Factors Promoting Sexual Desire One biological factor that promotes sexual desire in both sexes is the hormone testosterone, an androgen (masculinizing hormone). This fact has created a market for the legal and illegal use of androgens. The assumption is that if the goal is to increase sexual desire in women and men who complain of low libido, testosterone should be increased, like adding fuel to your gas tank; if the goal is to reduce sexual desire in sex offenders, testosterone should be lowered, perhaps through chemical castration. Yet these efforts often fail to produce the expected results (Berlin, 2003; M. Anderson, 2005). Why? A primary reason is that in primates, unlike other mammals, sexual motivation requires more than hormones; it is also affected by social experience and context (Randolph et al., 2014; Wallen, 2001). That is why desire can persist in sex offenders who have lost testosterone, and why desire might remain low in people who have been given testosterone. Indeed, artificially administered testosterone does not do much more than a placebo to increase sexual satisfaction in healthy people, nor does a drop in testosterone invariably cause a loss of sexual motivation or enjoyment. In studies of women who had had their uteruses or ovaries surgically removed or who were going through menopause, use of a testosterone patch increased their sexual activity to only one more time a month over the placebo group (Buster et al., 2005).

Men and Women: Same or Different? Despite the similarities in sexual response that Kinsey and then Masters and Johnson identified, the question of whether men and women are alike or different in some underlying, biologically based sex drive continues to provoke lively debate. Although women on average are certainly as capable as men of sexual pleasure, men do have higher rates of almost every kind of sexual behavior, including masturbation, erotic fantasies, casual sex, and orgasm (Peplau, 2003; Schmitt et al., 2012). These sex differences occur even when men are forbidden by cultural or religious rules to engage in sex at all; Catholic priests have more of these sexual experiences than Catholic nuns do (Baumeister, Catanese, & Vohs, 2001). Men are also more likely than women to admit to having sex because “the opportunity presented itself” (Meston & Buss, 2007).

Biological and evolutionary psychologists argue that these differences occur universally because the hormones and brain circuits involved in sexual behavior differ for men and women. They maintain that for men, the wiring for sex overlaps with that for dominance and aggression, which is why sex and aggression are more likely to be linked in men than in women. For women, the circuits and hormones governing sexuality and nurturance seem to overlap, which is why sex and love are more likely to be linked in women than in men (Diamond, 2008).

Other psychologists, however, believe that most gender differences in sexual behavior reflect women's and men's different roles and experiences in life (Ainsworth & Baumeister, 2012; Eagly & Wood, 1999; Tiefer, 2008). Women may have been more reluctant than men to have casual sex not because they have a lesser “drive” but because the experience is not as likely to be gratifying to them because of the greater risk of harm and risk of unwanted pregnancy, and because of the social stigma that may attach to women who have casual sex. Heterosexual women are just as likely as men to say they would accept a sexual offer from a great-looking person (or an unattractive famous person, which might explain the popularity of groupies). And both sexes are equally likely to say they would accept sex with friends or casual hook-ups who they think will be great lovers and give them a “positive sexual experience” (Conley et al., 2011; Rosin, 2012).

Of course, both views could be correct. It is possible that men's sexual behavior is more biologically influenced than is women's, increasing men's interest in having frequent, casual sex, whereas women's sexual desires and responsiveness are more affected by circumstances, the specific relationship, and cultural norms (Baumeister, 2000; Farr, Diamond, & Boker, 2014; Schmitt et al., 2012).

The Many Motives for Sex To most people, the primary motives for sex are pretty obvious: to enjoy the pleasure of it, to express love and intimacy, or to make babies. But other motives are not so positive, including money or perks, duty or feelings of obligation, rebellion, power over the partner, and submission to the partner to avoid his or her anger or rejection.

One survey of nearly 2,000 people yielded 237 motives for having sex, and nearly every one of them had been rated as the most important motive by someone. Most men and women listed the same top 10, including attraction to the partner, love, fun, and physical pleasure. But some said, “I wanted to feel closer to God,” “I was drunk,” “to get rid of a headache” (that was #173), “to help me fall asleep,” “to make my partner feel powerful,” “to return a favor,” “because someone dared me,” or “to hurt an enemy or a rival” (“I wanted to make him pay so I slept with his girlfriend”; “I wanted someone else to suffer from herpes as I do”). In this survey, men were more likely than women to say they use sex to gain status, enhance their reputation (e.g., because the partner was normally “out of my league”), or get things (such as a promotion) (Meston & Buss, 2007).

Across the many studies of motives for sex, there appear to be several major categories (Cooper, Shapiro, & Powers, 1998; Meston & Buss, 2007):

• Pleasure: the satisfaction and physical pleasure of sex.

• Intimacy: emotional closeness with the partner, spiritual transcendence.

• Insecurity: reassurance that you are attractive or desirable.

• Partner approval: the desire to please or appease the partner; the desire to avoid the partner's anger or rejection.

• Peer approval: the wish to impress friends, be part of a group, and conform to what everyone else seems to be doing.

