Men And Women - Sex, Romance, And Love

The Moral Animal - Whe We Are The Way We Are: The New Science of Evolutionary Psychology - Robert Wright 1995

Men And Women
Sex, Romance, And Love

Judging from the social habits of man as he now exists, and from most savages being polygamists, the most probable view is that primeval man aboriginally lived in small communities, each with as many wives as he could support and obtain, whom he would have jealously guarded against all other men. Or he may have lived with several wives by himself, like the Gorilla... .

The Descent of Man (1871)1

One of the more upbeat ideas to have emerged from an evolutionary view of sex is that human beings are a "pair-bonding" species. In its most extreme form, the claim is that men and women are designed for a lifetime of deep, monogamous love. This claim has not emerged from close scrutiny in pristine condition.

The pair-bond hypothesis was popularized by Desmond Morris in his 1967 book The Naked Ape. This book, along with a few other 1960s books (Robert Ardrey's The Territorial Imperative, for example), represent a would-be watershed in the history of evolutionary thought. That they found large readerships signaled a new openness to Darwinism, an encouraging dissipation of the fallout from its past political misuses. But there was no way, in the end, that these books could start a Darwinian renaissance within academia. The problem was simple: they didn't make sense. {55}

One example surfaced early in Morris's pair-bonding argument. He was trying to explain why human females are generally faithful to their mates. This is indeed a good question (if you believe they are, that is). For high fidelity would place women in a distinct minority within the animal kingdom. Though female animals are generally less licentious than males, the females of many species are far from prudes, and this is particularly true of our nearest ape relatives. Female chimpanzees and bonobos are, at times, veritable sex machines. In explaining how women came to be so virtuous, Morris referred to the sexual division of labor in an early hunter-gatherer economy. "To begin with," he wrote, "the males had to be sure that their females were going to be faithful to them when they left them alone to go hunting. So the females had to develop a pairing tendency."2

Stop right there. It was in the reproductive interests of the males for the females to develop a tendency toward fidelity? So natural selection obliged the males by making the necessary changes in the females? Morris never got around to explaining how, exactly, natural selection would perform this generous feat.

Maybe it's unfair to single Morris out for blame. He was a victim of his times. The trouble was an atmosphere of loose, hyper-teleological thinking. One gets the impression, reading Morris's book, and Ardrey's books, of a natural selection that peers into the future, decides what needs to be done to make things generally better for the species, and takes the necessary steps. But natural selection doesn't work that way. It doesn't peer ahead, and it doesn't try to make things generally better. Every single, tiny, blindly taken step either happens to make sense in immediate terms of genetic self-interest or it doesn't. And if it doesn't, you won't be reading about it a million years later. This was an essential message of George Williams's 1966 book, a message that had barely begun to take hold when Morris's book appeared.

One key to good evolutionary analysis, Williams stressed, is to focus on the fate of the gene in question. If a woman's "fidelity gene" (or her "infidelity gene") shapes her behavior in a way that helps get copies of itself into future generations in large numbers, then that gene will by definition flourish. Whether the gene, in the process, {56} gets mixed in with her husband's genes or with the mailman's genes is by itself irrelevant. As far as natural selection is concerned, one vehicle is as good as the next. (Of course, when we talk about "a gene" for anything — fidelity, infidelity, altruism, cruelty — we are usefully oversimplifying; complex traits result from the interaction of numerous genes, each of which, typically, was selected for its incremental addition to fitness.)

A new wave of evolutionists has used this stricter view of natural selection to think with greater care about the question that rightly interested Morris: Are human males and females born to form enduring bonds with one another? The answer is hardly an unqualified yes for either sex. Still, it is closer to a yes for both sexes than it is in the case of, say, chimpanzees. In every human culture on the anthropological record, marriage — whether monogamous or polygamous, permanent or temporary — is the norm, and the family is the atom of social organization. Fathers everywhere feel love for their children, and that's a lot more than you can say for chimp fathers and bonobo fathers, who don't seem to have much of a clue as to which youngsters are theirs. This love leads fathers to help feed and defend their children, and teach them useful things.3

At some point, in other words, extensive male parental investment entered our evolutionary lineage. We are, as they say in the zoology literature, high in MPI. We're not so high that male parental investment typically rivals female parental investment, but we're a lot higher than the average primate. We indeed have something important in common with the gibbons.

High MPI has in some ways made the everyday goals of male and female humans dovetail, and, as any two parents know, it can give them a periodic source of common and profound joy. But high MPI has also created whole new ways for male and female aims to diverge, during both courtship and marriage. In Robert Trivers's 1972 paper on parental investment, he remarked, "One can, in effect, treat the sexes as if they were different species, the opposite sex being a resource relevant to producing maximum surviving offspring."4 Trivers was making a specific analytical point, not a sweeping rhetorical one. But to a distressing extent — and an extent that was unclear before his paper — this metaphor does capture the overall situation; even {57} with high MPI, and in some ways because of it, a basic underlying dynamic between men and women is mutual exploitation. They seem, at times, designed to make each other miserable.


There is no shortage of clues as to why men are inclined to help rear their young. In our recent evolutionary past lie several factors that can make parental investment worthwhile from the point of view of the male's genes.5 In other words, because of these factors, genes inclining a male to love his offspring — to worry about them, defend them, provide for them, educate them — could flourish at the expense of genes that counseled continued remoteness.

One factor is the vulnerability of offspring. Following the generic male sexual strategy — roaming around, seducing and abandoning everything in sight — won't do a male's genes much good if the resulting offspring get eaten. That seems to be one reason so many bird species are monogamous, or at least relatively monogamous. Eggs left alone while the mother went out and hunted worms wouldn't last long. When our ancestors moved from the forests out onto the savanna, they had to cope with fleet predators. And this was hardly the only new danger to the young. As the species got smarter and its posture more upright, female anatomy faced a paradox: walking upright implied a narrow pelvis, and thus a narrow birth canal, but the heads of babies were larger than ever. This is presumably why human infants are born prematurely in comparison to other primates. From early on, baby chimps can cling to their mother while she walks around, her hands unencumbered. Human babies, though, seriously compromise a mother's food gathering. For many months, they're mounds of helpless flesh: tiger bait.

Meanwhile, as the genetic payoff of male investment was growing, the cost of investment was dropping. Hunting seems to have figured heavily in our evolution. With men securing handy, dense packages of protein, feeding a family was practical. It is probably no coincidence that monogamy is more common among carnivorous mammals than among vegetarians.

On top of all of this, as the human brain got bigger, it probably depended more on early cultural programming. Children with two {58} parents may have had an educational edge over children with only one.

Characteristically, natural selection appears to have taken this cost-benefit calculus and transmuted it into feeling — in particular, the sensation of love. And not just love for the child; the first step toward becoming a solid parental unit is for the man and woman to develop a strong mutual attraction. The genetic payoff of having two parents devoted to a child's welfare is the reason men and women can fall into swoons over one another, including swoons of great duration.

Until recently, this claim was heresy. "Romantic love" was thought to be an invention of Western culture; there were reports of cultures in which choice of mate had nothing to do with affection, and sex carried no emotional weight. But lately anthropologists mindful of the Darwinian logic behind attachment have taken a second look, and such reports are falling into doubt.6 Love between man and woman appears to have an innate basis. In this sense, the "pair-bonding" hypothesis stands supported, though not for all the reasons Desmond Morris imagined.

At the same time, the term pair bonding — and for that matter, the term love — conveys a sense of permanence and symmetry that, as any casual observer of our species can see, is not always warranted. To fully appreciate how large is the gap between idealized love and the version of love natural to people, we need to do what Trivers did in his 1972 paper: focus not on the emotion itself, but on the abstract evolutionary logic it embodies. What are the respective genetic interests of males and females in a species with internal fertilization, an extended period of gestation, prolonged infant dependence on mother's milk, and fairly high male parental investment? Seeing these interests clearly is the only way to appreciate how evolution not only invented romantic love, but, from the beginning, corrupted it.


