When two people are first together, their hearts are on fire and their passion is very great. After a while . . . they continue to love each other, but in a different way—warm and dependable.
—MARJORIE SHOSTAK, Nisa: The Life and Words of a !Kung Woman
TREMENDOUS BENEFITS FLOW to couples who remain committed. From this unique alliance come efficiencies that include complementary skills, a division of labor, and a sharing of resources, as well as mutual benefits such as a unified front against mutual enemies, a stable home environment for rearing children, and a more extended kin network. To reap these benefits, people must be able to retain the mates they have succeeded in attracting.
People who fail to stay together incur severe costs. Bonds between extended kin are ripped apart. Essential resources are lost. Children may be exposed to potentially dangerous stepparents. Failure to keep a committed mate can mean wasting all the effort expended in the selection, attraction, courting, and commitment process. Men who fail to prevent the defection of their mate risk losing access to valuable childbearing capabilities and maternal investment. Women who fail to retain their mate risk losing the mate’s resources, protection, and paternal investment. Both sexes incur opportunity costs, the lost opportunities for exploring other mating prospects.
Given the high rate of divorce in Western cultures, and the existence of divorce in all cultures, it is obvious that staying together is neither automatic nor inevitable. Rivals loom on the periphery, waiting for an opportunity to mate-poach. Existing mates sometimes fail to provide the promised benefits. Some start imposing costs that become too burdensome to bear. Couples are surrounded by people who have agendas at odds with their own and who attempt to loosen or fracture their bond. Staying together can be difficult unless the couple undertakes strategies designed to ensure a successful, committed union.
Mate-keeping tactics occupy an important place in nonhuman animal mating. Although they are phylogenetically far removed from humans, insects offer instructive contrasts with humans because of the great diversity of their tactics and because human ways of solving the adaptive problem of keeping a mate are strikingly analogous to those of insects.1 One of the most frequent strategies insects use is to conceal their mate from competitors. Concealment tactics include physically removing a mate from an area dense with competitors, covering up the attractive cues of a mate, and reducing the conspicuousness of the courtship display. Male wasps who successfully follow the scents of a female to her perch immediately whisk her away to prevent the mating attempts of other males who may also be tracking her scent.2 If the male wasp fails to remove the female, he risks a physical battle with other males who converge on the perch. Male beetles release a scent that reduces their mate’s attractiveness, preventing other males from noticing the female or making it more likely that other males will search for unmated females. A male cricket starts out with loud calls, but he softens them as he gets close to the female in order to avoid alerting competitors to their union and the interference that might follow.3 All of these concealment tactics reduce a mate’s contact with potential interlopers.
Another strategy is to physically prevent a takeover by other males. Many insects maintain close contact with the mate and repel interfering competitors. The male veliid water strider, for example, grasps his mate and sometimes rides on her back for hours or days, even while not copulating, to prevent encroachment. Faced with rival males, insects may use their antennae to lash out at them, turn and wrestle with them, or simply chase them off.4 Perhaps the most unusual form of physical interference is the insertion of copulatory plugs. One species of worm, for example, adds a special substance to the seminal fluid that coagulates once deposited in the female, preventing other males from inseminating her and literally cementing his own reproductive bond with her. And in one species of fly, the Johannseniella nitida, males leave their genitalia broken off from their own bodies after copulation to seal the reproductive opening of the female. These adaptations to hinder sperm competition illustrate the extraordinary lengths to which males go to prevent reproductive takeovers by rivals.5
Although humans and insects are only distantly related, the basic adaptive logic behind holding on to a mate shows striking parallels. Males in both cases strive to inseminate females and to prevent cuckoldry. Females in both cases strive to secure investments in return for mating access. But human mate retention tactics take on uniquely intricate forms of psychological manipulation that set them apart from the rest of the animal kingdom.
Humans differ from most nonhuman animals in forming long-term and highly committed mateships. Remaining bonded is crucial for women and men alike. Although mate-keeping tactics among insects are performed primarily by males, among humans both men and women use them. Indeed, women are equal to men in the effort they channel toward the adaptive problem of staying together. This equality follows from the evolutionary logic of the value of the reproductive resources that would be lost by a breakup compared with the potential gains an individual could accrue by a breakup. Because men and women who embark on a committed relationship tend to couple with individuals of equivalent desirability, the 8’s with other 8’s and the 6’s with other 6’s, both sexes lose equally, on average, as a result of a breakup.6
The Menace of Mate Poachers
One reason mate-keeping tactics are crucial is because mate poaching is an ever-present threat. Desirable mates are always in short supply. Glamorous, interesting, attractive, socially skilled people are heavily courted and rapidly removed from the mating pool. Those who succeed in attracting the 9’s and 10’s tend to hold on to them, escalating the effort they allocate to mate guarding.7 Transitions between relationships are brief for the beautiful. In modern monogamous societies, for those left on the sidelines of the mating dance, mate shortages get more severe with each passing year. In traditional polygynous societies, where most desirable women marry shortly after puberty, single men suffer the most. How can a person find a desirable mate when all these factors conspire to take attractive mates out of the mating market?
One unpretty solution to this recurrent quandary is mate poaching. Although many regard efforts to lure someone out of an existing mateship as morally reprehensible, it has a long recorded history.8 One of the earliest written records of mate poaching is the biblical account of King David and Bathsheba. One day King David happened to spy the beautiful Bathsheba, wife of Uriah, bathing on the roof of a neighboring house. David’s passion for her consumed him. He succeeded in seducing her, and consequently Bathsheba became pregnant. King David set out to destroy Uriah by sending him to the battle front and commanding his troops to retreat, exposing him to mortal danger. After Uriah was killed, King David married Bathsheba. Although their first child died, their union proved fruitful, and they went on to produce four children.