• Attaining a goal: to get status, money, revenge, or “even the score.”

People's motives for having sex affect many aspects of their sexual behavior, including whether they engage in sex in the first place, whether they enjoy it, whether they have unprotected or otherwise risky sex, and whether they have few or many partners (Browning et al., 2000; Muise, Impett, & Desmarais, 2013; Snapp et al., 2014). Extrinsic motives, such as having sex to gain approval from others or get some tangible benefit, are most strongly associated with risky sexual behavior, including having many partners, not using birth control, and pressuring a partner into sex (Hamby & Koss, 2003). For men, extrinsic motives include peer pressure, inexperience, a desire for popularity, or a fear of seeming unmasculine. Women's extrinsic motives include not wanting to lose the relationship; feeling obligated after the partner had spent time and money on them; feeling guilty about not doing what the partner demands; or wanting to avoid conflict and quarrels (Impett, Gable, & Peplau, 2005).

When one partner is feeling insecure about the relationship, he or she is also more likely to consent to unwanted sex. In a study of 125 college women, one-half to two-thirds of the Asian American, white, and Latina women had consented to having sex when they didn't really want to, and all of the African American women said they had. Do you remember the attachment theory of love discussed earlier? Anxiously attached women were the most willing to consent to unwanted sex, especially if they feared their partners were less committed than they were. They reported that they often had sex out of feelings of obligation and to prevent the partner from leaving. Securely attached women also occasionally had unwanted sex, but their reasons were different: to gain sexual experience, to satisfy their curiosity, or to actively please their partners and further the intimacy between them (Impett, Gable, & Peplau, 2005).

Sexual Coercion and Rape One of the most persistent differences in the sexual experiences of women and men has to do with sexual coercion. A U.S. government survey of rape and domestic violence, based on a nationally representative sample of 16,507 adults, reported that nearly one in five women said they had been raped or experienced attempted rape at least once. (The researchers defined rape as completed or attempted forced penetration, including forced penetration enabled by alcohol or drugs.) The investigators also found high rates of aggression not usually measured in studies of rape, including coercive efforts to control the woman's reproductive and sexual health. Men also reported being victimized, but the numbers were much lower: One in seven said they had been severely beaten at the hands of a partner, and 1 to 2 percent said they had been raped, most when they were younger than age 11 (Black et al., 2011).

However, many women who report a sexual assault that meets the legal definition of rape—being forced to engage in sexual acts against their will—do not label it as such (McMullin & White, 2006; Peterson & Muehlenhard, 2011). Most women define “rape” as being forced into intercourse by an acquaintance or stranger, as an act that caused them to fight back, or as having been molested as a child. They are least likely to call their experience rape if they were sexually assaulted by a boyfriend, had previously had consensual sex with him, were drunk or otherwise drugged, or were forced to have oral sex or unwanted manual stimulation. Other women are motivated to avoid labeling the experience with such a charged word because they are embarrassed, or simply because they don't want to think of someone they know personally as being a “rapist” (Koss, 2011; Perilloux, Duntley, & Buss, 2014; Peterson & Muehlenhard, 2011).

What causes some men to rape? Evolutionary arguments—that rape stems from the male drive to fertilize as many females as possible, the better to distribute their genes—have not been supported (Buss & Schmitt, 2011). Among human beings, rape is often committed by high-status men, including sports heroes and other celebrities, who could easily find consenting sexual partners. All too frequently its victims are children or older adults, who do not reproduce. And sadistic rapists often injure or kill their victims, hardly a way to perpetuate one's genes. The human motives for rape thus appear to be primarily psychological, and include these:

• Narcissism and hostility toward women. Sexually aggressive males often are narcissistic, are unable to empathize with women, and feel entitled to have sexual relations with whatever woman they choose. They misperceive women's behavior in social situations, equate feelings of power with sexuality, and accuse women of provoking them (Bushman et al., 2003; Malamuth et al., 1995; Widman & McNulty, 2010).

• A desire to dominate, humiliate, or punish the victim. This motive is apparent among soldiers who rape captive women during war and then often kill them (Olujic, 1998). Similarly, reports of the systematic rapes of female cadets at the U.S. Air Force Academy suggest that the rapists' motives were to humiliate the women and get them to drop out. Aggressive motives also occur in the rape of men by other men, usually by anal penetration (King & Woollett, 1997). This form of rape typically occurs in youth gangs, where the intention is to humiliate rival gang members, and in prison, where again the motive is to conquer and degrade the victim.

• Sadism. A minority of rapists are violent criminals who get pleasure out of inflicting pain on their victims and who often murder them in planned, grotesque ways (Healey, Lussier, & Beauregard, 2013; Turvey, 2008).

You can see that the answer to the question “Why do people have sex?” is not a simple matter of “doing what's natural.” In addition to the intrinsic motives of intimacy, pleasure, procreation, and love, extrinsic motives include intimidation, dominance, insecurity, appeasing the partner, approval from peers, and the wish to prove oneself a real man or a desirable woman.