For a species low in male parental investment, the basic dynamic of courtship, as we've seen, is pretty simple: the male really wants sex; the female isn't so sure.7 She may want time to (unconsciously) assess {59} the quality of his genes, whether by inspecting him or by letting him battle with other males for her favor. She may also pause to weigh the chances that he carries disease. And she may try to extract a precopulation gift, taking advantage of the high demand for her eggs. This "nuptial offering" — which technically constitutes a tiny male parental investment, since it nourishes her and her eggs — is seen in a variety of species, ranging from primates to black-tipped hanging flies. (The female hanging fly insists on having a dead insect to eat during sex. If she finishes it before the male is finished, she may head off in search of another meal, leaving him high and dry. If she isn't so quick, the male may repossess the leftovers for subsequent dates.)8 These various female concerns can usually be addressed fairly quickly; there's no reason for courtship to drag on for weeks.

But now throw high MPI into the equation — male investment not just at the time of sex, but extending up to and well beyond birth. Suddenly the female is concerned not only with the male's genetic investment, or with a free meal, but with what he'll bring to the offspring after it materializes. In 1989 the evolutionary psychologist David Buss published a pioneering study of mate preferences in thirty-seven cultures around the world. He found that in every culture, females placed more emphasis than males on a potential mate's financial prospects.9

That doesn't mean women have a specific, evolved preference for wealthy men. Most hunter-gatherer societies have very little in the way of accumulated resources and private property. Whether this accurately reflects the ancestral environment is controversial; hunter-gatherers have, over the last few millennia, been shoved off of rich land into marginal habitats and thus may not, in this respect, be representative of our ancestors. But if indeed all men in the ancestral environment were about equally affluent (that is, not very), women may be innately attuned not so much to a man's wealth as to his social status; among hunter-gatherers, status often translates into power — influence over the divvying up of resources, such as meat after a big kill. In modern societies, in any event, wealth, status, and power often go hand in hand, and seem to make an attractive package in the eyes of the average woman.

Ambition and industry also seem to strike many women as {60} auspicious — and Buss found that this pattern, too, is broadly international.10 Of course, ambition and industriousness are things a female might look for even in a low-MPI species, as indices of genetic quality. Not so, however, for her assessment of the male's willingness to invest. A female in a high-MPI species may seek signs of generosity, trustworthiness, and, especially, an enduring commitment to her in particular. It is a truism that flowers and other tokens of affection are more prized by women than by men.

Why should women be so suspicious of men? After all, aren't males in a high-MPI species designed to settle down, buy a house, and mow the lawn every weekend? Here arises the first problem with terms like love and pair bonding. Males in high-MPI species are, paradoxically, capable of greater treachery than males in low-MPI species. For the "optimal male course," as Trivers noted, is a "mixed strategy."11 Even if long-term investment is their main aim, seduction and abandonment can make genetic sense, provided it doesn't take too much, in time and other resources, from the offspring in which the male does invest. The bastard youngsters may thrive even without paternal investment; they may, for that matter, attract investment from some poor sap who is under the impression that they're his. So males in a high-MPI species should, in theory, be ever alert for opportunistic sex.

Of course, so should males in a low-MPI species. But this doesn't amount to exploitation, since the female has no chance of getting much more from another male. In a high-MPI species, she does, and a failure to get it from any male can be quite costly.

The result of these conflicting aims — the female aversion to exploitation, the male affinity for exploiting — is an evolutionary arms race. Natural selection may favor males that are good at deceiving females about their future devotion and favor females that are good at spotting deception; and the better one side gets, the better the other side gets. It's a vicious spiral of treachery and wariness — even if, in a sufficiently subtle species, it may assume the form of soft kisses, murmured endearments, and ingenuous demurrals.

At least it's a vicious spiral in theory. Moving beyond all this theoretical speculation and into the realm of concrete evidence — actually glimpsing the seamy underside of kisses and endearments — {61} is tricky. Evolutionary psychologists have made only meager progress. True, one study found that males, markedly more than females, report depicting themselves as more kind, sincere, and trustworthy than they actually are.12 But that sort of false advertising may be only half the story, and the other half is much harder to get at. As Trivers didn't note in his 1972 paper, but did note four years later, one effective way to deceive someone is to believe what you're saying. In this context, that means being blinded by love — to feel deep affection for a woman who, after a few months of sex, may grow markedly less adorable.13 This, indeed, is the great moral escape hatch for men who persist in a pattern of elaborate seduction and crisp, if anguished, abandonment. "I loved her at the time," they can movingly recall, if pressed on the matter.

This isn't to say that a man's affections are chronically delusional, that every swoon is tactical self-deception. Sometimes men do make good on their vows of eternal devotion. Besides, in one sense, an out-and-out lie is impossible. There's no way of knowing in mid-swoon, either at the conscious or unconscious level, what the future holds. Maybe some more genetically auspicious mate will show up three years from now; then again, maybe the man will suffer some grave misfortune that renders him unmarketable, turning his spouse into his only reproductive hope. But, in the face of uncertainty as to how much commitment lies ahead, natural selection would likely err on the side of exaggeration, so long as it makes sex more likely and doesn't bring counterbalancing costs.

There probably would have been some such costs in the intimate social environment of our evolution. Leaving town, or at least village, wasn't a simple matter back then, so blatantly false promises might quickly catch up with a man — in the form of lowered credibility or even shortened life span; the anthropological archives contain stories about men who take vengeance on behalf of a betrayed sister or daughter.14

Also, the supply of potentially betrayable women wasn't nearly what it is in the modern world. As Donald Symons has noted, in the average hunter-gatherer society, every man who can snare a wife does, and virtually every woman is married by the time she's fertile. There probably was no thriving singles scene in the ancestral environment, {62} except one involving adolescent girls during the fruitless phase between first menstruation and fertility. Symons believes that the lifestyle of the modern philandering bachelor — seducing and abandoning available women year after year after year, without making any of them targets for ongoing investment — is not a distinct, evolved sexual strategy. It is just what happens when you take the male mind, with its preference for varied sex partners, and put it in a big city replete with contraceptive technology.

Still, even if the ancestral environment wasn't full of single women sitting alone after one-night stands muttering "Men are scum," there were reasons to guard against males who exaggerate commitment. Divorce can happen in hunter-gatherer societies; men do up and leave after fathering a child or two, and may even move to another village. And polygamy is often an option. A man may vow that his bride will stay at the center of his life, and then, once married, spend half his time trying to woo another wife — or, worse still, succeed, and divert resources away from his first wife's children. Given such prospects, a woman's genes would be well served by her early and careful scrutiny of a man's likely devotion. In any event, the gauging of a man's commitment does seem to be part of human female psychology; and male psychology does seem inclined to sometimes encourage a false reading.

That male commitment is in limited supply — that each man has only so much time and energy to invest in offspring — is one reason females in our species defy stereotypes prevalent elsewhere in the animal kingdom. Females in low-MPl species — that is, in most sexual species — have no great rivalry with one another. Even if dozens of them have their hearts set on a single, genetically optimal male, he can, and gladly will, fulfill their dreams; copulation doesn't take long. But in a high-MPI species such as ours, where a female's ideal is to monopolize her dream mate — steer his social and material resources toward her offspring — competition with other females is inevitable. In other words: high male parental investment makes sexual selection work in two directions at once. Not only have males evolved to compete for scarce female eggs; females have evolved to compete for scarce male investment.

Sexual selection, to be sure, seems to have been more intense {63} among men than among women. And it has favored different sorts of traits in the two. After all, the things women do to gain investment from men are different from the things men do to gain sexual access to women. (Women aren't — to take the most obvious example — designed for physical combat with each other, as men are.) The point is simply that, whatever each sex must do to get what it wants from the other, both sexes should be inclined to do it with zest. Females in a high-MPI species will hardly be passive and guileless. And they will sometimes be the natural enemies of one another.


It would be misleading to say that males in a high-male-parental-investment species are selective about mates, but in theory they are at least selectively selective. They will, on the one hand, have sex with just about anything that moves, given an easy chance, like males in a low-MPI species. On the other hand, when it comes to rinding a female for a long-term joint venture, discretion makes sense; males can undertake only so many ventures over a lifetime, so the genes that the partner brings to the project — genes for robustness, brains, whatever — are worth scrutinizing.