Mate poaching is a common mating strategy. David Schmitt and I discovered that 60 percent of men and 53 percent of women admitted to having attempted to lure someone else’s mate into a committed relationship. Although more than half of these attempts failed, nearly half succeeded. This similarity between the genders in long-term poaching attempts contrasted with poaching efforts targeting brief sexual encounters—60 percent of the men but only 38 percent of the women reported attempting to lure someone else’s mate into a casual sexual encounter. Far higher percentages of both genders said that others had attempted to entice them to leave their own existing relationship—93 percent of the men and 82 percent of the women for long-term love, and 87 percent of the men and 94 percent of the women for a brief sexual encounter.
Somewhat smaller percentages reported that someone had attempted to poach their own mate, suggesting that poaching ploys are often initiated away from the prying eyes of the unsuspecting “victim.” Roughly one-third of our sample—35 percent of the men and 30 percent of the women—reported that a partner had been successfully taken away from them by a mate poacher. Although many attempts at mate poaching fail, nearly one-third appear to succeed. Schmitt has replicated these basic findings in a massive cross-cultural study involving more than thirty nations.9 Mate poaching has probably been successful often enough to have evolved as a distinct sexual strategy.
People poach for many of the same reasons they mate to begin with—to find emotional intimacy, experience passionate sex, secure protection, gain resources, enhance social status, fall in love, or have children. But mate poachers perceive additional benefits unique to the context of mate poaching. One is gaining revenge against a rival by stealing the rival’s mate. Vengeance could only have evolved as a motive, of course, if it served an adaptive function, such as inflicting a cost on a rival that lowered the rival’s relative reproductive success or deterring other potential rivals from inflicting costs. Another benefit is securing access to a pre-approved mate, one who has already established credibility by passing another’s screening criteria. Although enticing a mate who is already “taken” can provide these benefits, it sometimes comes at a price. Mate poachers risk violence—injury or even death—at the hands of the jealous partner. A poacher also incurs damage to his or her social reputation if branded as a deceiver. Poachers may be shunned when word of their deceit gets around, impairing their ability to attract other potential mates. Furthermore, if the mate poacher is successful, it might be costly to have a mate who is revealed to be potentially poachable and thus requires more expensive mate guarding.
Schmitt and I found that many of the tactics used to attract mates in other contexts—enhancing appearance, displaying resources, showing kindness, presenting a sense of humor, revealing empathy, and so on—are also effective for the purpose of poaching. Two tactics, however, are specially tailored to enticing mates away from others. The first is temporal invasion, which includes acts such as changing one’s schedule in order to be around the target more often than the target’s current partner, or dropping by when the current partner is off at work or out of town. The second is driving a wedge—infiltrating the existing mateship and actively promoting a breakup. One way to drive a wedge is to boost the target’s self-esteem, conveying messages that enhance their self-perceptions of their own desirability. At the same time, the poacher might communicate that the target is not appreciated by the regular partner: “He doesn’t treat you well,” or “You deserve better,” or “You’re too good for him.” The boost in self-esteem combined with the feeling of being underappreciated is sometimes enough to widen a small crack in a relationship. Through this double-pronged strategy, the mate poacher frees up an already taken mate and sits waiting in the wings when it happens.
Although not terribly admirable, there is good evidence that mate poaching can be an effective mating strategy. Indeed, those who pursue a mate poaching strategy have a larger number of lifetime sex partners and dating partners.10
Humans have evolved their own special strategies for defending against mate poachers and retaining a mate. Women in relationships are especially vigilant about rival women, whereas men tend to be more vigilant about monitoring their own partner.11 One of the most important mate retention strategies involves continuing to fulfill the desires of one’s mate—the desires that led to the mate selection to begin with. But merely fulfilling these desires may not be enough if rivals are attempting the same thing. Ancestral humans needed a psychological mechanism specifically designed to alert them to potential threats from the outside, an adaptation that would regulate when to deploy mate-guarding strategies. That mechanism was sexual jealousy.
Gender-Linked Adaptive Functions of Sexual Jealousy
Whenever males contribute to their offspring, they confront the problem of uncertain paternity. This problem occurs whenever fertilization and gestation occur inside the female’s body. It intensifies whenever males invest in offspring after they are born. Compared with many other male mammals, men invest tremendously in their offspring. Cuckoldry is therefore a serious adaptive problem that men have had to solve throughout human evolutionary history. The prevalence of the problem in the animal kingdom is reflected in the fact that so few mammalian males invest at all in their young.12 Among chimpanzees, our closest primate relatives, males defend their troop against chimp aggressors, but they invest little or nothing in their own offspring. Males would incur a double penalty if they invested in their children without some certainty of paternity: their parental effort would not only be wasted but might also get channeled to a rival’s offspring. The general failure of male mammals to invest in their young is telling—most have not solved the problem of ensuring paternity. The fact that men do invest heavily in their young provides powerful evidence that our ancestors evolved effective paternity assurance adaptations. Studies of sexual jealousy, in all its diverse manifestations, provide direct evidence that jealousy is one of the primary adaptations.
Imagine getting off work early and returning home. As you enter the house you hear sounds coming from the back room. You call your partner’s name, but no one answers. As you approach the back room, sounds of heavy breathing and moaning become louder. You open the bedroom door. On the bed is a stranger, naked and having passionate sexual intercourse with your partner. What emotions would you experience? If you are human, you would most likely experience some combination of humiliation, rage, betrayal, despondency, and grief.13
Sexual jealousy consists of emotions that are evoked by a perceived threat to a sexual relationship. The perception of a threat leads to actions designed to reduce or eliminate that threat.14 These can range from vigilance, which functions to monitor the mate for signs of extra-pair involvement, to violence, which inflicts a heavy cost on the mate or rival. Sexual jealousy is activated by cues that someone else has an interest in one’s mate or by indications of a mate’s defection, such as flirting with someone else. The rage, sadness, and humiliation following these cues motivate action intended either to cut off a rival, prevent the mate’s defection, or, sometimes, cut one’s losses.
