease) every day (WHO, 2013). 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 and Prevention 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).

Most Americans with AIDS have been 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 2012, the death of 1.6 million people with AIDS worldwide left behind countless grief-stricken partners and millions of orphaned children (UNAIDS, 2013). Sub-Saharan Africa is home to two-thirds of those infected with HIV, and medical treatment that extends life and care for the dying are sapping the region’s social resources.

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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 HPV infections can now be prevented with a vaccination administered before sexual contact.

11-8 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 (FIGURE 11.9). Thus, despite the shared biology that underlies sexual motivation, 281 expressed reasons for having sex ranged widely—from “to get closer to God” to “to get my boyfriend to shut up” (Buss, 2008; Meston & Buss, 2007).

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 indicates that it can:

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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. Lacking genital sensation because of a spinal-cord injury, people 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, which in a few women can produce orgasms (Komisaruk & Whipple, 2011). 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 (Schmitt et al., 2012). 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.

11-9 What factors influence teenagers’ sexual behaviors and use of contraceptives?

Sexual attitudes and behaviors vary dramatically across cultures. “Sex between unmarried adults” is “morally unacceptable,” agree 97 percent of Indonesians, 58 percent of Chinese, 30 percent of Americans, and 6 percent of Germans (Pew, 2014). We are all one species, but in some ways how differently we think. Compared with European teens, today’s American teens have a higher pregnancy rate—but a lower rate than their parents’ generation (CDC, 2011, 2012).

So, what produces these variations in teen sexuality and pregnancy? Twin studies show that genes influence teen sexual behavior—by influencing pubertal development and hormone levels, and also by influencing teen exposure to environments that stimulate sexual activity (Harden, 2014). Other influences include:

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 also reduce planning for birth control (Gerrard & Luus, 1995; MacDonald & Hynie, 2008).

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Alcohol use Most sexual hook-ups occur among people who are mildly to very intoxicated (Fielder et al., 2013; Garcia et al., 2013). 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? Sexual content appears in approximately 85 percent of movies, 82 percent of television programs, 59 percent of music videos, and 37 percent of music lyrics (Ward et al., 2014). And sexual partners on TV shows rarely have communicated any concern for birth control or STIs (Brown et al., 2002; Kunkel, 2001; Sapolski & Tabarlet, 1991). The more sexual content adolescents and young adults view or read (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; Kim & Ward, 2012; Parkes et al., 2014).

Media influences can either increase or decrease sexual risk taking. One study asked more than a thousand 12- to 14-year-olds what movies they had seen, and then after age 18 asked them about their teen sexual experiences (O’Hara et al., 2012). After controlling for various adolescent and family characteristics, the more the adolescents viewed movies with high sexual content, the greater was their sexual risk taking—with earlier debut, more partners, and inconsistent condom use. Another study analyzed the effect of MTV’s series 16 and Pregnant, which portrayed the consequences of unprotected sex and the challenges of having a child. By analyzing viewership and pregnancy rates over time in specific areas, the researchers concluded that the program led to a 6 percent reduction in the national teen pregnancy rate (Kearney & Levine, 2014).

Later sex may pay emotional dividends. One national study followed participants to about age 30. Even after controlling for several other factors, those who had later first sex reported greater relationship satisfaction in their marriages and partnerships (Harden, 2012). Several other factors also predict sexual restraint:

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11-10 What has research taught us about sexual orientation?

To motivate is to energize and direct behavior. So far, we have considered the energizing of sexual motivation but not its direction, which is 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). We experience this attraction in our interests, thoughts, and fantasies (who’s that person in your imagination?). Cultures vary in their attitudes toward same-sex attractions. “Should society accept homosexuality?” Yes, say 88 percent of Spaniards, 80 percent of Canadians, 60 percent of Americans, 39 percent of South Koreans, 21 percent of Chinese, and 1 percent of Nigerians, with women everywhere being more accepting than men (Pew, 2013). Yet whether a culture condemns or accepts same-sex unions, heterosexuality prevails.

