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 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. When we feel an attraction, we hardly stop to think of ourselves as guided by ancestral genes. We may crave our partner’s presence, with a brain response similar to when someone struggles with an alcohol craving (Acevedo et al., 2012). 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.

15-1 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:

In most mammals, nature neatly synchronizes sex with fertility. Females become sexually receptive (in nonhumans, “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 readily affect the sexual behavior of male animals (Feder, 1984). Nevertheless, male rats that have had their testosterone-making testes surgically removed will gradually lose much of their interest in receptive females. They slowly regain it if injected with testosterone.

Hormones do influence human sexual behavior, but in a looser way. Researchers are exploring and debating whether women’s mate preferences change across the menstrual cycle (Gildersleeve et al., 2014; Wood et al., 2014). Some evidence suggests that, 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).

Women have much less testosterone than men do. 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 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 (Edelstein et al., 2015; Gettler et al., 2013; Gray et al., 2006).

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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.

15-2 What is the human sexual response cycle, and how do sexual dysfunctions and paraphilias differ?

The scientific process often begins with careful observations of complex behaviors. When gynecologist-obstetrician William Masters and his collaborator Virginia Johnson (1966) applied this process to human sexual intercourse in the 1960s, they made headlines. They recorded 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 their partner during sex (Bancroft et al., 2003).

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. Some more modestly effective drug treatments for female sexual interest/arousal disorder are also 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.

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15-3 How can sexually transmitted infections be prevented?

Every day, more than 1 million people worldwide acquire a sexually transmitted infection (STI; also called STD for sexually transmitted disease) (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 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, by that point in time, has had sex with the same number of partners as has Pat. How many partners—including “phantom” sexual 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 for Sexual Health, 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. Half of all humans with HIV are women, and their proportion is increasing, 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 in a woman’s vagina and cervix, increasing the time of exposure (Allen & Setlow, 1991; WHO, 2015).

Most Americans with AIDS have been in midlife and younger—ages 25 to 44 (CDC, 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 is 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 papillomavirus (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.