Men who fail to solve this adaptive problem risk not only suffering direct reproductive costs but also losing status and reputation, which can seriously impair their ability to attract other mates. Consider the reaction in Greek culture to cuckoldry: “The wife’s infidelity . . . brings disgrace to the husband who is then a Keratas—the worst insult for a Greek man—a shameful epithet with connotations of weakness and inadequacy. . . . While for the wife it is socially acceptable to tolerate her unfaithful husband, it is not socially acceptable for a man to tolerate his unfaithful wife and if he does so, he is ridiculed as behaving in an unmanly manner.”15 Cuckolded men are universal objects of derision. The penalties for failure to keep a mate, including the loss of social status, are compounded by diminished future success in the game of mating.
Most research on jealousy has focused on male sexual jealousy, probably because of the asymmetry between men’s and women’s confidence about their parenthood. Nonetheless, women experience jealousy just as intensely; a mate’s contact with other women can lead him to redirect his resources and commitment away from her and her children and toward another woman and her children. Men and women do not differ in either the frequency or the magnitude of their jealousy experience. In one study, 300 individuals who were partners in 150 romantic relationships were asked to rate how jealous they were in general, how jealous they were of their partner’s relationships with members of the other sex, and the degree to which jealousy was a problem in their relationship. Men and women admitted to equal amounts of jealousy, confirming that both experience jealousy and overall do not differ in the intensity of their jealous feelings.16
These reactions are not limited to the United States. Over 2,000 individuals from Hungary, Ireland, Mexico, the Netherlands, the United States, and the former Soviet Union and Yugoslavia were asked their reactions to a variety of different sexual scenarios. Men and women in all of these places expressed identically negative emotional reactions to thoughts of their partner’s flirting with someone else or having sexual relations with someone else. The genders are also the same in their jealous reactions to a sexual partner’s hugging someone else or dancing with someone else, although their responses to these events are less negative than to flirting and sexual relations. For both men and women worldwide, as in the United States, jealousy is an adaptation that becomes activated in response to a threat to a valued relationship.17
Despite these similarities, there are intriguing gender differences in the content and focus of jealousy and in the specific events that trigger jealousy. In one study, twenty men and twenty women were asked to play a role in a scenario in which they became jealous.18 But first the participants had to choose their scenario from among a group of possible scenarios, which typically involved either jealousy over a partner’s sexual involvement with someone else or jealousy over a partner’s devotion of time and resources to someone else. Seventeen women chose infidelity in the allocation of either resources or time as the jealousy-inducing event, and only three women chose sexual infidelity. In marked contrast, sixteen of the twenty men chose sexual infidelity as the jealousy-inducing event, and only four men chose the diversion of time or resources. This study provides the first clue that, although both men and women have the jealousy adaptation, it is triggered by different events, and that those events correspond to the adaptive problems of ensuring paternity for men and ensuring resources and commitment for women.
In another study, fifteen couples listed situations that would make them jealous. Men identified sexual involvement between their partner and another man as the primary cause of jealousy, and secondarily comparison between themselves and a rival. Women, in contrast, indicated that they would become jealous primarily in response to their partner’s spending time with other women, talking with a rival, and kissing a female rival.19 Women’s jealousy, in short, is triggered by cues to the possible diversion of their mate’s investment to another woman, whereas men’s jealousy is triggered primarily by cues to their mate’s having sex with another man.
These gender differences reveal themselves both psychologically and physiologically. In a study of sex differences in jealousy, my colleagues and I asked 511 college men and women to compare two distressing events—if their partner had sexual intercourse with someone else and if their partner formed a deep emotional attachment to someone else.20 Fully 83 percent of the women found their partner’s emotional infidelity more upsetting, whereas only 40 percent of the men did. In contrast, 60 percent of the men experienced their partner’s sexual infidelity as more upsetting, whereas only 17 percent of the women did.
To evaluate a different group—this one comprising sixty men and women—on their physiological distress in response to sexual and emotional infidelity, we placed electrodes on the corrugator muscle in the brow, which contracts when people frown; on the first and third fingers of the right hand to measure skin conductance, or sweating; and on the thumb to measure heart rate. Then we asked people to imagine two types of infidelity, sexual and emotional. Men became more physiologically distressed by the sexual infidelity. Their heart rates accelerated by nearly five beats per minute, equivalent to drinking three cups of coffee in one sitting. Their skin conductance increased 1.5 micro-siemens with the thought of sexual infidelity, but showed little change from baseline in response to the thought of emotional infidelity. And their frowning increased, showing 7.75 microvolt units of contraction in response to sexual infidelity, as compared with only 1.16 units in response to emotional infidelity. Women tended to show the opposite pattern, exhibiting greater physiological distress at the thought of emotional infidelity. Women’s frowning, for example, increased to 8.12 microvolt units of contraction in response to emotional infidelity, from only 3.03 units of contraction in response to sexual infidelity. Other researchers have replicated the physiological gender differences using multiple measures.21 The coordination of psychological distress with physiological distress illustrates the precision with which humans have adapted over time to the specific threats they have faced to keeping a mate.
Sex differences in the causes of jealousy are not limited to Americans. In one study of jealous men and women in central Europe, 80 percent of the men expressed fears of a sexual nature, such as worrying about their mate’s having intercourse with another man or worrying about their own sexual adequacy.22 Only 22 percent of the jealous women expressed sexual concerns, the majority focusing instead on the emotional relationship, such as the degree of closeness between their mate and another woman. Men in Hungary, Ireland, Mexico, the Netherlands, the Soviet Union, the United States, and Croatia all showed more intense jealousy than women in response to their partner’s having sexual fantasies about another person.23 These gender differences in the triggers of jealousy appear to characterize the entire human species.
Competing Explanations for Gender Differences in Jealousy
The evolutionary interpretation of sex differences in jealousy has been challenged.24 Some have proposed that sexual infidelity and emotional infidelity are often correlated, and indeed they are. People tend to get emotionally involved with those with whom they have sex and, conversely, tend to become sexually involved with those with whom they are emotionally close. But men and women might differ in their beliefs about the correlation. According to this hypothesis, perhaps women get more upset about a partner’s emotional involvement with someone else because they think it implies that their partner will also become sexually involved. Women might believe that men can have sex, in contrast, without getting emotionally involved, and so imagining a partner’s sexual involvement with another person is less upsetting. Men’s beliefs might differ. Perhaps men get more upset about a partner’s sexual involvement with another man because they think that she is likely to have sex with him only if she is also emotionally involved, whereas they think that a woman can easily become emotionally involved without having sex with a man.