How many people are exclusively homosexual? About 10 percent, as the popular press has often assumed? Or 20 percent, as the average American estimated in a 2013 survey (Jones et al., 2014)? According to more than a dozen national surveys that have explored sexual orientation in Europe and the United States, a better estimate is about 3 or 4 percent of men and 2 percent of women (Chandra et al., 2011; Herbenick et al., 2010a; Savin-Williams et al., 2012). When Gallup asked 121,290 Americans about their sexual identity—“Do you, personally, identify as lesbian, gay, bisexual, or transgender?”—3.4 percent answered Yes (Gates & Newport, 2012). When the National Center for Health Statistics asked 34,557 Americans about their sexual identity, they found essentially the same result: All but 3.4 percent answered “straight,” with 1.6 percent answering “gay” or “lesbian” and 0.7 percent saying “bisexual” (Ward et al., 2014).

Survey methods that absolutely guarantee people’s anonymity reveal another percent or two of gay people (Coffman et al., 2013). Moreover, people in less tolerant places are more likely to hide their sexual orientation. About 3 percent of California men express a same-sex preference on Facebook, for example, as do only about 1 percent in Mississippi. Yet about 5 percent of Google pornography searches in both states are for gay porn. And Craigslist ads for males seeking “casual encounters” with other men tend to be at least as large in less tolerant states, where there are also more Google searches for “Is my husband gay?” (Stephens-Davidowitz, 2013).

Fewer than 1 percent of people—for example, only 12 people out of 7076 Dutch adults in one survey (Sandfort et al., 2001)—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. In laboratory assessments, some self-identified bisexual men show a homosexual arousal pattern by responding with genital arousal mostly to male erotic images. Others exhibit increased viewing time and genital arousal to both male and female images (Cerny & Janssen, 2011; Lippa, 2013; Rieger et al., 2013; Rosenthal et al., 2012).

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What does it feel like to be homosexual 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 lifestyle and to enter into a homosexual marriage?

Facing such reactions, some individuals struggle with their sexual attractions, especially during adolescence and if feeling rejected by parents or harassed by peers. If lacking social support, the result may be lower self-esteem and higher anxiety and depression (Jager & Davis-Kean, 2011; Kwon, 2013; Oswalt & Wyatt, 2011), as well as an increased risk of contemplating suicide (Plöderl et al., 2013; Ryan et al., 2009; Wang et al., 2012). They may at first try to ignore or deny their desires, hoping they will go away. But they don’t. Then they 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). Moreover, as we noted in Chapter 6, people’s sexual orientation is so basic to who they are that it operates unconsciously, as seen in experiments that draw their attention toward particular flashed nude images not consciously perceived.

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

This conclusion is most strongly established for men. Women’s sexual orientation tends to be less strongly felt and potentially more fluid and changing (Chivers, 2005; Diamond, 2008; Dickson et al., 2013). In general, men are sexually simpler. Their lesser sexual variability is apparent in many ways, notes Roy Baumeister (2000). Across time, across cultures, across situations, and across differing levels of education, religious observance, and peer influence, adult women’s sexual drive and interests are more flexible and varying than are adult men’s. Women, for example, more often prefer to alternate periods of high sexual activity with periods of almost none (Mosher et al., 2005). In their pupil dilation and genital responses to erotic videos, and in their implicit attitudes, heterosexual women exhibit more bisexual attraction than do men (Rieger & Savin-Williams, 2012; Snowden & Gray, 2013). Baumeister calls women’s more varying sexuality a difference in erotic plasticity.

In men, a high sex drive is associated with increased attraction to women (if heterosexual), or men (if homosexual). In women, a high sex drive is generally associated with increased attraction to both men and women (Lippa, 2006, 2007a; Lippa et al., 2010). When shown sexually explicit film clips, men’s genital and subjective sexual arousal is mostly to preferred sexual stimuli (for heterosexual viewers, depictions of women). Women respond more nonspecifically to depictions of sexual activity involving males or females (Chivers et al., 2007).

