Hidden Desires

The Dangerous Passion: Why Jealousy Is Necessary in Love and Sex
by David M. Buss, Ph.D.

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One Sunday morning William burst into the living room and said, "Dad! Mom! I have some great news for you! I'm getting married to the most beautiful girl in town. She lives a block away and her name is Susan." After dinner, William's dad took him aside. "Son, I have to talk with you. Your mother and I have been married 30 years. She's a wonderful wife, but has never offered much excitement in the bedroom, so I used to fool around with women a lot. Susan is actually your half-sister, and I'm afraid you can't marry her."

William was heartbroken. After eight months he eventually started dating again. A year later he came home and proudly announced, "Dianne said yes! We're getting married in June." Again, his father insisted on a private conversation and broke the sad news. "Dianne is your half-sister too, William. I'm awfully sorry about this."

William was furious. He finally decided to go to his mother with the news. "Dad has done so much harm. I guess I'm never going to get married," he complained. "Every time I fall in love, Dad tells me the girl is my half-sister."

His mother just shook her head. "Don't pay any attention to what he says, dear. He's not really your father."

We find this story funny not simply because the ending carries a surprise. It's amusing because the mother ultimately gets payback for the "father's" philandering. Cuckolds are universal objects of laughter and derision, and a constant source of engaging tales from the tragedy of William Shakespeare's Othello to the middle-class marital dramas portrayed in the novels of John Updike.

To understand the origins of sexual passion we must introduce a disturbing difference between the sexes. Everyday observation tells us that men are more promiscuously inclined than women. "Men found to desire more sex partners than women desire" would be no more likely to make the headlines than "Dog bites man." But scientific verification is always useful, since common sense, which tells us that the earth is flat, sometimes turns out to be wrong. Science, in this case, has verified the everyday knowledge that men do display a greater passion for playing around. In one of our recent studies of more than 1,000 men and women, men reported desiring eight sex partners over the next three years, whereas women reported desiring only one or two. In another study, men were four times more likely than women to say that they have imagined having sex with 1,000 or more partners.

Observing that men and women differ, however, is not the same as explaining why they differ. There are compelling evolutionary reasons for the fact that this difference in desire for sexual variety is universal, found not just in cultures saturated with media images of seductive models, not just among Hugh Hefner's generation of Playboy readers, and not just in studies conducted by male scientists. To explain this desire, we must introduce another key fact about human reproductive biology.

To produce a single child, women bear the burdens and pleasures of nine months of pregnancy — an obligatory form of parental investment that men cannot share. Men, to produce the same child, need only devote a few hours, a few minutes, or even a few seconds. Wide is the gulf between men and women in the effort needed to bring forth new life. Over time, therefore, a strategy of casual mating proved to be more reproductively successful for men than for women. Men who succeeded in the arms of many women out-reproduced men who succeeded with fewer. An ancestral woman, in contrast, could have had sex with hundreds of partners in the course of a single year and still have produced only a single child. Unless a woman's regular partner proved to be infertile, additional sex partners did not translate into additional children. As a consequence, men evolved a more powerful craving for sex with a variety of women.

This sex difference in desire creates an intriguing puzzle. Sexual encounters require two people. Mathematically, the number of heterosexual encounters must be identical for the sexes. Men cannot satisfy their lust for sex partners without willing women. Indeed, men's passion for multiple partners could never have evolved unless there were some women who shared that desire. Is casual sex a recent phenomenon, perhaps created by the widespread prevalence of birth control devices that liberated women from the previous risks of pregnancy? Or did ancestral women do it too?

Three scientific clues, when taken together, provide a compelling answer. Men's sexual jealousy provides the first clue, the ominous passion that led us to this mystery. If ancestral women were naturally inclined to be flawlessly faithful, men would have had no evolutionary catalyst for jealousy. Men's jealousy is an evolutionary response to something alarming: the threat of a loved one's infidelity. The intensity of men's jealousy provides a psychological clue that betrays women's desire for men other than their regular partners.

Second, affairs are known in all cultures, including tribal societies, pointing to the universal prevalence of infidelity. Prevalence rates vary from culture to culture (high in Sweden and low in China), but affairs occur everywhere. Sexual infidelity causes divorce worldwide more than any other marital violation, being closely rivaled only by the infertility of the union. The fact that women have affairs in cultures from the Tiwi of northern Australia to the suburbs of Los Angeles reveals that some women refuse to limit themselves to a single partner despite men's attempts to control them and despite the risk of divorce if discovered.