My team and I conducted four empirical studies in three cultures to test predictions from the competing evolutionary and belief hypotheses.25 The first study involved 1,122 undergraduates at a liberal arts college in the southeastern United States. The original infidelity scenarios were altered to render the two types of infidelity mutually exclusive. Participants reported their relative distress in response to a partner’s sexual infidelity with no emotional involvement and their response to the partner’s emotional involvement with no sexual infidelity. A large gender difference emerged, as predicted by the evolutionary model. If the belief hypothesis were correct, then the gender difference should have disappeared. It did not.
Our second study provided four additional tests of the predictions from the two models, using three research strategies. One strategy employed three versions of rendering the two types of infidelity mutually exclusive. A second strategy posited that both types of infidelity had occurred and asked participants to indicate which aspect they found more upsetting. A third strategy used a statistical procedure to test the independent predictive value of sex and beliefs in accounting for which form of infidelity would be more distressing. The results were conclusive: large gender differences were discovered, precisely as predicted by the evolutionary model. No matter how the questions were worded, no matter which methodological strategy was employed, and no matter how stringently the conditional probabilities were controlled, the gender differences remained robust.
Our third study replicated the infidelity dilemmas in a non-Western sample of native Koreans. The original sex differences were replicated. With two strategies to control for the co-occurrence of sexual and emotional infidelity, the gender differences again remained robust. The evolutionary hypothesis survived this empirical hurdle. In our fourth study, we tested the predictions about jealousy and about the nature of beliefs in a non-Western Japanese sample. The results again provided support for the evolutionary hypothesis. In yet another study, Brooke Scelza surveyed a small-scale population, the Himba of Namibia, and also found that men more than women were more distressed by the sexual aspect of the infidelity when both forms of infidelity occurred.26 And finally, the evolutionary psychologist Barry Kuhle analyzed spontaneous jealous interrogations following the discovery of an actual infidelity. He found that men more than women wanted to know, “Did you have sex with him?” whereas women more than men wanted to know, “Do you love her?”27
Perhaps more important than the details of any one study is evaluation by the key scientific criterion—the weight of the evidence.28 The sex differences in jealousy have now been discovered using an astonishingly wide array of diverse methods. The sex differences in jealousy using the forced-choice method are robust across cultures such as Brazil, England, Romania, Korea, Japan, the Netherlands, Norway, and Sweden, suggesting universality.29 The sex differences remain robust when participants are asked “which aspect” of the infidelity would be most distressing when both sexual infidelity and emotional infidelity have occurred. Most, although not all, researchers examining physiological responses to jealousy have replicated the sex differences in physiological distress. The sex differences become even more pronounced among those who have experienced an actual infidelity in their lives and when participants undergo a procedure that requires them to vividly imagine the experience of infidelity. Men have more difficulty than women in forgiving a sexual than an emotional infidelity and indicate a greater likelihood of ending a relationship following a sexual infidelity than an emotional one.
Cognitively, men, compared to women, show greater memorial recall of cues to sexual than to emotional infidelity; preferentially search for cues to sexual rather than to emotional infidelity; involuntarily focus attention on cues to sexual rather than to emotional infidelity; and show faster decision times in response to cues to sexual than to emotional infidelity.
A study of brain activation, using fMRI brain scans of participants as they viewed imagery of sexual and emotional infidelity, found striking sex differences.30 Men showed far greater activation than women in the amygdala and hypothalamus—brain regions involved in sexuality and aggression. Women, in contrast, showed greater activation than men in the posterior superior sulcus—a brain region involved in the process of mind-reading, such as inferring a partner’s future intentions. These findings are precisely what we would expect if male and female jealousy adaptations were designed to solve somewhat different adaptive problems.31 In sum, the sex differences in jealousy remain robust across cultures and across a wide range of methods, including psychological dilemmas, physiological recordings, cognitive experiments, and fMRI recordings of brain activation.
Several other gender-differentiated design features of the jealousy adaptation have been documented. Men’s jealousy is especially attuned to rivals who have status and resources; women’s jealousy is especially attuned to rivals who are physically attractive.32 One man said, “The thought of my ex having sex with another was excruciating. . . . I would stay awake with this thought going through my head, could feel my temperature rise to boiling point.”33 One woman said that, “with girls, if they are pretty, or if he says they are pretty, I don’t like it at all.”34 Interestingly, these sex differences in distress over the attributes of rivals—women’s anger at a rival’s attractiveness and men’s anger at a rival’s status and resources—show up even in women and men diagnosed as having “pathological” jealousy.35
Men more than women display an “infidelity overperception bias” in overestimating their partner’s likelihood of sexual infidelity.36 Finally, among women and men who are prone to chronic jealousy and worry a lot about relationship threats, the gender differences in response to sexual versus emotional infidelity are especially large.37
Mate Retention Through Fulfilling a Partner’s Desires
Once jealousy has become activated by threats to the security of one’s mateship, it can motivate tactics directed at the mate, at the rival, or at oneself. Men and women use an astonishing variety of tactics to keep a mate. A partner’s original mate preferences form the basis for one major strategy: fulfilling the partner’s preferences—that is, providing the sorts of resources he or she initially sought—should be a highly effective method of preserving the relationship.
To investigate this possibility, I initiated mate retention studies.38 First, I asked dating men and women to describe specific behaviors they had observed in people trying to hold on to partners to prevent them from becoming involved with someone else. They came up with 104 identifiable acts, which a team of four investigators classified into nineteen discrete clusters. The cluster called “vigilance,” for example, included calling a partner at unexpected times to see who he or she was with, having one’s friends check up on the partner, snooping through personal belongings, and dropping by unexpectedly to see what the partner was doing. Finally, I asked 102 college students who were involved in dating relationships and 210 newlyweds to rate how frequently they performed each of these acts of vigilance. In their fifth year of marriage, the newlywed couples again reported on their use of mate retention tactics. A separate panel of judges evaluated the tactics for their effectiveness in keeping a mate when performed by a man and when performed by a woman.