Is there truth to the homosexual-as-child-molester stereotype? No. Measuring men’s genital response to various sexual images indicates that sexual orientation is unrelated to pedophilia (Blanchard et al., 2009; Herek, 2014). A Canadian research team led by Ray Blanchard (2012; Dreger, 2011) outfitted 2278 men (mostly sex offenders) with a device that measured their “phallometric response” to viewing nude photos of adults and children of both sexes, accompanied by sexual audio stories. Most of the men responded not to children, but to adult men (if gay) or to adult women (if straight). Some of the men exhibited pedophilia, by instead responding mostly to young boys or girls, and much less to adults.

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So, our sexual orientation is something we do not choose and (especially for males) cannot change. Where, then, do these preferences come from? See if you can anticipate the conclusions that have emerged from hundreds of research studies by responding Yes or No to the following questions:

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, dating experiences (Bell et al., 1981; Hammersmith, 1982). Their findings: Homosexuals are no more likely than heterosexuals to have been smothered by maternal love or neglected by their father. In one national survey of nearly 35,000 adults, those with a same-sex attraction were somewhat more likely to report having experienced child sexual abuse. But 86 percent of the men and 75 percent of the women with same-sex attraction reported no such abuse (Roberts et al., 2013).

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).

So, what else might influence sexual orientation? One theory has proposed that people develop same-sex erotic attachments if segregated by sex at the time their sex drive matures (Storms, 1981). Indeed, gay men tend to recall going through puberty somewhat earlier, when peers are more likely to be all males (Bogaert et al., 2002). But even in tribal cultures in which homosexual behavior is expected of all boys before marriage, heterosexuality prevails (Hammack, 2005; Money, 1987). (As this illustrates, homosexual behavior does not always indicate a homosexual orientation.) Moreover, though peer network attitudes predict teens’ sexual attitudes and behavior, they do not predict same-sex attraction. “Peer influence has little or no effect” on sexual orientation (Brakefield et al., 2014).

The bottom line 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. This reality has motivated researchers to explore same-sex behaviors in animals and to consider gay-straight brain differences, genetics, and prenatal influences.

Same-Sex Attraction in Other Species In Boston’s Public Gardens, caretakers solved the mystery of why a much-loved swan couple’s eggs never hatched. Both swans were female. In New York City’s Central Park Zoo, penguins Silo and Roy spent several years as devoted same-sex partners. Same-sex sexual behaviors have also been observed in several hundred other species, including grizzlies, gorillas, monkeys, flamingos, and owls (Bagemihl, 1999). Among rams, for example, some 7 to 10 percent display same-sex attraction by shunning ewes and seeking to mount other males (Perkins & Fitzgerald, 1997). Homosexual behavior seems a natural part of the animal world.

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Gay-Straight Brain Differences Researcher Simon LeVay (1991) studied sections of the hypothalamus taken from deceased heterosexual and homosexual people. As a gay scientist, LeVay wanted to do “something connected with my gay identity.” To avoid biasing the results, he did a blind study, not knowing which donors were gay. For nine months he peered through his microscope at a cell cluster he thought might be important. Then, one morning, he broke the code: One 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 in other ways, too, brains differ with sexual orientation (Bao & Swaab, 2011; Savic & Lindström, 2008). Remember our maxim: Everything psychological is simultaneously biological. But when do such brain differences begin? At conception? In the womb? During childhood or adolescence? Does experience produce these differences? Or is it genes or prenatal hormones (or genes via prenatal hormones)?

LeVay does not view the hypothalamus as a sexual orientation center; rather, he sees it as an important part of the neural pathway engaged in sexual behavior. He acknowledges that sexual behavior patterns may influence the brain’s anatomy. In fish, birds, rats, and humans, brain structures vary with experience—including sexual experience, reports sex researcher Marc Breedlove (1997). But LeVay believes it more likely that brain anatomy influences sexual orientation. His hunch seems confirmed by the discovery of a similar hypothalamic difference between the male sheep that do and don’t display same-sex attraction (Larkin et al., 2002; Roselli et al., 2002, 2004). Moreover, report University of London psychologists Qazi Rahman and Glenn Wilson (2003), “the neuroanatomical correlates of male homosexuality differentiate very early postnatally, if not prenatally.”

