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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 a high priority from puberty on. Our sexual feelings and behaviors reflect both physiological and psychological influences.

Unlike hunger, sex is not an actual need. (Without it, we may feel like dying, but we will not.) Yet sex is a 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. As the pleasure we take in eating is nature’s method of ensuring we nourish our bodies, so the desires and pleasures of sex are nature’s way of driving us to preserve and spread our genes. Life is sexually transmitted.

Hormones and Sexual Behavior

Among the forces driving sexual behavior are the sex hormones. As we noted earlier, 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, sexual interest and fertility overlap. Females become sexually receptive when their estrogen levels peak at ovulation. By injecting female animals with estrogens, researchers can increase their sexual interest. Hormone injections do not affect male animals’ sexual behavior as easily because male hormone levels are more constant (Piekarski et al., 2009). Nevertheless, male hamsters that have had their testosterone-making testes surgically removed gradually lose much of their interest in receptive females. They gradually regain it if injected with testosterone.

Hormones do influence human sexuality, but more loosely. Researchers are exploring and debating whether women’s mating preferences change across the menstrual cycle (Gildersleeve et al., 2014; Wood et al., 2014). At ovulation, women’s estrogens surge, as does their testosterone, though not as much. (Recall that women have testosterone, though less than men have.) Some evidence suggests that, among women with mates, sexual desire rises slightly at this time—a change men can sometimes detect in women’s behaviors and voices (Haselton & Gildersleeve, 2011).

More than other mammalian females, women are responsive to their testosterone levels (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 plummet (Davison & Davis, 2011; Lindau et al., 2007). But testosterone-replacement therapy can often restore sexual desire, arousal, and activity (Braunstein et al., 2005; Buster et al., 2005; Petersen & Hyde, 2011).

Testosterone-replacement therapy also increases sexual functioning in men with abnormally low testosterone levels (Khera et al., 2011). But normal ups and downs in testosterone levels (from man to man and hour to hour) have little effect on sexual drive (Byrne, 1982). In fact, male hormones sometimes vary in response to sexual stimulation (Escasa et al., 2011). One Australian study tested whether the presence of an attractive woman would affect heterosexual male skateboarders’ performance. The result? Their testosterone surged, as did their riskier moves and crash landings (Ronay & von Hippel, 2010). Thus, sexual arousal can be a cause as well as a result of increased testosterone.

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

To recap, 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 at least adequate, adding more won’t change how the car runs. This isn’t a perfect comparison, because hormones and sexual motivation influence each other. But it does suggest that biology alone cannot fully explain human sexual behavior. Hormones are the essential fuel for our sex drive. But psychological stimuli turn on the engine, keep it running, and shift it into high gear. Let’s now see just where that drive usually takes us.

The Sexual Response Cycle

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As we noted in Chapter 1, science often begins by carefully observing behavior. Sexual behavior is no exception. In the 1960s, two researchers—gynecologist-obstetrician William Masters and his colleague, Virginia Johnson (1966)—made headlines with their observations of sexual behavior. They recorded the physiological responses of 382 female and 312 male volunteers who came to their lab to masturbate or have intercourse. With the help of this atypical sample of people able and willing to display arousal and orgasm while scientists observed, the researchers identified a four-stage sexual response cycle:

Sexual Dysfunctions

Masters and Johnson had two goals: to describe the human sexual response cycle, and to understand and treat problems that prevent people from completing that cycle. Sexual dysfunctions consistently impair sexual arousal or functioning. Some involve sexual motivation—the person lacks sexual energy and/or does not become aroused. For men, one common problem (and the subject of many TV commercials) is erectile disorder, an inability to have or maintain an erection. Another is premature ejaculation, reaching a sexual climax before the man or his partner wishes. For some women, pain during intercourse may prevent them from completing the sexual response cycle. Others may experience female orgasmic disorder, distress over rarely or never having an orgasm. In surveys of some 35,000 American women, about 4 in 10 reported a sexual problem, such as female orgasmic disorder or low desire. Only about 1 in 8 said that the problem caused them personal distress (Lutfey et al., 2009; Shifren et al., 2008). Most women who have reported sexual distress have connected it with their emotional relationship with their sexual partner (Bancroft et al., 2003).