The distinction was nicely drawn by a study in which both men and women were asked about the minimal level of intelligence they would accept in a person they were "dating." The average response, for both male and female, was: average intelligence. They were also asked how smart a person would have to be before they would consent to sexual relations. The women said: Oh, in that case, markedly above average. The men said: Oh, in that case, markedly below average.15

Otherwise, the responses of male and female moved in lockstep. A partner they were "steadily dating" would have to be much smarter than average, and a marriageable partner would have to be smarter still. This finding, published in 1990, confirmed a prediction Trivers had made in his 1972 paper on parental investment. In a high-MPI species, he wrote, "a male would be selected to differentiate between a female he will only impregnate and a female with whom he will also raise young. Toward the former he should be more eager for sex and less discriminating in choice of sex partner than the female toward {64} him, but toward the latter he should be about as discriminating as she toward him."16

As Trivers knew, the nature of the discrimination, if not its intensity, should still differ between male and female. Though both seek general genetic quality, tastes may in other ways diverge. Just as women have special reason to focus on a man's ability to provide resources, men have special reason to focus on the ability to produce babies. That means, among other things, caring greatly about the age of a potential mate, since fertility declines until menopause, when it falls off abruptly. The last thing evolutionary psychologists would expect to find is that a plainly postmenopausal woman is sexually attractive to the average man. They don't find it. (According to Bronislaw Malinowski, Trobriand Islanders considered sex with an old woman "indecorous, ludicrous, and unaesthetic.")17 Even before menopause, age matters, especially in a long-term mate; the younger a woman, the more children she can bear. In every one of Buss's thirty-seven cultures, males preferred younger mates (and females preferred older mates).

The importance of youth in a female mate may help explain the extreme male concern with physical attractiveness in a spouse (a concern that Buss also documented in all thirty-seven cultures). The generic "beautiful woman" — yes, she has actually been assembled, in a study that collated the seemingly diverse tastes of different men — has large eyes and a small nose. Since her eyes will look smaller and her nose larger as she ages, these components of "beauty" are also marks of youth, and thus of fertility.18 Women can afford to be more open-minded about looks; an oldish man, unlike an oldish woman, is probably fertile.

Another reason for the relative flexibility of females on the question of facial attractiveness may be that a woman has other things to (consciously or unconsciously) worry about. Such as: Will he provide for the kids? When people see a beautiful woman with an ugly man, they typically assume he has lots of money or status. Researchers have actually gone to the trouble of showing that people make this inference, and that the inference is often correct.19

When it comes to assessing character — to figuring out if you can trust a mate — a male's discernment may again differ from a female's, {65} because the kind of treachery that threatens his genes is different from the kind that threatens hers. Whereas the woman's natural fear is the withdrawal of his investment, his natural fear is that the investment is misplaced. Not long for this world are the genes of a man who spends his time rearing children who aren't his. Trivers noted in 1972 that, in a species with high male parental investment and internal fertilization, "adaptations should evolve to help guarantee that the female's offspring are also his own."20

All of this may sound highly theoretical — and of course it is. But this theory, unlike the theory about male love sometimes being finely crafted self-delusion, is readily tested. Years after Trivers suggested that anticuckoldry technology might be built into men, Martin Daly and Margo Wilson found some. They realized that if indeed a man's great Darwinian peril is cuckoldry, and a woman's is desertion, then male and female jealousy should differ.21 Male jealousy should focus on sexual infidelity, and males should be quite unforgiving of it; a female, though she'll hardly applaud a partner's extracurricular activities, since they consume time and divert resources, should be more concerned with emotional infidelity — the sort of magnetic commitment to another woman that could eventually lead to a much larger diversion of resources.

These predictions have been confirmed — by eons of folk wisdom and, over the past few decades, by considerable data. What drives men craziest is the thought of their mate in bed with another man; they don't dwell as much as women do on any attendant emotional attachment, or the possible loss of the mate's time and attention. Wives, for their part, do find the sheerly sexual infidelity of husbands traumatic, and do respond harshly to it, but the long-run effect is often a self-improvement campaign: lose weight, wear makeup, "win him back." Husbands tend to respond to infidelity with rage; and even after it subsides, they often have trouble contemplating a continued relationship with the infidel.22

Looking back, Daly and Wilson saw that this basic pattern had been recorded (though not stressed) by psychologists before the theory of parental investment came along to explain it. But evolutionary psychologists have now confirmed the pattern in new and excruciating detail. David Buss placed electrodes on men and women and had {66} them envision their mates doing various disturbing things. When men imagined sexual infidelity, their heart rates took leaps of a magnitude typically induced by three successive cups of coffee. They sweated. Their brows wrinkled. When they imagined instead a budding emotional attachment, they calmed down, though not quite to their normal level. For women, things were reversed: envisioning emotional infidelity — redirected love, not supplementary sex — brought the deeper physiological distress.23

The logic behind male jealousy isn't what it used to be. These days some adulterous women use contraception and thus don't, in fact, dupe their husbands into spending two decades shepherding another man's genes. But the weakening of the logic doesn't seem to have weakened the jealousy. For the average husband, the fact that his wife inserted a diaphragm before copulating with her tennis instructor will not be a major source of consolation.

The classic example of an adaptation that has outlived its logic is the sweet tooth. Our fondness for sweetness was designed for an environment in which fruit existed but candy didn't. Now that a sweet tooth can bring obesity, people try to control their cravings, and sometimes they succeed. But their methods are usually roundabout, and few people find them easy; the basic sense that sweetness feels good is almost unalterable (except by, say, repeatedly pairing a sweet taste with a painful shock). Similarly, the basic impulse toward jealousy is very hard to erase. Still, people can muster some control over the impulse, and, moreover, can muster much control over some forms of its expression, such as violence, given a sufficiently powerful reason. Prison, for example.


Before further exploring the grave imprint that cuckoldry has left on the male psyche, we might ask why it would exist. Why would a woman cheat on a man, if that won't increase the number of her progeny — and if, moreover, she thus risks incurring the wrath, and losing the investment, of her mate? What reward could justify such a gamble? There are more possible answers to this question than you might imagine.

First, there is what biologists call "resource extraction." If female {67} humans, like female hanging flies, can get gifts in exchange for sex, then the more sex partners, the more gifts. Our closest primate relatives act out this logic. Female bonobos are often willing to provide sex in exchange for a hunk of meat. Among common chimpanzees, the food-for-sex swap is less explicit but is evident; male chimps are more likely to give meat to a female when she exhibits the red vaginal swelling that signifies ovulation.24

Human females, of course, don't advertise their ovulation. One theory about this "cryptic ovulation" sees it as an adaptation designed to expand the period during which they can extract resources. Men may lavish gifts on them well before or past ovulation and receive sex in return, blissfully oblivious to the fruitlessness of their conquest. Nisa, a woman in a !Kung San hunter-gatherer village, spoke candidly with an anthropologist about the material rewards of multiple sex partners. "One man can give you very little. One man gives you only one kind of food to eat. But when you have lovers, one brings you something and another brings you something else. One comes at night with meat, another with money, another with beads. Your husband also does things and gives them to you."25

Another reason women might copulate with more than one man — and another advantage of concealed ovulation — is to leave several men under the impression that they might be the father of particular offspring. Across primate species, there is a rough correlation between a male's kindness to youngsters and the chances that he is their father. The dominant male gorilla, with his celestial sexual stature, can rest pretty much assured that the youngsters in his troop are his; and, although not demonstrative by comparison with a human father, he is indulgent of them and reliably protective. At the other end of the spectrum, male langur monkeys kill infants sired by others as a kind of sexual icebreaker, a prelude to pairing up with the (former) mother.26 What better way to return her to ovulation — by putting an emphatic end to her breast-feeding — and to focus her energies on the offspring to come?

Anyone tempted to launch into a sweeping indictment of langur morality should first note that infanticide on grounds of infidelity has been acceptable in various human societies. In two societies men have been known to demand, upon marrying women with a past, {68} that their babies be killed.27 And among the Ache hunter-gatherers of Paraguay, men sometimes collectively decide to kill a newly fatherless child. Even leaving murder aside, life can be hard on children without a devoted father. Ache children raised by stepfathers after their biological fathers die are half as likely to live to age fifteen as children whose parents stay alive and together.28 For a woman in the ancestral environment, then, the benefits of multiple sex partners could have ranged from their not killing her youngster to their defending or otherwise aiding her youngster.