A third line of evidence comes from new research on human sperm competition. Sperm competition occurs when the sperm from two different men inhabit a woman's reproductive tract at the same time. Human sperm remain viable within the woman's tract for up to seven days, not merely one or two days as scientists previously believed. Indeed, my colleagues have discovered hundreds of "crypts" recessed within the vaginal walls of women in which they store a man's sperm and then release it several days later to enter a marathon race to her egg. If a woman has sex with two men within the course of a week, sperm competition can ensue, as the sperm from different men scramble and battle for the prize of fertilizing the egg. Research on sperm competition reveals that men's sperm volume, relative to their body weight, is twice that which occurs in primate species known to be monogamous, a clue that hints at a long evolutionary history of human sperm competition.

Human sperm, moreover, come in different "morphs," or shapes, designed for different functions. Most common are the "egg getters," the standard government-issue sperm with conical heads and sinewy tails designed for swimming speed — the Mark Spitzes of the sperm world. But a substantial minority of sperm have coiled tails. These so-called kamikaze sperm are poorly designed for swimming speed. But that's not their function. When the sperm from two different men are mixed in the laboratory, kamikaze sperm wrap themselves around the egg getters and destroy them, committing suicide in the process. These physiological clues reveal a long evolutionary history in which men battled with other men, literally within the woman's reproductive tract, for access to the vital egg needed for transporting their genes into the next generation. Without a long history of sperm competition, evolution would have favored neither the magnitude of human sperm volume nor the specialized sperm shapes designed for battle.

All these clues — the universality of infidelity, men's sexual jealousy, and the hallmarks of sperm competition — point to a disturbing answer to the question of ancestral women's sexual strategies. They reveal the persistent expression of women's passion for men other than their husbands, a phenomenon that must have occurred repeatedly over the long course of human evolution. Modern women have inherited this passion from their ancestral mothers.

Why Women Have Affairs

Because scientists have focused primarily on the obvious reproductive benefits of men's desire for sexual variety, the potential benefits to women of short-term sexual passion languished for years unstudied. The puzzle is compounded by the fact that a woman's infatuation with another man comes laden with danger. An unfaithful woman, if discovered, risks damage to her social reputation, the loss of her partner's commitment, physical injury, and occasionally death at the hands of a jealous man. Undoubtedly, many women weigh these risks, and choose not to act on their sexual desires. The benefits to women who do act on their passion for other men, given the possibility of catastrophic costs, must be perceived as sufficiently great to make it worth the risk.

For the past seven years, Heidi Greiling and I have been studying why women have affairs. Our lab has focused on the benefits that are so alluring that women from all walks of life are willing to take great risks to pursue sex and love outside of marriage. Our research centered on three questions: What benefits do women reap from affairs? What circumstances are most likely to drive a woman into another man's arms? And which women are most prone to affairs?

Historically, women may have benefited from an affair in countless ways. The first and most obvious benefit comes from the direct resources that an affair partner may provide. A few expensive dinners may not seem like much today, but an extra supply of meat from the hunt would have made the difference between starving and surviving during ancestral winters when the land lay bare, or between merely surviving and robustly thriving during more plentiful times.

Women also can benefit from affairs in the currency of quality genes. The puzzle of the peacock's tail provided the telltale clue to this benefit. A peahen's preference for peacocks with brilliant plumage may signal selection for genes for good health. When peacocks carry a high load of parasites, their diminished health is revealed in duller displays. By selecting for luminescence, peahens secure good genes for health that benefit their offspring. Research by Steve Gangestad and Randy Thornhill of the University of New Mexico reveals that women may be choosing affair partners with especially healthy genes. Women who have sex with different men can also produce more genetically diverse children, providing a sort of "hedge" against environmental change.

Although genetic and resource benefits may flow to women who express their hidden sexual side, our studies uncovered one benefit that overshadowed the others in importance, a benefit we call "mate insurance." During ancestral times, disease, warfare, and food shortages made survival a precarious proposition. The odds were not trivial that a husband would succumb to a disease, become debilitated by a parasite, or incur injury during a risky hunt or a tribal battle. The paleontological and cross-cultural records reveal this clue — the skulls and skeletons show injuries mostly on males. A woman's husband, in short, stood a significant chance of suffering a debilitating or lethal wound.

Ancestral women who failed to have mate insurance, a backup replacement in the event that something happened to her regular partner, would have suffered greatly compared to women who cultivated potential replacements. Modern women have inherited the desires of their ancestral mothers for replacement mates. In the words of one woman in our study, "Men are like soup — you always want to have one on the back burner." Mate insurance provides a safeguard against reasonable risks of losing a partner.

And mate insurance remains relevant today, even though we've conquered many of the hazards that felled our forebears. American divorce rates now approach 67 percent for those currently getting married, up from the mere 50 percent figure that alarmed many over the past two decades. Remarriage is rapidly becoming the norm. The Dangerous Passion explores how women's desire for additional partners is ancestral wisdom that, however alarming to husbands, continues to serve a critical insurance function for women today.