Fulfilling the initial mating desires of the partner did indeed prove to be an effective mate-keeping tactic. Because women desire love and kindness in their initial selection of a mate, continuing to provide love and kindness is a highly effective tactic for men who want to keep their mates. Men who tell their mates that they love them, are helpful when their mates need assistance, and regularly display kindness and affection succeed in retaining their mates. These were judged to be the most effective acts that men could perform, with an effectiveness rating of 6.23 on a 7-point scale, and were significantly more effective for men than the same acts performed by women, which received an effectiveness rating of 5.39. Furthermore, the performance of these acts was directly linked with the length of the relationship among dating couples and with the duration of marriage after five years. Husbands who failed to perform acts of love and commitment were more likely to have a wife who was contemplating or seeking divorce than husbands who were kind and loving. Acts of love and kindness succeed because they signal an emotional commitment to the relationship, they bestow a benefit rather than inflicting a cost, and they fulfill women’s psychological preferences for a desirable partner.
Because women also value economic and material resources, continuing to provide them is another highly effective tactic for men to keep their mates. In the service of this goal, men reported spending a lot of money on their mates and buying them expensive gifts. Among committed dating couples, men provided these external resources more often than women did. Providing resources was the second most effective tactic for men in retaining a mate, with an average effectiveness rating of 4.50, in contrast to a rating of 3.76 when used by women. Men more than women provided resources in the service of keeping their mates during the newlywed phase of marriage, and they continued to use this tactic more often than their wives after five years of marriage.39 Like successful tactics for attracting a mate, successful tactics for keeping a mate fulfill the mate’s desires—in this case, providing the economic and material resources on which women place a premium.
Analogously, men value physical attractiveness, and I found that women were most likely to cite enhancing their appearance as one of their primary mate retention tactics—a finding that is replicable across cultures, including Brazil, Croatia, and the United States.40 Out of the nineteen clusters evaluated, enhancing their appearance was the second most effective tactic used by women, after love and kindness. Women go out of their way to make themselves attractive to their partners, making up their faces to look nice, dressing to maintain a partner’s interest, and acting sexy to distract a partner’s attention from other women. Not only newlywed women but also women married for five years enhance their physical appearance in the service of keeping a mate, which shows that, for women, continuing to fulfill men’s initial mating desires is a key to staying together.
The importance of appearance was dramatically illustrated by a study in which men and women watched a videotape of a couple sitting on a couch talking.41 After forty-five seconds, during which the couple cuddle, kiss, and touch one another, one of the partners gets up and leaves the room to refill their wineglasses. Seconds later, an interloper enters and is introduced as the previous girlfriend or boyfriend of the partner who has remained on the couch. (The men watched the version with a previous boyfriend as the interloper, and the women watched the version with an old girlfriend as the interloper.) The partner stands up and briefly hugs the interloper, then the two sit down on the couch. Over the next minute they perform intimate actions, such as kissing and touching. The absent partner then returns, stops, and looks down at the two people who are showing affection to each other on the couch. The tape ends there. Women who saw the tape were nearly twice as likely as men to report that, in response to this threat to keeping a mate, they would try to make themselves more attractive to their partner. Men, in contrast, were more likely to say that they would become angry, suggesting a more aggressive strategy for keeping a mate. Women enhance their appearance because it activates men’s evolved desires.
Emotional Manipulation
When tactics such as providing resources, love, and kindness fail, people sometimes resort to desperate emotional tactics to retain their mates, particularly if they are lower in mate value. Examples are crying when the partner indicates interest in others, making the partner feel guilty about such interest, and telling the partner that they are hopelessly dependent on him or her.
Submission or self-abasement is another tactic of emotional manipulation. For example, people may go along with everything their mate says, let that person have his or her way, and promise to change—a desperation tactic if there ever was one. In spite of the common stereotype that women are more submissive than men, the mate retention studies show the opposite in mate retention tactics. Men submit to, and abase themselves before, their mates roughly 25 percent more than women do. This gender difference shows up among college dating couples, among newlywed couples, and even among couples after several years of marriage. The gender difference in self-abasement cannot be attributed to a male reporting bias, because their spouses corroborate those reports.
Precisely why men submit and self-abase more than women remains a puzzle. Perhaps a man who perceives himself to be lower in mate value than his partner uses submission to try to prevent her from ditching him. Perhaps the tactic represents an attempt to placate a woman who is on the verge of leaving. But these speculations are not satisfactory because they do not answer the question of why men need to resort to this tactic more than women. Only future research can unravel the answer to this mystery.
Another emotional manipulation is intentionally trying to provoke sexual jealousy with the goal of keeping a mate. This tactic includes actions such as dating others to make a mate jealous, talking with people of the opposite sex at parties to incite jealousy, and showing an interest in people of the opposite sex to make a mate angry. People perceive these tactics to be nearly twice as effective for women as for men. A woman who flirts with other men in order to elicit jealousy and thereby hold on to a mate, however, is walking a fine line: eliciting jealousy injudiciously might provoke either violence or abandonment if her mate perceives her as promiscuous.
Although women admit to inducing jealousy more than men do, not all women resort to this tactic. One study has identified a key context in which women do intentionally elicit jealousy.42 It examined discrepancies between partners in their emotional commitment to the relationship. Such discrepancies can signal differences in the desirability of the partners, since the less involved person is generally higher in mate value. Whereas 50 percent of the women who viewed themselves as more involved than their partner intentionally provoked jealousy, only 26 percent of the women who were equally or less involved used this tactic. These women acknowledged that they were motivated to elicit jealousy in order to increase the closeness of the relationship, to test the strength of the relationship, to see if the partner still cared, and to inspire possessiveness. Discrepancies between partners in desirability, as indicated by differences in involvement in the relationship, apparently cause women to provoke jealousy as a tactic to gain information about, and to increase, men’s commitment.