This logic doesn't depend on the sex partners' consciously mulling it over. Male gorillas and langurs, like the Trobriand Islanders as depicted by Malinowski, are not conscious of biological paternity. Still, the behavior of males in all three cases reflects an implicit recognition. Genes making males unconsciously sensitive to cues that certain youngsters may or may not be carrying their genes have flourished. A gene that says, or at least whispers, "Be nice to children if you've had a fair amount of sex with their mothers" will do better than a gene that says, "Steal food from children even if you were having regular sex with their mothers months before birth."

This "seeds of confusion" theory of female promiscuity has been championed by the anthropologist Sarah Blaffer Hrdy. Hrdy has described herself as a feminist sociobiologist, and she may take a more than scientific interest in arguing that female primates tend to be "highly competitive ... sexually assertive individuals."29 Then again, male Darwinians may get a certain thrill from saying males are built for lifelong sex-a-thons. Scientific theories spring from many-sources. The only question in the end is whether they work.

Both of these theories of female promiscuity — "resource extraction" and "seeds of confusion" — could in principle apply to a mate-less woman as well as a married one. Indeed, both would make sense for a species with little or no male parental investment, and thus may help explain the extreme promiscuity of female chimpanzees and bonobos. But there is a third theory that grows uniquely out of the dynamics of male parental investment, and thus has special application to wives: the "best of both worlds" theory.

In a high-MPI species, the female seeks two things: good genes and high ongoing investment. She may not find them in the same {69} package. One solution would be to trick a devoted but not especially brawny or brainy mate into raising the offspring of another male. Again, cryptic ovulation would come in handy, as a treachery facilitator. It's fairly easy for a man to keep rivals from impregnating his mate if her brief phase of fertility is plainly visible; but if she appears equally fertile all month, surveillance becomes a problem. This is exactly the confusion a female would want to create if her goal is to draw investment from one man and genes from another.30 Of course, the female may not consciously "want" this "goal." And she may not be consciously aware of when she's ovulating. But at some level she may be keeping track.

Theories involving so much subconscious subterfuge may sound too clever by half, especially to people not steeped in the cynical logic of natural selection. But there is some evidence that women are more sexually active around ovulation.31 And two studies have found that women going to a singles bar wear more jewelry and makeup when near ovulation.32 These adornments, it seems, have the advertising value of a chimpanzee's pink genital swelling, attracting a number of men for the woman to choose from. And these decked-out women did indeed tend to have more physical contact with men in the course of the evening.

Another study, by the British biologists R. Robin Baker and Mark Bellis, found that women who cheat on their mates are more likely to do so around ovulation. This suggests that often the secret lover's genes, not just his resources, are indeed what they're after.33

Whatever the reason(s) women cheat on their mates (or, as biologists value-neutrally put it, have "extra-pair copulations"), there's no denying that they do. Blood tests show that in some urban areas more than one fourth of the children may be sired by someone other than the father of record. And even in a !Kung San village, which, like the ancestral environment, is so intimate as to make covert liaison tricky, one in fifty children was found to have misassigned paternity.34 Female infidelity appears to have a long history.

Indeed, if female infidelity weren't a long-standing part of life in this species, why would distinctively maniacal male jealousy have evolved? At the same time, that men so often invest heavily in the children of their mates suggests that cuckoldry hasn't been rampant; {70} if it had, genes encouraging this investment would long ago have run into a dead end.35 The minds of men are an evolutionary record of the past behavior of women. And vice versa.

If a "psychological" record seems too vague, consider a more plainly physiological bit of data: human testicles — or, more exactly, the ratio of average testes weight to average male body weight. Chimpanzees and other species with high relative testes weights have "multimale breeding systems," in which females are quite promiscuous.36 Species with low relative testes weights are either monogamous (gibbons, for example) or polygynous (gorillas), with one male monopolizing several families. (Polygamous is the more general term, denoting a male or a female that has more than one mate.) The explanation is simple. When females commonly breed with many different males, male genes can profit by producing lots of semen for their transportation. Which male gets his DNA into a given egg may be a question of sheer volume, as competing hordes of sperm do subterranean battle. A species' testicles are thus a record of its females' sexual adventure over the ages. In our species, relative testes weight falls between that of the chimpanzee and the gorilla, suggesting that women, while not nearly as wild as chimpanzee females, are, by nature, somewhat adventurous.

Of course, adventurous doesn't mean unfaithful. Maybe women in the ancestral environment had their wild, unattached periods — during which fairly weighty testicles paid off for men — as well as their devoted, monogamous periods. Then again, maybe not. Consider a truer record of female infidelity: variable sperm density. You might think that the number of sperm cells in a husband's ejaculate would depend only on how long it's been since he last had sex. Wrong. According to work by Baker and Bellis, the quantity of sperm depends heavily on the amount of time a man's mate has been out of his sight lately.37 The more chances a woman has had to collect sperm from other males, the more profusely her mate sends in his own troops. Again: that natural selection designed such a clever weapon is evidence of something for the weapon to combat.

It is also evidence that natural selection is fully capable of designing equally clever psychological weapons, ranging from furious jealousy to the seemingly paradoxical tendency of some men to be {71} sexually aroused by the thought of their mate in bed with another man. Or, more generally: the tendency of men to view women as possessions. In a 1992 paper called "The Man Who Mistook His Wife for a Chattel," Wilson and Daly wrote that "men lay claim to particular women as songbirds lay claim to territories, as lions lay claim to a kill, or as people of both sexes lay claim to valuables. ... [Referring to man's view of woman as 'proprietary' is more than a metaphor: Some of the same mental algorithms are apparently activated in the rharital and mercantile spheres."38

The theoretical upshot of all this is another evolutionary arms race. As men grow more attuned to the threat of cuckoldry, women should get better at convincing a man that their adoration borders on awe, their fidelity on the saintly. And they may partly convince themselves too, just for good measure. Indeed, given the calamitous fallout from infidelity uncovered — likely desertion by the offended male, and possible violence — female self-deception may be finely honed. It could be adaptive for a married woman to not feel chronically concerned with sex, even if her unconscious mind is keeping track of prospects and will notify her when ardor is warranted.


Anticuckoldry technology could come in handy not just when a man has a mate, but earlier, in choosing her. If available females differ in their promiscuity, and if the more promiscuous ones tend to make less faithful wives, natural selection might incline men to discriminate accordingly. Promiscuous women would be welcome as short-term sex partners — indeed, preferable, in some ways, since they can be had with less effort. But they would make poor wife material, a dubious conduit for male parental investment.

What emotional mechanisms — what complex of attractions and aversions — would natural selection use to get males to uncomprehendingly follow this logic? As Donald Symons has noted, one candidate is the famed Madonna-whore dichotomy, the tendency of men to think in terms of "two kinds of women" — the kind they respect and the kind they just sleep with.39 {72}

One can imagine courtship as, among other things, a process of placing a woman in one category or the other. The test would run roughly as follows. If you find a woman who appears genetically suitable for investment, start spending lots of time with her. If she seems quite taken by you, and yet remains sexually aloof, stick with her. If, on the other hand, she seems eager for sex right away, then by all means oblige her. But if the sex does come that easily, you might want to shift from investment mode into exploitation mode. Her eagerness could mean she'll always be an easy seduction — not a desirable quality in a wife.

Of course, in the case of any particular woman, sexual eagerness may not mean she'll always be an easy seduction; maybe she just finds this one man irresistible. But if there is any general correlation between the speed with which a woman succumbs to a man and her likelihood of later cheating on him, then that speed is a statistically valid cue to a matter of great genetic consequence. Faced with the complexity and frequent unpredictability of human behavior, natural selection plays the odds.

Just to add a trifle more ruthlessness to this strategy: the male may actually encourage the early sex for which he will ultimately punish the woman. What better way to check for the sort of self-restraint that is so precious in a woman whose children you may invest in? And, if self-restraint proves lacking, what faster way to get the wild oats sown before moving on to worthier terrain?