Urges of Ovulation

Women's attraction to lovers has another mysterious ingredient: the puzzle of concealed ovulation. Unlike chimpanzees, women's genitals do not become engorged when they ovulate. Women have "lost estrus" and engage in sex throughout their ovulatory cycle. Conventional scientific wisdom has declared that a woman's ovulation is cryptic, concealed even from the woman herself. But have the urges associated with ovulation totally vanished?

In the most extensive study of ovulation and women's sexuality, several thousand married women were asked to record their sexual desires every day for a period of twenty-four months. The methods were crude but straightforward: women simply placed an X on the recording sheet on each day that they experienced sexual desire. Basal body temperature was recorded to determine the phase of the menstrual cycle. These thousands of data points yielded a startling pattern. On the first day of a woman's period, practically no women reported experiencing sexual desire. The numbers rose dramatically across the ovarian cycle, peaking precisely at the point of maximum fertility, and then declining rapidly during the luteal phase after ovulation. Women, of course, can experience sexual desire at any phase of their cycle. Nonetheless, they are five times more likely to experience sexual desire when they are ovulating than when they are not.

Women sometimes act on their passions. A recent survey of 1,152 women, many of whom were having affairs, revealed a startling finding. Women who stray tend to time their sexual liaisons with their affair partners to coincide with the peak of their sexual desire, when they are most likely to conceive. Sex with husbands, in sharp contrast, is more likely to occur when women are not ovulating, a strategy that may be aimed at keeping a man rather than conceiving with him. None of this is conscious, of course. Women do not think "I'll try to time sex with my affair partner when I'm ovulating so that I'll bear his child and not my husband's." Psychologically, women simply experience sexual desire more when they are ovulating, and if they have an affair partner, have urges to have sex with him during this phase. Ovulation may seem concealed to outside observers, but women appear to act on the impulses that spring from it. And when that desire for men other than their husbands occurs, it's difficult for most men to tell when their mates are straying or may be likely to stray. I call this the signal detection problem.

The Signal Detection Problem

Across cultures, people have affairs that are specifically designed to avoid detection. In Arizona, one motel marquee boasts that it is the "No-Tell Motel." In states across America, you can rent some hotel rooms at an hourly rate. The woman returning from a business trip does not make her brief fling on the road the first topic of conversation. The husband who conceals his finances from his wife may be funneling resources to support a mistress on the side.

Spouses experience a signal detection problem. Consider camping in the woods at night and hearing a sound somewhere in the dark. Was that the sound of a twig snapping, merely the wind blowing, or the unfamiliar night sounds playing tricks on your ears? Assuming that you have correctly detected the signal as a twig snap, the possible causes of this event are many, but they are not infinite. It could be a rock that somehow got dislodged. But it could also be a dangerous animal or a hostile human. The signal detection problem is not merely about picking up accurate signals in the face of an uncertain and ambiguous welter of information. It is also about making correct inferences about the cause of the signal.

Since sexual infidelities are almost invariably secret, the signals they might emit are intentionally muted. An unfamiliar scent, the purchase of a sharp new jacket, the running of a yellow light, a new interest in Beethoven or the Beastie Boys, an unexplained absence — all of these can be signals, but they can originate from many causes other than infidelity. The jealous person experiences an elevated sensitivity to signals of infidelity: "He may see a red flush on his wife's cheek, she may appear to be standing awkwardly, or sitting sideways on a chair, she has put on a clean dress, there is a cigarette-end in the fireplace... the jealous man sees a handkerchief on the floor, a wet cloth in the bathroom, newspapers in a ditch, and attaches to all the same import."

Consider the case of a European psychiatrist who counseled many couples referred to him in which one of the spouses experienced "morbid jealousy." Most cases were husbands who had delusions that their wives were sexually unfaithful, and these delusions destroyed the fabric of trust required for harmonious marriage. Because he believed that extreme jealousy was a psychiatric illness that could not be cured, his most common recommendation was that the couples separate or divorce. Many couples followed his recommendations. Because he was keenly interested in the subsequent fate of his patients, he routinely contacted them after a number of months had passed. To his astonishment, he discovered that many of the wives of his patients had subsequently become sexually involved with the very men about whom their husbands had been jealous! Some of these women actually married the men who were the objects of their husbands' suspicions. In many cases, the husbands must have been sensing signs of infidelity. But because the wives proclaimed innocence and declared that their husbands' jealousy was irrational, the husbands ended up believing that the problem was in their heads. The problem of signal detection is how to identify and correctly interpret a partner's betrayal in an uncertain social world containing a chaos of conflicting clues.

