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.
Like hunger, sexual arousal depends on the interplay of internal and external stimuli. To understand sexual motivation, we must consider both.
FOCUS QUESTION: What is the human sexual response cycle, and what dysfunctions disrupt it?
In the 1960s, gynecologist-obstetrician William Masters and his collaborator Virginia Johnson (1966) made headlines by recording the physiological responses of volunteers who masturbated or had 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 being observed in a laboratory – Masters and Johnson monitored or filmed more than 10,000 sexual”cycles.” Their description of the sexual response cycle identified four stages. During the initial excitement phase, men’s and women’s genital areas become engorged with blood, a woman’s vagina expands and secretes lubricant, and her breasts and nipples may enlarge.
In the plateau phase, excitement peaks as breathing, pulse, and blood pressure rates continue to increase. The penis becomes fully engorged and some fluid – frequently containing enough live sperm to enable conception – may appear at its tip. Vaginal secretion continues to increase.
Masters and Johnson observed muscle contractions all over the body during orgasm; these were accompanied by further increases in breathing, pulse, and blood pressure rates.
At orgasm, pulse rate surges from about 70 to 115 beats per minute Gackson, 2009). A woman’s arousal and orgasm facilitate conception by positioning the uterus to receive sperm, and drawing the sperm further inward. A woman’s orgasm therefore not only reinforces intercourse, which is essential to natural reproduction, it also increases retention of deposited sperm (Furlow & Thornhill, 1996).
The pleasurable feeling of sexual release apparently is much the same for both sexes. In one study, a panel of experts could not reliably distinguish between descriptions of orgasm written by men and those written by women (Vance & Wagner, 1976). University of Groningen neuroscientist Gert Holstege and his colleagues (2003a,b) understand why. They discovered that when men and women undergo PET scans while having orgasms, the same subcortical brain regions glow. And when people who are 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).
The body gradually returns to its unaroused state as the engorged genital blood vessels release their accumulated blood – 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.) During this resolution phase, the male enters a refractory period, lasting from a few minutes to a day or more, during which he is incapable of another orgasm. The female’s much shorter refractory period may enable her to have more orgasms if restimulated during or soon after resolution.
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 experience sexual distress relate it to their emotional relationship with the partner during sex (Bancroft et al., 2003).
Men and women with sexual dysfunctions can often be helped through therapy. 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.
Sexual dysfunction involves problems with arousal or sexual functioning. People with paraphilias such as exhibitionism, fetishism, and pedophilia, do experience sexual arousal, but they direct it in unusual ways. The American Psychiatric Association (2013) only classifies such behavior as disordered if
FOCUS QUESTION: How do hormones, and external and internal stimuli, influence human sexual motivation?
Sex hormones have two effects: They direct the physical development of male and female sex characteristics, and (especially in nonhuman animals) they activate sexual behavior. In most mammals, nature neatly synchronizes sex with fertility. The female becomes sexually receptive (in other animals, being “in heat”) when secretion of the female hormones, the estrogens (such as estradiol), peaks during ovulation. In experiments, researchers can stimulate receptivity by injecting female animals with an estrogen. Male hormone levels are more constant, and hormone injection does not so easily manipulate the sexual behavior of male animals (Feder, 1984). Nevertheless, castrated male rats – having lost their testes, which manufacture the male sex hormone testosterone – gradually lose much of their interest in receptive females. They gradually regain it if injected with testosterone.
In humans, hormones more loosely influence sexual behavior, although sexual desire rises slightly at ovulation among women with mates (Pillsworth et al., 2004). When at peak fertility in their menstrual cycle, women express increased preference for masculine faces and ability to detect sexual orientation, but also increased apprehensiveness of men perceived as potentially sexually coercive (Eastwick, 2009; Little et al., 2008; Navarrete et al., 2009; Rule et al., 2011). 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’s sexuality differs from that of other mammalian females in being more responsive to testosterone level (van Anders & Dunn, 2009). If a woman’s natural testosterone level drops, as happens with removal of the ovaries or adrenal glands, her sexual interest may wane. But testosterone-replacement therapy sometimes restores diminished sexual appetite. That is the finding of experiments with hundreds of surgically or naturally menopausal women, for whom a testosterone-replacement patch restored sexual activity, arousal, and desire more than did a placebo (Braunstein et al., 2005; Buster et al., 2005; Petersen & Hyde, 2011). For men with abnormally low testosterone levels, testosterone-replacement therapy often increases sexual desire and also energy and vitality (yates, 2000).
In men, normal fluctuations in testosterone levels, from man to man and hour to hour, have little effect on sexual drive (Byrne, 1982). Indeed, fluctuations in male hormones are partly a response to sexual stimulation. In the presence of an attractive female, Australian skateboarders’testosterone surges, which contributes 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.
Although normal short-term hormonal changes have little effect on men’s and women’s desire, large hormonal shifts over the life span have a greater effect. A person’s interest in dating and sexual stimulation usually increases with the pubertal surge in sex hormones. 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 – the normal development of sex characteristics and sexual desire does not occur (Peschel & Peschel, 1987). When adult men are castrated, sex drive typically falls as testosterone levels decline sharply (Hucker & Bain, 1990). Male sex offenders taking Depo-Provera, a drug that reduces testosterone levels to that of a prepubertal boy, similarly lose much of their sexual urge (Bilefsky, 2009; Money et al., 1983). In later life, as sex hormone levels decline, the frequency of sexual fantasies and intercourse declines as well (Leitenberg & Henning, 1995).
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 ex planation 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.
Hunger and sex are different sorts of motivations. Hunger responds to a need. If we do not eat, we die. Sex is not in this sense a need. (We may feel like dying, but we do not.) Nevertheless, there are similarities between hunger and sexual motivation. Both depend on internal physiological factors. Both reflect the interplay of excitatory and inhibitory responses – the body’s acceleration and braking systems (Bancroft et al., 2009). And both are influenced by external and imagined stimuli, and by cultural expectations (FIGURE 39.1).
Many studies confirm that men become aroused when they see, hear, or read erotic material. Surprising to many (because sexually explicit materials are marketed mostly to men) is that most women – at least the less-inhibited women who volunteer to participate in such studies – report or exhibit nearly as much arousal to the same stimuli (Heiman, 1975; Stockton & Murnen, 1992). (Their brains do, however, respond differently, with fMRI scans revealing a more active amygdala in men viewing erotica [Hamann et al., 2004].) In 132 such 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 hemlines first reached the knee, an exposed leg was a mildly erotic stimulus.
Can sexually explicit material have adverse effects? Research indicates that it can. Depictions of women being sexually coerced – and liking it – tend to increase viewers’ acceptance of the false idea that women enjoy rape, and they tend to increase 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 similarly tends 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.
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 (W’lilmuth, 1987). Consider, too, the erotic potential of dreams. Sleep researchers have discovered that genital arousal accompanies all types of dreams, even though most dreams have no sexual content. But in nearly all men and some 40 percent of women, dreams sometimes contain sexual imagery that leads to orgasm (Wells, 1986). In men, nighttime orgasm and nocturnal emissions (“wet dreams”) are more likely when orgasm has not occurred recently.
About 95 percent of both men and women say 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.