In its extreme, pathological form — the Madonna-whore complex — this dichotomization of women leaves a man unable to have sex with his wife, so holy does she seem. Obviously, this degree of worship isn't likely to have been favored by natural selection. But the more common, more moderate version of the Madonna-whore distinction has the earmarks of an efficient adaptation. It leads men to shower worshipful devotion on the sexually reserved women they want to invest in — exactly the sort of devotion these women will demand before allowing sex. And it lets men guiltlessly exploit the women they don't want to invest in, by consigning them to a category that merits contempt. This general category — the category of reduced, sometimes almost subhuman, moral status — is, as we'll see, {73} a favorite tool of natural selection's; it is put to especially effective use during wars.

In polite company, men sometimes deny that they think differently of a woman who has slept with them casually. And wisely so. To admit that they do would sound morally reactionary. (Even to admit as much to themselves might make it hard to earnestly assure such a woman that they'll still respect her in the morning — sometimes a vital part of foreplay.)

As many modern wives can attest, sleeping with a man early in courtship doesn't doom the prospect of long-term commitment. A man's (largely unconscious) assessment of a woman's likely fidelity presumably involves many things — her reputation, how she looks at other men, how honest she seems generally. And anyway, even in theory the male mind shouldn't be designed to make virginity a prerequisite for investment. The chances of finding a virgin wife vary from man to man and from culture to culture — and to judge by some hunter-gatherer societies, they would have been quite low in the ancestral environment. Presumably males are designed to do the best they can under the circumstances. Though in prudish Victorian England some men may have insisted on virgin wives, the term Madonna-whore dichotomy is actually a misnomer for what is surely a more flexible mental tendency.40

Still, the flexibility is bounded. There is some level of female promiscuity above which male parental investment plainly makes no genetic sense. If a woman seems to have an unbreakable habit of sleeping with a different man each week, the fact that all women in that culture do the same thing doesn't make her any more logical a spouse. In such a society, men should in theory give up entirely on concentrated parental investment and focus solely on trying to mate with as many women as possible. That is, they should act like chimpanzees.


The Madonna-whore dichotomy has long been dismissed as an aberration, another pathological product of Western culture. In particular, the Victorians, with their extraordinary emphasis on virginity {74} and their professed disdain for illicit sex, are held responsible for nourishing, even inventing, the pathology. If only men in Darwin's day had been more relaxed about sex, like the men in non-Western, sexually liberated societies. How different things would be now!

The trouble is, those idyllic, non-Western societies seem to have existed only in the minds of a few misguided, if influential, academics. The classic example is Margaret Mead, one of several prominent anthropologists who early this century reacted to the political misuses of Darwinism by stressing the malleability of the human species and asserting the near absence of human nature. Mead's best-known book, Coming of Age in Samoa, created a sensation upon its appearance in 1928. She seemed to have found a culture nearly devoid of many Western evils: status hierarchies, intense competition, and all kinds of needless anxieties about sex. Here in Samoa, Mead wrote, girls postpone marriage "through as many years of casual love-making as possible." Romantic love "as it occurs in our civilisation," bound up with ideas of "exclusiveness, jealousy and undeviating fidelity," simply "does not occur in Samoa."41 What a wonderful place!

It is hard to exaggerate the influence of Mead's findings on twentieth-century thought. Claims about human nature are always precarious, vulnerable to the discovery of even a single culture in which its elements are fundamentally lacking. For much of this century, such claims have been ritually met with a single question: "What about Samoa?"

In 1983 the anthropologist Derek Freeman published a book called Margaret Mead and Samoa: The Making and Unmaking of an Anthropological Myth. Freeman had spent nearly six years in Samoa (Mead had spent nine months, and hadn't spoken the language when she arrived), and was well versed in accounts of its earlier history, before Western contact had much changed it. His book left Mead's reputation as a great anthropologist in serious disarray. He depicted her as a naif, a twenty-three-year-old idealist who went to Samoa steeped in fashionable cultural determinism, chose not to live among the natives, and then, dependent for her data on scheduled interviews, was duped by Samoan girls who made a game of misleading her. Freeman assaulted Mead's data broadly — the supposed dearth of status {75} competition, the simple bliss of Samoan adolescence — but for present purposes what matters is the sex: the purportedly minor significance of jealousy and male possessiveness, the seeming indifference of men to the Madonna-whore dichotomy.

Actually, on close examination, Mead's point-by-point findings turn out to be less radical than her glossy, well-publicized generalizations. She conceded that Samoan males took a certain pride in the conquest of a virgin. She also noted that each tribe had a ceremonial virgin — a girl of good breeding, often a chief's daughter, who was carefully guarded until, upon marriage, she was manually deflowered, with the blood from her hymen proving her purity. But this girl, Mead insisted, was an aberration, "excepted" from the "free and easy experimentation" that was the norm. Parents of lower rank "complacently ignore" their daughters' sexual experimentation.42 Mead granted, almost under her breath, that a virginity test was "theoretically" performed "at weddings of people of all ranks," but she dismissed the ceremony as easily and often evaded.

Freeman raised the volume of Mead's more hushed observations and pointed out some things she had failed entirely to note. The value of virgins was so great in the eyes of marriageable men, he wrote, that an adolescent female of any social rank was monitored by her brothers, who would "upbraid, and sometimes beat" her if they found her with "a boy suspected of having designs on her virginity." As for the suspected boy, he was "liable to be assaulted with great ferocity." Young men who fared poorly in the mating game sometimes secured a mate by sneaking in at night, forcibly deflowering a woman, and then threatening to disclose her corruption unless she agreed to marriage (perhaps in the form of elopement, the surest way to avoid a virginity test). A woman found on her wedding day not to be a virgin was publicly denounced with a term meaning, roughly, "whore." In Samoan lore, one deflowered woman is described as a "wanton woman, like an empty shell exposed by the ebbing tide!" A song performed at defloration ceremonies went like this: "All others have failed to achieve entry, all others have failed to achieve entry... . He is first by being foremost, being first he is foremost; O to be foremost!"43 These are not the hallmarks of a sexually liberated culture. {76}

It now appears that some of the supposed Western aberrations that Mead found lacking in Samoa had if anything been suppressed by Western influence. Missionaries, Freeman noted, had made virginity testing less public — performed in a house, behind a screen. In "former days," as Mead herself wrote, if the tribe's ceremonial virgin was found at her wedding to be less than virginal, "her female relatives fell upon and beat her with stones, disfiguring and sometimes fatally injuring the girl who had shamed their house."44

So too with the Samoan jealousy that, Mead stressed, was so muted by Western standards: Westerners may have done the muting. Mead noted that a husband who caught his wife in adultery might be appeased by a harmless ritual that, as she depicted it, would end in an air of bonhomie. The male offender would bring men of his family, sit outside the victimized husband's house in supplication, offering finery in recompense, until forgiveness was forthcoming and everyone buried the hatchet over dinner. Of course, "in olden days," Mead observed, the offended man might "take a club and together with his relatives go out and kill those who sit without."45

That violence became less frequent under Christian influence is, of course, a testament to human malleability. But if we are ever to fathom the complex parameters of that malleability, we must be clear about which is the core disposition and which is the modifying influence. Time and again, Mead, along with her whole cohort of mid-twentieth-century cultural determinists, got things backwards.

Darwinism helps set the record straight. A new generation of Darwinian anthropologists is combing old ethnographies and conducting new field studies, finding things past anthropologists didn't stress, or even notice. Many candidates for "human nature" are emerging. And one of the more viable is the Madonna-whore dichotomy. In exotic cultures from Samoa to Mangaia to the land of the Ache in South America, a reputation for extreme promiscuity is something men actively avoid in a long-term mate.46 And an analysis of folklore reveals the "good girl/bad girl" polarity to be a chronically recurring image — in the Far East, in Islamic states, in Europe, even in pre-Columbian America.47

Meanwhile, in the psychology laboratory, David Buss has found {77} evidence that men do dichotomize between short-term and long-term mates. Cues suggesting promiscuity (a low-cut dress, perhaps, or aggressive body language) make a woman more attractive as a short-term mate and less attractive as a long-term mate. Cues suggesting a lack of sexual experience work the other way around.48

For now, the hypothesis that the Madonna-whore dichotomy has at least some inherent basis rests on strong theoretical expectation and considerable, though hardly exhaustive, anthropological and psychological evidence. There is also, of course, the testimony of experienced mothers from many eras who have warned their daughters what will happen if a man gets the impression that they're "that kind of girl": he won't "respect" them anymore.