Jealousy is often triggered by circumstances that signal a real threat to a relationship, such as differences in the desirability of the partners, as illustrated by the following case. The man was 35 years old, working as a foreman, when he was referred to a psychiatrist and diagnosed with "morbid jealousy." He had married at age 20 to a woman of 16 whom he deeply loved. During their first two years of marriage, he was stationed in military service in England. During this two-year separation, he received several anonymous letters saying that his wife was carrying on an affair. When he returned to America to rejoin her, he questioned her intensely about the allegations, but she denied them. Their own sexual relations proved disappointing. He became obsessed with the earlier time in their marriage, repeatedly accused his wife of infidelity, and hit her from time to time, especially after a bout of drinking. He tried to strangle her twice, and several times he threatened to kill himself.

He openly admitted his problems to the psychiatrist: "I'm so jealous that when I see anyone near her I want to hurt her. I have always loved her but do not think she has returned my affection. This jealousy is something I feel in my stomach and when it comes out of me there is nothing I can do about it. That is why I behave so madly.... My wife is always telling me that other men are stronger and can beat me.... I'm not a big chap or a handsome chap but my wife is so pretty and I don't think I come up to her high standards." In other words, he perceived a difference in their level of desirability; she was attractive and alluring, and he saw himself as beneath her. When the psychiatrist questioned the wife in private, she admitted to meeting and having an affair with a married man. The affair was carried on in secret, and throughout the duration of her affair she insisted that her husband's jealousy was delusional. The affair began roughly one year before the husband was referred to the psychiatrist to treat "his problem."

Differences in desirability — when an "8" is married to a "10" — can heighten sensitivity to signals of infidelity in the partner who has fewer outside mating options. Elaine Hatfield and her colleagues at the University of Hawaii discovered that the more desirable partner in the couple in fact is more likely to stray. Those who have been in relationships with both more attractive and less attractive partners have an acute awareness of how jealousy is attuned to these differences. These differences represent one among many signs of actual or impending infidelity explored in depth later in the book.

Emotional Wisdom

Jealousy is necessary because of the real threat of sexual treachery. In a hazardous world where rivals lurk, partners harbor passions for other people, and infidelity threatens to destroy what could have been a lifelong love, it would be surprising if evolution had not forged elaborate defenses to detect and fend off these threats. Exposing these threats, and the psycho-logical arms we have to combat them, is a first step toward comprehending the wisdom of passions that sometimes seem so destructive.

The Dangerous Passion takes us on a journey through the rationality of these seemingly irrational emotions, examining the fundamental desires of what men and women want, and why these longings so often produce conflict. Chapter 2 introduces the jealousy paradox — why an emotion that evolved to protect love can rip a relationship apart. It explores the evolution of conflict between men and women, why painful emotions are necessary in resolving conflicts, and why men and women are locked in a never-ending spiral of love and strife.

Chapter 3 focuses on why men and women differ in their underlying psychology of jealousy. It reveals that men and women are neither unisex equivalents nor aliens from different planets. When it comes to adaptive problems that differ for men and women, passions diverge; for adaptive problems that are the same, their emotions joyfully commingle.

Chapter 4, "The Othello Syndrome," investigates seemingly bizarre clinical cases in which a jealous person becomes untethered, resulting in delusional suspicions about a partner's infidelity. We explore why our minds are designed not merely to pick up on infidelities that have already occurred, but also to detect circumstances that signal an increased likelihood that a partner will stray in the future. Chapter 5 delves into the frightening abuses produced by the dangerous passion — battering, stalking, and killing — and identifies when women are most vulnerable to these violations.

Although I call jealousy the dangerous passion, it cannot be disentangled from the risky cravings that men and women harbor for other lovers. Chapter 6 examines the qualities of relationships that make a person susceptible to infidelity, the personality characteristics that predict who's likely to cheat, and why some people unwittingly drive their partners into the arms of a paramour. Chapter 7 explores why women have affairs, and why modern women have inherited from their ancestral mothers a roving eye.

Chapter 8 identifies the strategies we use to cope with jealousy and infidelity and why some therapeutic efforts to eradicate jealousy are often misguided. The final chapter reveals the positive uses of jealousy for enhancing sexual passion and life-long love, and examines how we can harness emotional wisdom to enrich our relationships.

Copyright © 2000 by David M. Buss

About the Author

David M. Buss, Ph.D., is a professor of psychology at the University of Texas, Austin, and previously taught at Harvard University and the University of Michigan. He is the author of the highly praised The Evolution of Desire and is internationally known for his expertise on sex, emotions, and the strategies of human mating. He lives in Austin, Texas.