Responses to hormone-derived sexual scents also point to a brain difference (Savic et al., 2005). When straight women were given a whiff of a scent derived from men’s sweat, their hypothalamus activated in an area governing sexual arousal. Gay men’s brains responded similarly to the men’s scent. But straight men’s brains showed the arousal response only to a female hormone derivative. Other studies of brain responses to sex-related sweat odors and to pictures of male and female faces have found similar gay-straight differences, including differing responses between lesbians and straight women (Kranz & Ishai, 2006; Martins et al., 2005).

Genetic Influences Evidence indicates a genetic influence on sexual orientation. “First, homosexuality does appear to run in families,” noted Brian Mustanski and Michael Bailey (2003). “Second, twin studies have established that genes play a substantial role in explaining individual differences in sexual orientation.” Identical twins are somewhat more likely than fraternal twins to share a homosexual orientation (Alanko et al., 2010; Lángström et al., 2008, 2010). (Because sexual orientations differ in many identical twin pairs, especially female twins, we know that other factors besides genes are also at work.)

By genetic manipulations, experimenters have created female fruit flies that during courtship act like males (pursuing other females) and males that act like females (Demir & Dickson, 2005). “We have shown that a single gene in the fruit fly is sufficient to determine all aspects of the flies’ sexual orientation and behavior,” explained Barry Dickson (2005). With humans, it’s likely that multiple genes, possibly in interaction with other influences, shape sexual orientation. A genome-wide study of 409 pairs of gay brothers identified sexual orientation links with areas of two chromosomes, one maternally transmitted (Sanders et al., 2014). And molecules attached to genes may affect their expression. Some scientists speculate that, by affecting sensitivity to testosterone, such epigenetic influences might also cause the masculinization of females in the womb, or the feminization of males (Rice et al., 2012).

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Researchers have speculated about possible reasons why “gay genes” might exist in the human gene pool, given that same-sex couples cannot naturally reproduce. One possible answer is kin selection. Recall from Chapter 4 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).

A fertile females theory offers further support for the idea that maternal genetics may be at work (Bocklandt et al., 2006). Recent Italian studies confirm what others have found—that homosexual men tend to have more homosexual relatives on their mother’s side than on their father’s (Camperio-Ciani et al., 2004, 2009, 2012; VanderLaan et al., 2011, 2012). 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 (or attractive) to men, and therefore to have more children, also dispose some men to be attracted to men (LeVay, 2011). Thus, the decreased reproduction by gay men appears offset by the increased reproduction by their maternal extended family.

Prenatal Influences Elevated rates of homosexual orientation in identical and fraternal twins suggest the influence not only of shared genes but also a shared prenatal environment. In animals and some human cases, prenatal hormone conditions have altered a fetus’ sexual orientation. German researcher Gunter Dorner (1976, 1988) pioneered research on the influence of prenatal hormones by manipulating a fetal rat’s exposure to male hormones, thereby “inverting” its sexual orientation. In other studies, when pregnant sheep were injected with testosterone during a critical period of fetal development, their female offspring later showed homosexual behavior (Money, 1987).

A critical period for the human brain’s neural-hormonal control system may exist 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 time appears to predispose the person (whether female or male) to be attracted to males in later life. “Prenatal sex hormones control the sexual differentiation of brain centers involved in sexual behaviors,” notes Simon LeVay (2011, p. 216). Thus, female fetuses most exposed to testosterone, and male fetuses least exposed to testosterone, appear most likely later to exhibit gender-atypical traits and to experience same-sex desires.

The mother’s immune system may also play a role in the development of sexual orientation. Men who have older brothers are somewhat more likely to be gay, report Ray Blanchard (2004, 2008a,b, 2014) and Anthony Bogaert (2003)—about one-third more likely for each additional older brother. 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 (see FIGURE 11.10). The reason for this curious phenomenon—the older brother or fraternal birth-order effect—is unclear. Blanchard suspects a defensive maternal immune response to foreign substances produced by male fetuses. With each pregnancy with a male fetus, the maternal antibodies may become stronger and may prevent the fetus’ brain from developing in a male-typical pattern. Consistent with this biological explanation, the fraternal birth-order effect occurs only in men with older brothers born to the same mother (whether raised together or not). Sexual orientation is unaffected by adoptive brothers (Bogaert, 2006b). The birth-order effect on sexual orientation is not found among women with older sisters, women who were wombmates of twin brothers, and men who are not right-handed (Rose et al., 2002).