The Madonna-whore distinction is a dichotomy imposed on a continuum. In real life, women aren't either "fast" or "slow"; they are promiscuous to various degrees, ranging from not at all to quite. So the question of why some women are of one type and others of the other has no meaning. But there is meaning in the question of why women are nearer one end of the spectrum than the other — why women differ in their general degree of sexual reserve. And for that matter, what about men? Why do some men seem capable of unswerving monogamy, and others so inclined to depart from that ideal to various degrees? Is this difference — between Madonnas and whores, between dads and cads — in the genes? The answer is a definite yes. But the only reason the answer is definite is that the phrase "in the genes" is so ambiguous as to be essentially meaningless.

Let's start with the popular conception of "in the genes." Are some women, from the moment their father's sperm meets their mother's egg, all but destined to be Madonnas, while others are almost certain to be whores? Are some men equally bound to be cads, and others dads?

For both men and women, the answer is: unlikely, but not impossible. As a rule, two extremely different alternative traits will not both be preserved by natural selection. One or the other is usually {78} at least slightly more conducive to genetic proliferation. However marginal its edge, it should win out, given enough time.49 That's why almost all of the genes in you are also in the average inhabitant of any land in the world. But there is something called "frequency-dependent" selection, in which the value of a trait declines as it becomes more common, so that natural selection places a ceiling on its predominance, thus leaving room for the alternative.

Consider the bluegill sunfish.50 The average bluegill male grows up, builds a bunch of nests, waits for females to lay eggs, then fertilizes the eggs and guards them. He is an upstanding member of the community. But he may have as many as 150 nests to tend, a fact that leaves him vulnerable to a second, less responsible kind of male, a drifter. The drifter sneaks around, surreptitiously fertilizes eggs, and then darts off, leaving them to be tended by their duped custodian. At a certain stage in life, drifters even don the color and behavior of females to mask their covert operations.

You can see how the balance between drifters and their victims is maintained. The drifters must do fairly well in reproductive terms; otherwise they wouldn't be around. But as this success makes their fraction of the population grow, the success itself diminishes, because the relative supply of upstanding, exploitable males — the drifters' meal ticket — shrinks. This is a situation in which success is its own punishment. The more drifters there are, the fewer offspring per drifter there are.

In theory, the drifter fraction of the population should grow until the average drifter is having as many offspring as the average upstanding bluegill. At that point, any shift in the fraction — growth or shrinkage — will change the value of the two strategies in a way that tends to reverse the shift. This equilibrium is known as an "evolutionarily stable" state, a term coined by the British biologist John Maynard Smith, who, during the 1970s, fully developed the idea of frequency-dependent selection.51 Bluegill drifters, presumably, long ago reached their evolutionarily stable fraction of the population, which seems to be about one fifth.

The dynamics of sexual treachery are different for humans than for bluegill sunfish, in part because of the mammalian penchant for {79} internal fertilization. But Richard Dawkins has shown, with an abstract analysis applicable to our species, that Maynard Smith's logic can, in principle, fit us too. In other words: one can imagine a situation in which neither coy nor fast women, and neither cads nor dads, have a monopoly on the ideal strategy. Rather, the success of each strategy varies with the prevalence of the three other strategies, and the population tends toward equilibrium. For example, with one set of assumptions, Dawkins found that five-sixths of the females would be coy, and five-eighths of the males would be faithful.52

Now, having comprehended this fact, you are advised to forget it. Don't just forget the fractions themselves, which, obviously, grow out of arbitrary assumptions within a highly artificial model. Forget the whole idea that each individual would be firmly bound to one strategy or the other.

As Maynard Smith and Dawkins have noted, evolution equilibrates to an equally stable state if you assume that the magic proportions are found within individuals — that is, if each female is coy on five-sixths of her mating opportunities, and each male is coy on five-eighths of his. And that's true even if the fractions are randomly realized — if each person just rolls dice on each encounter to decide what to do. Imagine how much more effective it is for the person to ponder each situation (consciously or unconsciously) and make an informed guess as to which strategy is more propitious under the circumstances.

Or imagine a different kind of flexibility: a developmental program that, during childhood, assesses the local social environment and then, by adulthood, inclines the person toward the strategy more likely to pay off. To put this in bluegill terms: imagine a male that during its early years checks out the local environment, calculates the prevalence of exploitable, upstanding males, and then decides — or, at least, "decides" — whether to become a drifter. This plasticity should eventually dominate the population, pushing the two more rigid strategies into oblivion.

The moral of the story is that limberness, given the opportunity, usually wins out over stiffness. In fact, limberness seems to have won a partial victory even in the bluegill sunfish, which isn't exactly known {80} for its highly developed cerebral cortex. Though some genes incline a male bluegill to one strategy, and others to the other, the inclination isn't complete; the male absorbs local data before "deciding" which strategy to adopt.53 Obviously, when you move from fish to us, the likely extent of flexibility grows. We have huge brains whose whole reason for being is deft adjustment to variable conditions. Given the many things about a person's social environment that can alter the value of being a Madonna versus a whore, a cad versus a dad — including the way other people react to the person's particular assets and liabilities — natural selection would be uncharacteristically obtuse not to favor genes that build brains sensitive to these things.

So too in many other realms. The value of being a given "type" of person — cooperative, say, or stingy — has depended, during evolution, on things that vary from time to time, place to place, person to person. Genes that irrevocably committed our ancestors to one personality type should in theory have lost out to genes that let the personality solidify gracefully.

This is not a matter of consensus. There are in the literature a few articles with titles like "The Evolution of the 'Con Artist.' "54 And, to return to the realm of Madonnas and whores, there is a theory that some women are innately inclined to pursue a "sexy son" strategy: they mate promiscuously with sexually attractive men (handsome, brainy, brawny, and so on), risking the high male parental investment they might extract if more Madonnaish but gaining the likelihood that any sons will be, like their fathers, attractive and therefore prolific. Such theories are interesting, but they all face the same obstacle: with con artists as with promiscuous women, however effective the strategy, it is even more effective when flexible — when it can be abandoned amid signs of likely failure.55 And the human brain is a pretty flexible thing.

To stress this flexibility isn't to say that all people are born psychologically identical, that all differences in personality emerge from environment. There plainly are important genetic differences for such traits as nervousness and extroversion. The "heritability" of these traits is around .4; that is, about 40 percent of individual differences in these traits (within the particular populations geneticists have studied) {81} can be explained by genetic differences. (By comparison, the heritability of height is around .9; about 10 percent of the difference in height among individuals is due to nutritional and other environmental differences.) The question is why the undoubtedly important genetic variation in personality exists. Do different degrees of genetic disposition toward extroversion represent different personality "types," the products of a very elaborate process of frequency-dependent selection? (Though frequency dependence is classically analyzed in terms of two or three distinct strategies, it could also yield a more finely graded array.) Or are the differing genetic dispositions just "noise" — some incidental by-product of evolution, not specifically favored by natural selection? No one knows, and evolutionary psychologists differ in their suspicions.56 What they agree on is that a big part of the story of personality differences is the evolution of malleability, of "developmental plasticity."

This emphasis on psychological development doesn't leave us back where social scientists were twenty-five years ago, attributing everything they saw to often unspecified "environmental forces." A primary — perhaps the primary — promise of evolutionary psychology is to help specify the forces, to generate good theories of personality development. In other words: evolutionary psychology can help us see not only the "knobs" of human nature, but also how the knobs are tuned. It not only shows us that (and why) men in all cultures are quite attracted to sexual variety, but can suggest what circumstances make some men more obsessed with it than others; it not only shows us that (and why) women in all cultures are more sexually reserved, but promises to help us figure out how some women come to defy this stereotype.

A good example lies in Robert Trivers's 1972 paper on parental investment. Trivers noted two patterns that social scientists had already uncovered: (1) the more attractive an adolescent girl, the more likely she is to "marry up" — marry a man of higher socioeconomic status; and (2) the more sexually active an adolescent girl, the less likely she is to marry up.