Repelling Mate Poachers
Like many species, humans show proprietary attitudes toward their possessions and toward their mates. One method for signaling ownership is a public marking that tells rivals to stay away. Public signals of possession can be verbal, as in introducing a person as a spouse or lover and bragging about a mate to friends. Public signals can be physical, such as holding hands with or putting one’s arm around a mate in front of other people. Public signals can also be ornamental, such as asking a mate to wear one’s jacket, giving jewelry that signifies that the person is taken, and displaying a Facebook relationship status to signify that the person is taken.
Although men and women do not differ in how often they use these public measures to repel mate poachers, our expert panel judged the signals to be significantly more effective when used by men than by women.43 The reason may be that public signals provide a strong cue to the woman of a man’s intent to commit. An engagement ring, for example, sends a strong signal to potential mate poachers. Verbal, physical, and ornamental displays attain their effectiveness by dissuading potential competitors, just as the male insect who mingles his scent with that of the female causes rivals to seek other mates who are uncontested. These displays also communicate a commitment that fulfills women’s long-term desires.
Maintaining vigilance is an additional means that both sexes use to keep their mates away from others. An animal analogue occurs among the male elephant seals on the California coast. They maintain a vigil against rivals and female defections by patrolling the perimeter of their harem. Calling a mate at unexpected times, reading his or her email, or monitoring Facebook posts are human exercises of vigilance. Vigilance represents an effort to detect whether there are any signs of defection in the mate. Vigilance also conveys a message to a mate that evidence of consorting with rivals will be detected and acted upon. People in our evolutionary past who were not vigilant experienced more defections than those who kept a watchful eye.
The tactic of concealing mates is closely related to vigilance. The male wasp whisks his mate away from the path that might be tracked by other males. Men and women conceal their mates by refusing to take them to parties where competitors will be present, refusing to introduce them to friends who might mate-poach, and taking them away from gatherings filled with potential competitors. Concealment attains its effectiveness by reducing the contact of mates with rivals and reducing the opportunities for mates to assess alternative mating prospects.
A close cousin of concealment is monopolizing a mate’s time—insisting that all free time be spent together and monopolizing the mate at social gatherings. Monopolizing a mate prevents that person from having contact with potential rivals who could poach him or her or be appealing as an attractive alternative.
These forms of mate retention have historical and cross-cultural precedents. Claustration, or the concealment of women to prevent their contact with potential sexual partners, provides a vivid example of mate monopolization. Historically, Indian men have secluded women in the interior of dwellings. Arab men have concealed the faces and bodies of women with veils or burkas. Japanese men have bound the feet of women to restrict their mobility. In societies that practice veiling, weddings are the venue for the most extreme forms of this variety of concealment—that is, those in which the greatest surface area of the skin is covered—because a woman getting married is close to peak fertility. Young prepubescent girls and older postmenopausal women are less severely concealed because they are viewed as less enticing to other men.44
Another common practice throughout human history has been for men to gather women into guarded harems. The term harem means “forbidden.” Indeed, it was often as difficult for women to leave harems as it was for outside men to get in. Kings and other rulers used eunuchs to guard their harems. In India during the sixteenth century, merchants supplied rich men with a steady supply of Bengali slave eunuchs, who not only were castrated but had their entire genitalia cut off.45
The number of women collected into harems is staggering by any standard. The Indian emperor Bhuponder Singh had 332 women in his harem when he died, “all of [whom] were at the beck and call of the Maharaja. He could satisfy his sexual lust with any of them at any time of day or night.”46 In India, the harems of sixteenth-century kings were estimated to have between 4,000 and 12,000 occupants.47 In Imperial China, emperors around 771 BC kept one queen, three consorts or wives of the first rank, nine wives of the second rank, twenty-seven wives of the third rank, and eighty-one concubines.48 In Peru, an Inca lord kept a minimum of 700 women “for the service of his house and on whom to take his pleasure . . . [having] many children by these women.”49
All of these public signals of mate retention served the sole goal of preventing contact between the mates and potential rivals. Because high-status men historically have been in a position of power, their ability to apply extreme tactics has reduced women’s freedom of choice. In modern industrial societies, with greater gender equality, both sexes deploy public signals—albeit typically less drastic ones than those used by medieval lords—to retain their mates.
Cost-Inflicting Mate Retention Tactics
Another mate retention tactic is to inflict costs on competitors or on mates through derogation, threats, and violence. These contrast sharply with benefit-conferring tactics such as providing resources or bestowing love and kindness. Destructive tactics acquire their effectiveness by deterring interlopers from poaching and deterring mates from straying.
One set of these cost-inflicting tactics is aimed at rivals. Verbal denigration of competitors is perhaps the mildest form of this tactic, although in Ecclesiasticus (28:17) it is noted that “the blow of a whip raises a welt, but a blow of the tongue crushes the bones.” To dissuade their mates from becoming attracted to a rival, both men and women may belittle the rival’s appearance or intelligence or start damaging rumors about him or her. Derogation of competitors continues even after vows of undying commitment are taken, because mate switching is always a possibility. When used judiciously, it is an effective method for rendering rivals less attractive and lowers the odds of a mate’s defection.
A costlier tactic is subjecting a rival to verbal threats and violence. Just as chimpanzees bare their teeth in a threat that sends rivals scurrying from a female, newlywed men yell at rivals who look at their bride, threaten to strike those who make passes at her, and stare coldly at men who look too long at her. These threatening retention tactics are performed almost exclusively by men. Although they are not performed often, we discovered that 46 percent of married men had threatened an intrasexual competitor within the past year, whereas only 11 percent of the married women had done so. These tactics convey strong signals that mate poaching attempts come with large risks.
Men sometimes inflict even more extreme costs on their rivals. Married men may hit men who make passes at their wives, get their friends to beat up a rival, slap men who show too much interest in their wives, or vandalize their property. These acts impose heavy costs in the form of bodily injury or, in rare cases, death. The reputation that these acts earn for their perpetrator deters other men. Most men would think twice before flirting with the girlfriend of a large, rough-looking, burly man with a reputation for hair-trigger violence.