Psychological and medical therapies can help people with sexual dysfunctions (Frühauf et al., 2013). Behaviorally oriented therapy, for example, can help men learn ways to control their urge to ejaculate, or help women learn to bring themselves to orgasm. Starting with the introduction of Viagra in 1998, erectile disorder has been routinely treated by taking a pill. Some modestly effective drug treatments for female sexual interest/arousal disorder are also available.

Sexually Transmitted Infections

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Every day, more than 1 million people worldwide acquire a sexually transmitted infection (STI; also called STD, for sexually transmitted disease) (WHO, 2013). “Compared with older adults,” reports the Centers for Disease Control (2016b), “sexually active adolescents aged 15–19 years and young adults aged 20–24 years are at higher risk.” Teenage girls, for example, are at heightened risk because their anatomy is not fully mature and their level of protective antibodies is lower (Dehne & Riedner, 2005; Guttmacher Institute, 1994).

To understand the mathematics of infection, imagine this scenario. Over the course of a year, Pat has sex with 9 people. By that time, each of Pat’s partners has had sex with 9 other people, who in turn have had sex with 9 others. How many partners—including “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 are very effective in blocking the spread of some STIs. The effects were clear when Thailand promoted condom use by commercial sex workers. Over a 4-year period, condom use soared from 14 to 94 percent. During that time, the number of bacterial STIs reported each year plummeted 93 percent—from 410,406 to 27,362 (WHO, 2000).

Condoms offer only limited protection against certain skin-to-skin STIs, such as herpes. But their ability to reduce other risks has saved lives (NIH, 2001). When used by people with an infected partner, condoms have been 80 percent effective in preventing transmission of HIV (human immunodeficiency virus—the virus that causes AIDS—acquired immune deficiency syndrome) (Weller & Davis-Beaty, 2002; WHO, 2003). AIDS can be transmitted by other means, such as needle sharing during drug use, but its sexual transmission is most common. Many people think oral sex is “safe sex,” but it carries a significant risk. It is linked to STIs, such as the human papillomavirus (HPV), and risks rise with the number of sexual partners (Ballini et al., 2012; Gillison et al., 2012). Most HPV infections can now be prevented if people are vaccinated before they become sexually active.

Half of all humans with HIV (and one-fourth of Americans with HIV) are women. Women’s proportion of the worldwide AIDS population has grown, for several reasons. 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 in a woman’s vagina and cervix, increasing her exposure time (Allen & Setlow, 1991; WHO, 2015).

Just over half of Americans with AIDS are between ages 30 and 49 (CDC, 2013). Given AIDS’ long incubation period, this means that many were infected in their teens and twenties. 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). In sub-Saharan Africa, home to two-thirds of those with HIV, medical treatment to extend life and care for the dying is sapping social resources.

Biological factors powerfully influence our sexual motivation and behavior. But despite our shared biology, human sexual motivation and behavior vary widely—over time, across place, and among individuals. So social and psychological factors exert a great influence as well (FIGURE 4.4).

What motivates people to have sex? The 281 (by one count) expressed reasons have ranged widely—from “to get closer to God” to “to get my boyfriend to shut up” (Buss, 2008; Meston &Buss, 2007). One thing is certain: Our most important sex organ may be the one resting above our shoulders. Our sophisticated brain enables sexual arousal both from what is real and from what is imagined.

External Stimuli

Men and women become aroused when they see, hear, or read erotic material (Heiman, 1975; Stockton & Murnen, 1992). In men more than in women, feelings of sexual arousal closely mirror their (more obvious) physical genital responses (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 arousing material, just as those wishing to avoid overeating limit their exposure to tempting food cues.) With repeated exposure to any stimulus, including an erotic stimulus, our response lessens—we habituate. During the 1920s, when Western women’s rising hemlines first reached the knee, many male hearts fluttered when viewing a woman’s leg. Today, many men wouldn’t notice.