To begin with, these two patterns make Darwinian sense independently. A wealthy, high-status male often has a broad range of aspiring wives to choose from. So he tends to choose a good-looking {82} woman who is also relatively Madonnaish. Trivers took the analysis further. Is it possible, he asked, "that females adjust their reproductive strategies in adolescence to their own assets"?57 In other words, maybe adolescent girls who get early social feedback affirming their beauty make the most of it, becoming sexually reserved and thus encouraging long-term investment by high-status males who are looking for pretty Madonnas. Less attractive women, with less chance to hit the jackpot via sexual reserve, become more promiscuous, extracting small chunks of resources from a series of males. Though this promiscuity may somewhat lower their values as wives, it wouldn't, in the ancestral environment, have doomed their chances of finding a husband. In the average hunter-gatherer society, almost any fertile woman can find a husband, even if he's far from ideal, or she has to share him with another woman.


The Trivers scenario doesn't imply a conscious decision by attractive women to guard their jewels (though that may play a role, and, what's more, parents may be genetically inclined to encourage a daughter's sexual reserve with special force when she is pretty). By the same token, we aren't necessarily talking about unattractive women who "realize" they can't be choosy and start having sex on less than ideal Darwinian terms. The mechanism at work might well be subconscious, a gradual molding of sexual strategy — read: "moral values" — by adolescent experience.

Theories like this one matter. There has been much talk about the problem of unwed motherhood among teenagers, especially poor teenagers. But no one really knows how sexual habits get shaped, or how firmly fixed they then are. There is much talk about boosting "self-esteem," but little understanding of what self-esteem is, what it's for, or what it does.

Evolutionary psychology can't yet confidently provide the missing basis for these discussions. But the problem isn't a shortage of plausible theories; it's a shortage of studies to test the theories. The Trivers theory has sat in limbo for two decades. In 1992, one psychologist did find what the theory predicts — a correlation between {83} a woman's self-perception and her sexual habits: the less attractive she thinks she is, the more sex partners she has had. But another scholar didn't find the predicted correlation — and, more to the point, neither study was conducted specifically to test Trivers's theory, of which both scholars were unaware.58 For now, this is the state of evolutionary psychology: so much fertile terrain, so few farmers.

Eventually, the main drift of Trivers's theory, if not the theory itself, will likely be vindicated. That is: women's sexual strategies probably depend on the likely (genetic) profitability of each strategy, given prevailing circumstances. But those circumstances go beyond what Trivers stressed — a particular woman's desirability. Another factor is the general availability of male parental investment. This factor surely fluctuated in the ancestral environment. For example, a village that had just invaded a neighboring village might have a suddenly elevated ratio of women to men — not just because of male casualties, but because victorious warriors commonly kill or vanquish enemy men and keep their women.59 Overnight, a young woman's prospects for receiving a man's undivided investment could thus plummet. Famine, or sudden abundance, might also alter investment patterns. Given these currents of change, any genes that helped women navigate them would, in theory, have flourished.

There is tentative evidence that they did. According to a study by the anthropologist Elizabeth Cashdan, women who perceive men in general as pursuing no-obligation sex are more likely to wear provocative clothes and have sex often than women who see men as generally willing to invest in offspring.60 Though some of these women may be conscious of the connection between local conditions and their lifestyle, that isn't necessary. Women surrounded by men who are unwilling or unable to serve as devoted fathers may simply feel a deepened attraction to sex without commitment — feel, in other words, a relaxation of "moral" constraint. And perhaps if market conditions suddenly improve — if the male to female ratio rises, or if men for some other reason shift toward a high-investment strategy — women's sexual attractions, and moral sensibilities, shift accordingly.

All of this is necessarily speculative at this early stage in evolutionary psychology's growth. But already we can see the sort of light that will increasingly be shed. For example, "self-esteem" almost {84} certainly isn't the same, either in its sources or in its effects, for boys and girls. For teenage girls, feedback reflecting great beauty may, as Trivers suggested, bring high self-esteem, which in turn encourages sexual restraint. For boys, extremely high self-esteem could well have the opposite effect: it may lead them to seek with particular intensity the short-term sexual conquests that are, in fact, more open to a good-looking, high-status male. In many high schools, a handsome, star athlete is referred to, only half-jokingly, as a "stud." And, for those who insist on scientific verification of the obvious: good-looking men do have more sex partners than the average man.61 (Women report putting more emphasis on a sex partner's looks when they don't expect the relationship to last; they are apparently willing, unconsciously, to trade off parental investment for good genes.)62

Once a high-self-esteem male is married, he may not be notable for his devotion. Presumably his various assets still make philandering a viable lifestyle, even if it's now covert. (And you never know when an outside escapade will take on a life of its own, and lead to desertion.) Men with more moderate self-esteem may make more committed, if otherwise less desirable, husbands. With fewer chances at extramarital dalliance, and perhaps more insecurity about their own mate's fidelity, they may focus their energy and attention toward family. Meanwhile, men with extremely low self-esteem, given continued frustration with women, may eventually resort to rape. There is ongoing debate within evolutionary psychology over whether rape is an adaptation, a designed strategy that any boy might grow up to adopt, given sufficiently discouraging feedback from his social environment. Certainly rape surfaces in a wide variety of cultures, and often under the expected circumstances: when men have had trouble finding attractive women by legitimate means. One (non-Darwinian) study found the typical rapist to possess "deep-seated doubts about his adequacy and competency as a person. He lacks a sense of confidence in himself as a man in both sexual and nonsexual areas."63

A second sort of light shed by the new Darwinian paradigm may illuminate links between poverty and sexual morality. Women living in an environment where few men have the ability and/or desire to support a family might naturally grow amenable to sex without commitment. (Often in history — including Victorian England — women {85} in the "lower classes" have had a reputation for loose morals.)64 It is too soon to assert this confidently, or to infer that inner-city sexual mores would change markedly if income levels did. But it is noteworthy, at least, that evolutionary psychology, with its emphasis on the role of environment, may wind up highlighting the social costs of poverty, and thus at times lend strength to liberal policy prescriptions, defying old stereotypes of Darwinism as right-wing.

Of course, one could argue that various policy implications ensue from any given theory. And one can dream up wholly different kinds of Darwinian theories about how sexual strategies get shaped.65 The one thing one can't do, I submit, is argue that evolutionary psychology is irrelevant to the whole discussion. The idea that natural selection, acutely attentive to the most subtle elements of design in the lowliest animals, should build huge, exquisitely pliable brains and not make them highly sensitive to environmental cues regarding sex, status, and various other things known to figure centrally in our reproductive prospects — that idea is literally incredible. If we want to know when and how a person's character begins to assume distinct shape, if we want to know how resistant to change the character will subsequently be, we have to look to Darwin. We don't yet know the answers, but we know where they'll come from, and that knowledge helps us phrase the questions more sharply.


Much of the attention paid to the "short-term" sexual strategies of women — whether unattached women willing to settle for a one-night stand, or attached women sneaking out on a mate — is fairly recent. Sociobiological discussion during the 1970s, at least in its popular form, tended to depict men as wild, libidinous creatures who roamed the landscape looking for women to dupe and exploit; women were often depicted as dupes and exploitees. The shift in focus is due largely to the growing number of female Darwinian social scientists, who have patiently explained to their male colleagues how a woman's psyche looks from the inside.

Even after this restoration of balance, there remains one important sense in which men and women will tend to be, respectively, exploiter {86} and exploited. As a marriage progresses, the temptation to desert should — in the average case — shift toward the man. The reason isn't, as people sometimes assume, that the Darwinian costs of marital breakup are greater for the woman. True, if she has a young child and her marriage dissolves, that child may suffer — whether because she can't find a husband willing to commit to a woman with another man's child, or because she finds one who neglects or mistreats the child. But, in Darwinian terms, this cost is borne equally by the deserting husband; the child who will thus suffer is his child too, after all.

The big difference between men and women comes, rather, on the benefits side of the desertion ledger. What can each partner gain from a breakup in the way of future reproductive payoff? The husband can, in principle, find an eighteen-year-old woman with twenty-five years of reproduction ahead. The wife — even aside from the trouble she'll have finding a husband if she already has a child — cannot possibly find a mate who will give her twenty-five years worth of reproductive potential. This difference in outside opportunity is negligible at first, when both husband and wife are young. But as they age, it grows.