Another set of cost-inflicting tactics are sometimes directed at mates to deter them from straying. Male baboons and other primates wound females through bites or beatings when they consort with other males.50 Married men and women become angry when their partner flirts with others, yell if the partner flirts, and threaten to break up if their partner ever cheats. Men perceived by their partner as lower in mate value are especially likely to verbally insult her, possibly because these men, with fewer benefits to bestow, are trying to reduce her perception that she is the more desirable partner.51 Moreover, both genders sometimes threaten never to speak with their partner again if they catch the partner with someone else. Occasionally they hit their partner when he or she flirts with others. Men in committed dating relationships and men who are married inflict these costs nearly twice as often as men who do not expect to be with their current mate in the future.
Punishing a mate who shows signs of interest in others acquires its effectiveness from the deterrent value of the threatened costs. Some of these costs are physical, such as bodily injury. Other costs are psychological, such as the lowered self-esteem that comes from being yelled at or otherwise verbally abused.52 Since a person’s self-esteem is, in part, an internal tracking device reflecting self-perceived mate value, these forms of abuse may be functional in mate retention, however morally abhorrent they are.
Clues to the functionality of partner abuse come from the specific contexts in which it is deployed.53 One circumstance is the perception or discovery of sexual infidelity. A second is when there exists a mate value discrepancy: the less desirable partner may abuse the other to reduce that partner’s perceptions of the discrepancy. A third is when a woman is pregnant and the man suspects that the child might not be his. Blows in this circumstance are often directed to the woman’s abdomen, suggesting the disturbing possibility that the function of this form of violence is to abort a rival’s child.
These vicious sorts of spousal abuse are seen across cultures. Studies of the Baiga, for example, reveal cases in which husbands have attacked their wives with blazing logs as punishment for flirting with another man, a form of violence against women in which jealousy is the key motive.54 In studies of abused women in Canada, 55 percent of the women reported that jealousy was one of the reasons for their husband’s assault on them; half of the women who reported jealousy as a motive acknowledged that their sexual infidelity had provoked the violence.55
Brutal beatings occur not just after an infidelity is suspected but to prevent its occurrence. Moreover, several culturally created means have been developed to prevent extramarital sexual activity through genital mutilation in various cultures across northern and central Africa, Arabia, Indonesia, and Malaysia. Clitoridectomy, the surgical removal of the clitoris to prevent a woman from experiencing sexual pleasure, is practiced on millions of African women. Another practice common in Africa is infibulation, the sewing shut of the labia majora. According to one estimate, 200 million women living today in twenty-three countries in northern and central Africa have been genitally mutilated through infibulation.56
Infibulation effectively prevents sexual intercourse. It is sometimes performed by the woman’s kin as a guarantee to a potential husband that the bride is virginal. After marriage, infibulated women must be cut open to allow for sexual intercourse. If the husband goes away for a while, his wife may be reinfibulated. In the Sudan, the woman is reinfibulated after she bears a child and then must be reopened to allow for intercourse. Although the decision to reinfibulate a woman usually rests with her husband, some women demand reinfibulation after delivery, in the belief that it increases the husband’s pleasure. A Sudanese woman who fails to please her husband risks being divorced, thus losing her children, losing economic support, and bringing disgrace on her entire family.57
There are no cultures in which men are not sexually jealous. In every supposedly nonjealous culture previously thought to contain no barriers to sexual conduct beyond the incest taboo, evidence for sexual jealousy has now been found. The Marquesa Islanders, for example, were once thought to impose no formal or informal prohibitions on adultery. This notion is contradicted by the ethnographic report: “When a woman undertook to live with a man, she placed herself under his authority. If she cohabited with another man without his permission, she was beaten or, if her husband’s jealousy was sufficiently aroused, killed.”58
Another presumed example of the absence of sexual jealousy is the Inuit, a culture that practices wife sharing. Contrary to popular myth, however, male sexual jealousy is a leading cause of spousal homicide among the Inuit, and these homicides occur at an alarmingly high rate.59 Inuit men share their wives only under highly circumscribed conditions, such as when there is a reciprocal expectation that the favor will be returned in kind. Wife swapping apparently can mitigate the onset of men’s jealousy. All of these findings demonstrate that there are no paradises populated with sexually liberated people who share mates freely and do not get jealous.