Can exposure to sexually explicit material have lingering negative effects? Researchers have found that it can, in two areas especially.

Imagined Stimuli

Sexual arousal and desire can also be products of our imagination. People left with no genital sensation after a spinal cord injury can still feel sexual desire, and many engage in sexual intercourse (Donohue & Gebhard, 1995; Sipski et al., 1999; Willmuth, 1987).

Both men and women (about 95 percent of each) report having sexual fantasies, which for a few women can produce orgasms (Komisaruk & Whipple, 2011). Men, regardless of sexual orientation, tend to have more frequent, more physical, and less romantic fantasies (Schmitt et al., 2012). They also prefer less personal and faster-paced sexual content in books and videos (Leitenberg & Henning, 1995).

Does fantasizing about sex indicate a sexual problem or dissatisfaction? No. If anything, sexually active people have more sexual fantasies.

Sexual Risk Taking and Teen Pregnancy

Thanks to decreased sexual activity and increased protection, teen pregnancy rates are declining (CDC, 2016b). Yet compared with European teens, American teens have a higher pregnancy rate (Sedgh et al., 2015). What environmental factors contribute to sexual risk taking among teens?

MINIMAL COMMUNICATION ABOUT BIRTH CONTROL Many teens are uncomfortable discussing birth control with parents, partners, and peers. But teens who talk freely and openly with their parents and with their partner in an exclusive relationship are more likely to use contraceptives (Aspy et al., 2007; Milan & Kilmann, 1987).

IMPULSIVE SEXUAL BEHAVIOR Among sexually active 12- to 17-year-old American girls, 72 percent said they regretted having had sex (Reuters, 2000). When sexually aroused, people perform poorly on measures of impulse control (Macapagal et al., 2011). If passion overwhelms intentions (either to delay using contraceptives or to delay having sex), unplanned sexual activity may result in pregnancy (Ariely & Loewenstein, 2006; Gerrard & Luus, 1995; MacDonald & Hynie, 2008).

ALCOHOL USE Among late teens and young adults, most sexual hook-ups (casual encounters outside of a relationship) occur after alcohol use, often without knowing consent (Fielder et al., 2013; Garcia et al., 2013; Johnson & Chen, 2015). Those who use alcohol prior to sex are also less likely to use condoms (Kotchick et al., 2001). Alcohol disarms normal restraints by depressing the brain centers that control judgment, inhibition, and self-awareness.

MASS MEDIA INFLUENCES The more sexual content adolescents and young adults view or read, 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., 2013). These perceptions of peer norms (what “everybody else” is doing) influence teens’ sexual behavior (Lyons et al., 2015; van de Bongardt et al., 2015). And they come in part from the popular media, which help write the social scripts that shape our views of how to act in certain situations. So what sexual scripts do today’s media write on our minds? Sexual content appears in approximately 85 percent of movies, 82 percent of TV programs, 59 percent of music videos, and 37 percent of music lyrics (Ward et al., 2014). Twenty percent of middle school students, and 44 percent of 18- to 24-year-olds, report having received a “sext”—a sexually explicit text (Lenhart & Duggan, 2014; Rice et al., 2014). Online dating sites, such as Tinder, enable young people to seek out quick hook-ups with little emotional investment.

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Media influences can either increase or decrease sexual risk taking. One long-term study asked more than a thousand 12- to 14-year-olds what movies they had seen. Then, after those teens reached age 18, researchers again asked them about their sexual experiences (O’Hara et al., 2012). (The study controlled for other factors that predict early sexual activity, such as personal and family characteristics.) The result? The more the adolescents had viewed movies with high sexual content, the greater was their sexual risk taking. They started earlier, had more partners, and used condoms inconsistently. 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, researchers concluded that the program led to a 6 percent reduction in the national teen pregnancy rate (Kearney & Levine, 2014).

What are the characteristics of teens who delay having sex?

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In the rest of this chapter, we will consider two special topics: sexual orientation (the direction of our sexual interests), and evolutionary psychology’s explanation of our sexuality.