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On several traits, gays and lesbians appear to fall midway between straight females and males (TABLE 11.2; see also LeVay, 2011; Rahman & Koerting, 2008). For example, lesbians’ cochlea and hearing systems develop in a way that is intermediate between those of heterosexual females and heterosexual males, which seems attributable to prenatal hormonal influence (McFadden, 2002). Gay men tend to be shorter and lighter, even at birth, than straight men, while women in same-sex marriages were mostly heavier than average at birth (Bogaert, 2010; Frisch & Zdravkovic, 2010). Fingerprint ridge counts may also differ: Although most people have more fingerprint ridges on their right hand than on their left, some studies find a greater right-left difference in heterosexual males than in females and gay males (Hall & Kimura, 1994; Mustanski et al., 2002; Sanders et al., 2002). Given that fingerprint ridges are complete by the sixteenth fetal week, this difference may be due to prenatal hormones.

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Another you-never-would-have-guessed-it gay-straight difference appears in studies showing that gay men’s spatial abilities resemble those typical of straight women (Cohen, 2002; Gladue, 1994; McCormick & Witelson, 1991; Sanders & Wright, 1997). On mental rotation tasks such as the one illustrated in FIGURE 11.11, straight men tend to outscore straight women. (So do women who were wombmates of a male co-twin [Vuoksimaa et al., 2010].) Studies by Qazi Rahman and colleagues (2004, 2008) find that, as on a number of other measures, the scores of gays and lesbians fall between those of heterosexual males and females. 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).

***

The consistency of the brain, genetic, and prenatal findings has swung the pendulum toward a biological explanation of sexual orientation (Rahman & Wilson, 2003; Rahman & Koerting, 2008). Still, some people wonder: Should the cause of sexual orientation matter? Perhaps it shouldn’t, but people’s assumptions matter. To justify his signing a 2014 bill that made some homosexual acts punishable by life in prison, the president of Uganda, Yoweri Museveni, declared that homosexuality is not inborn but rather is a matter of “choice” (Balter, 2014; Landau et al., 2014).

However, the new biological research is a double-edged sword (Diamond, 1993; Roan, 2010). If sexual orientation, like skin color and sex, is genetically influenced, that offers a further rationale for civil rights protection. At the same time, this research raises the troubling possibility that genetic markers of sexual orientation could someday be identified through fetal testing, that a fetus could be aborted simply for being predisposed to an unwanted orientation, or that hormonal treatment in the womb might engineer a desired orientation.

11-11 Is scientific research on sexual motivation value free?

Recognizing that values are both personal and cultural, most sex researchers and educators strive to keep their writings value free. But the very words we use to describe behavior can reflect our personal values. Whether we label certain sexual behaviors as “perversions” or as an “alternative sexual lifestyle” depends on our attitude toward the behaviors. Labels describe, but they also evaluate.

Scientific research on sexual motivation does not aim to define the personal meaning of sex in our own lives. You 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 the respiration rate to 40 breaths per minute—but fail to understand the human significance of sexual intimacy.

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Surely one significance of such 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, such as marriage, not only reported greater relationship satisfaction and stability but also better sex (Busby et al., 2010; Galinsky & Sonenstein, 2013). For both men and women, but especially for women, orgasm occurs more often (and with less morning-after regret) when sex happens in a committed relationship rather than a sexual hook-up (Garcia et al., 2012, 2013). Partners who share regular meals are more likely than one-night dinner guests to have educated one another about what seasoning touches suit their food tastes; so likewise with the touches of loyal partners who share a bed.

The benefits of commitment—of “vow power”—apply regardless of sexual orientation. Gay and straight couples experience almost identical stability in their relationships, if they have married or entered into a civil union—and almost identical instability, if they have not (Rosenfeld, 2014). Thus, as the opportunity for straight and gay people to marry becomes more equal, we can expect the stability of their relationships to become more similar.

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). Thanks to their overlapping brain reward areas, sexual desire and love feed each other (Cacioppo et al., 2012). Sex at its human best is life uniting and love renewing.