Some societies require the mate poacher to pay reparation to the husband when he is caught having intercourse with the wife. Even in the United States, monetary payments to the husband have been imposed on mate poachers for “alienation of affection.” In North Carolina, for example, an ophthalmologist was required to pay a woman’s ex-husband $200,000 for having enticed her away from him. These legal strictures reflect an intuitive understanding of human evolutionary psychology: in men’s eyes, cuckoldry represents the unlawful stealing of another’s resources. When it comes to sex, men everywhere seem to regard wives as “theirs” to be owned and controlled. Men react to cuckoldry as they would to theft and sometimes leave a trail of destruction in their wake.60
A Dangerous and Deadly Passion
Men’s sexual jealousy is neither a trivial nor a peripheral emotion in human life. It sometimes becomes so powerful that it causes the person who experiences it to kill a mate or an interloper. In one case a wife killing was apparently fueled by an awareness of the reproductive damage of cuckoldry, as the husband explained:
You see, we were always arguing about her extramarital affairs. That day was something more than that. I came home from work and as soon as I entered the house I picked up my little daughter and held her in my arms. Then my wife turned around and said to me: “You are so damned stupid that you don’t even know she is someone else’s child and not yours.” I was shocked! I became so mad. I took the rifle and shot her.61
A wife’s infidelity is sometimes viewed as so extreme a provocation that a “reasonable man” may legally respond with lethal violence. In Texas until 1974, for example, it was legal for a husband to kill his wife and her lover if he did so while the adulterers were engaging in the act of intercourse; their murder was considered a reasonable response to a powerful provocation. Laws exonerating men from killing adulterous wives are found worldwide and throughout human history. Among the Yaps, for example, rules permit husbands to kill wives and their lovers and to burn them up in the house if caught in the act of adultery. Similar provisions are made for offended husbands among the Toba-Batak of Sumatra. Old Roman law granted the husband the right to kill only if the adultery occurred in his own house, and similar laws remain in effect in some European countries today.62
Male sexual jealousy is the single most frequent cause of all types of violence directed at wives, including physical abuse and actual murder. In one study of forty-four physically abused wives seeking refuge, 55 percent stated that jealousy was their husband’s key motive for assaulting them.63 Sexual jealousy is a major motive for murder. In a study of homicides among the Tiv, Soga, Gisu, Nyoro, Luyhia, and Luo in British colonial Africa, of seventy murders of wives by their husbands, 46 percent were explicitly over sexual matters, including adultery, the woman’s abandonment of the husband, and the woman’s refusal of sex with the husband.64
Many of the homicides perpetrated by women also appear to have male sexual jealousy at their root. Women who kill men frequently do so to defend themselves against an enraged, threatening, and abusive husband from whom they fear bodily harm. In a sample of forty-seven homicide cases precipitated by a jealous man, sixteen women were killed by men for real or suspected infidelity, seventeen male rivals were killed by enraged men, and nine men were killed in self-defense by women whom the men had accused of infidelity.65
Homicidal jealousy is by no means limited to American or even to Western cultures. Sexual jealousy is a leading motive behind homicide in Sudan, Uganda, and India.66 One study in the Sudan, for example, found that the leading motive for 74 of 300 male-perpetrated murders was sexual jealousy.67 Most cases of spousal homicide in every society studied are apparently precipitated by male accusations of adultery or by the woman’s leaving or threatening to leave the husband. Furthermore, about 20 percent of the homicides of men by men have as their motive rivalry over a woman or offense taken at advances made to a spouse, daughter, or female relative.68
The adaptive functions of jealousy to prevent infidelity and ensure paternity are hard to reconcile with the seemingly maladaptive act of killing one’s wife, which interferes with reproductive success by destroying a key reproductive resource. There are several possible explanations. Because the overwhelming majority of unfaithful wives are not killed, the actual killing of a wife might represent an accidental slip of the adaptation, in which violent jealousy becomes pathological, is carried too far, and intentionally or accidentally results in death.69 Although this explanation fits some cases of wife killing, it does not square with the seeming intentionality of the many men who acknowledge that they intended to kill their partner and even hunted the woman down to do so.
An alternative explanation is that the killings that stem from jealousy represent extreme but nonetheless evolved manifestations of the adaptation. Killing one’s wife would not necessarily have been reproductively damaging under all conditions during human evolutionary history. In the first place, if a wife is going to abandon her husband, not only will he lose her reproductive resources anyway, but he also may suffer the additional cost of finding that those resources are channeled to a competitor, which is a double blow to relative reproductive success.
Men who allow themselves to be cuckolded are subject to ridicule and damage to their reputation, especially if they take no retaliatory action. In a polygynous marriage, for example, killing an unfaithful wife might salvage a man’s honor and also serve as a powerful deterrent to infidelity by his other wives. Polygynous men who took no action may have risked being cuckolded with impunity in the future. In some circumstances in our evolutionary past, killing a wife may have represented an effort at damage containment designed to stop the hemorrhaging of reproductive resources.
In the face of the conflicting costs and benefits of homicide, it is reasonable to speculate that, in some circumstances, killing a spouse who was unfaithful or was determined to leave would have been reproductively more beneficial than allowing oneself to be cuckolded or abandoned with impunity. Thoughts of killing and occasional actual killings may have been adaptive over human evolutionary history and hence may be part of men’s evolved psychological machinery. This is a horrifying possibility and obviously does not justify or excuse murder, but if society is ever going to grapple successfully with the serious problem of spousal homicide, it must confront the psychological mechanisms that give rise to it, especially the circumstances that activate violent adaptations and make them especially dangerous.
The Fragile Union
It is a remarkable human achievement that a man and a woman who have no genes in common can stay together in a union of solidarity over years, decades, or a lifetime. Because of the many forces that pull couples apart, however, staying together is a fragile proposition that poses a unique set of adaptive problems. Successful solutions typically incorporate several ingredients. First, the mate is supplied with the adaptively relevant resources needed to prevent defection. Second, competitors are kept at bay, for example, by public signals of possession or through concealing the mate from others. Third, mate guarders use emotional manipulation, for example, by provoking jealousy to increase perceptions of desirability, submitting or abasing oneself to the mate, or convincing the mate that alternatives are undesirable. Fourth, cost-inflicting measures come into play, such as punishing a mate for signals of defection or physically assaulting a rival.
These diverse tactics for retaining mates succeed by exploiting the psychological adaptations of mates and rivals. The beneficial tactics, such as giving love and resources, work for a man because they fulfill the psychological desires that led the woman to choose him to begin with. For a woman, enhancing her physical appearance and providing sexual resources succeeds because they match men’s psychology of desire. Indeed, our study of married couples found that men intensify their mate retention efforts when they perceive their partner to be attractive, just as women ramp up their mate retention efforts with partners who are higher in status and income.70
Unfortunately, the tactics of threats and violence, which inflict costs on mates and rivals, also work by exploiting the psychological adaptations of others. Just as physical pain leads people to avoid the environmental hazards that can harm them, psychological fear causes people to avoid the wrath of an angry mate. Aggression sometimes pays.
Male sexual jealousy, a key adaptation underlying many tactics of mate retention, is also the dangerous emotion responsible for a majority of men’s acts of violence against their mates. It may seem paradoxical that this adaptation, which is designed to keep a mate, causes so much destruction. It does so because the reproductive stakes are so high and the reproductive interests of the players can diverge so dramatically. The goals of a married man conflict with those of his rival, who seeks to lure his desirable wife away. A man’s goals can conflict with those of his wife or girlfriend, who may become the victim of violent sexual jealousy. And when one partner wants to stay together while the other wants to break up, both parties are in for suffering—which brings us to the broader topic of conflict between the sexes.