Circumstance can subdue or heighten it. A poor, low-status husband may not have a chance to desert and may, indeed, provide his wife with reason to desert, especially if she has no children and can thus find another mate readily. A husband who rises in status and wealth, on the other hand, will thus strengthen his incentive to desert while weakening his wife's. But all other things being equal, it is the husband's restlessness that will tend to grow as the years pass.

All this talk of "desertion" may be misleading. Though divorce is available in many hunter-gatherer cultures, so is polygyny; in the ancestral environment, gaining a second wife didn't necessarily mean leaving the first. And so long as it didn't, there was no good Darwinian reason to desert. Staying near offspring, giving protection and guidance, would have made more genetic sense. Thus, males may be designed less for opportune desertion than for opportune polygyny. But in the modern environment, with institutionalized monogamy, a polygynous impulse will find other outlets, such as divorce.

As a mother's children grow self-sufficient, the urgency of hanging {87} onto male parental investment drops. There are plenty of middle-aged women who, especially if they're financially secure, can take or leave their husbands. Still, there is no Darwinian force driving them to leave their husbands, nothing about leaving him that will sharply advance their genetic interests. The thing most likely to drive a post-menopausal woman out of a marriage is her husband's malicious marital discontent. Many a woman seeks divorce, but that doesn't mean her genes are ultimately the problem.

Among all the data on contemporary marriage, two items stand out as especially telling. First is the 1992 study which found that the husband's dissatisfaction with a marriage is the single strongest predictor of divorce.66 Second is that men are much more likely than women to remarry after a divorce.67 The second fact — and the biological force behind the second fact — is probably a good part of the reason for the first.

Objections to this sort of analysis are predictable: "But people leave marriages for emotional reasons. They don't add up the number of their children and pull out their calculators. Men are driven away by dull, nagging wives, or by the profound soul-searching of a mid-life crisis. Women are driven out by abusive or indifferent husbands, or lured away by a sensitive, caring man."

All true. But, again: emotions are just evolution's executioners. Beneath all the thoughts and feelings and temperamental differences that marriage counselors spend their time sensitively assessing are the stratagems of the genes — cold, hard equations composed of simple variables: social status, age of spouse, number of children, their ages, outside opportunities, and so on. Is the wife really duller and more nagging than she was twenty years ago? Possibly, but it's also possible that the husband's tolerance for nagging has dropped now that she's forty-five and has no reproductive future. And the promotion he just got, which has already drawn some admiring glances from a young woman at work, hasn't helped. Similarly, we might ask the young childless wife who finds her husband intolerably insensitive why the insensitivity wasn't so oppressive a year ago, before he lost his job and she met the kindly, affluent bachelor who seems to be flirting with her. Of course, maybe her husband's abuses are quite real, in which case they signal his disaffection, and perhaps his impending {88} departure — and merit just the sort of preemptive strike the wife is now mustering.

Once you start seeing everyday feelings and thoughts as genetic weapons, marital spats take on new meaning. Even the ones that aren't momentous enough to bring divorce are seen as incremental renegotiations of contract. The husband who on his honeymoon said he didn't want an "old-fashioned wife" now sarcastically suggests that it wouldn't be too taxing for her to cook dinner once in a while. The threat is as clear as it is implicit: I'm willing and able to break the contract if you're not willing to renegotiate.


All told, things are not looking good for Desmond Morris's version of the pair-bond hypothesis. We do not seem to be too much like our famously, just about unswervingly, monogamous primate relatives, the gibbons, to which we have been optimistically compared. This should come as no great surprise. Gibbons aren't very social. Each family lives on a large home range — sometimes more than a hundred acres — that buffers it from extramarital dalliances. And gibbons chase off any intruders that might want to steal or borrow a mate.68 We, by contrast, have evolved in large social groups that are rife with genetically profitable alternatives to fidelity.

We do have, to be sure, the earmarks of high male parental investment. For hundreds of thousands of years, and maybe longer, natural selection has been inclining males to love their children, thus giving them a feeling females had been enjoying for the previous several hundred million years of mammalian evolution. Natural selection has also, during that time, been inclining men and women to love each other (or, at least, to "love" each other, with the meaning of that word varying greatly, and seldom approaching the constancy of devotion it reaches between parent and offspring). Still, love or no love, gibbons we aren't.

So what are we? Just how far from being naturally monogamous is our species? Biologists often answer this question anatomically. We've already seen anatomical evidence — testes weight and the fluctuations in sperm density — suggesting that human females are not devoutly monogamous by nature. There is also anatomical evidence {89} bearing on the question of precisely how far from monogamous males naturally are. As Darwin noted, in highly polygynous species the contrast in body size between male and female — the "sexual dimorphism" — is great. Some males monopolize several females, while other males get shut out of the genetic sweepstakes altogether, so there is immense evolutionary value in being a big male, capable of intimidating other males. Male gorillas, who mate with lots of females if they win lots of fights and no females if they win none, are gargantuan — twice as heavy as females. Among the monogamous gibbons, small males breed about as prolifically as bigger ones, and sexual dimorphism is almost imperceptible. The upshot is that sexual dimorphism is a good index of the intensity of sexual selection among males, which in turn reflects how polygynous a species is. When placed on the spectrum of sexual dimorphism, humans get a "mildly polygynous" rating.69 We're much less dimorphic than gorillas, a bit less than chimps, and markedly more than gibbons.

One problem with this logic is that competition among human, and even prehuman, males has been largely mental. Men don't have the long canine teeth that male chimps use to fight for alpha rank and thus supreme mating rights. But men do employ various stratagems to raise their social status, and thus their attractiveness. So some, and maybe much, of the polygyny in our evolutionary past would be reflected not in gross physiology but in distinctively male mental traits. If anything, the less than dramatic difference in size between men and women paints an overly flattering picture of men's monogamous tendencies.70

How have societies over the years coped with the basic sexual asymmetry in human nature? Asymmetrically. A huge majority — 980 of the 1,154 past or present societies for which anthropologists have data — have permitted a man to have more than one wife.71 And that number includes most of the world's hunter-gatherer societies, societies that are the closest thing we have to a living example of the context of human evolution.

The more zealous champions of the pair-bond thesis have been known to minimize this fact. Desmond Morris, hell-bent on proving the natural monogamy of our species, insisted in The Naked Ape that the only societies worth paying much attention to are modern {90} industrial societies, which, coincidentally, fall into the 15 percent of societies that have been avowedly monogamous. "[A]ny society that has failed to advance has in some sense failed, 'gone wrong,' " he wrote. "Something has happened to it to hold it back, something that is working against the natural tendencies of the species. ..." So "the small, backward, and unsuccessful societies can largely be ignored." In sum, said Morris (who was writing back when Western divorce rates were about half what they are now): "whatever obscure, backward tribal units are doing today, the mainstream of our species expresses its pair-bonding character in its most extreme form, namely long-term monogamous matings."72

Well, that's one way to get rid of unsightly, inconvenient data: declare them aberrant, even though they vastly outnumber the "mainstream" data.

Actually, there is a sense in which polygynous marriage has not been the historical norm. For 43 percent of the 980 polygynous cultures, polygyny is classified as "occasional." And even where it is "common," multiple wives are generally reserved for a relatively few men who can afford them or qualify for them via formal rank. For eons and eons, most marriages have been monogamous, even though most societies haven't been.

Still, the anthropological record suggests that polygyny is natural in the sense that men given the opportunity to have more than one wife are strongly inclined to seize it. The record also suggests something else: that polygyny has its virtues as a way of handling the basic imbalance between what men and women want. In our culture, when a man whose wife has given him a few children grows restless and "falls in love" with a younger woman, we say: Okay, you can marry her, but we insist that you desert your first wife and that a stigma be placed on your kids and that, if you don't make much money, your kids and former wife suffer miserably. Some other cultures have tended to say: Okay, you can marry her, but only if you can really afford a second family; and you can't desert your first family, and there won't be any stigma placed on your kids.

Maybe some of today's nominally monogamous societies, those in which half of all marriages actually fail, should just go whole hog. Maybe we should fully erase the already fading stigma of divorce. {91}

Maybe we should simply make sure that men who stray from their families remain legally responsible for them, and keep supporting them in the manner to which they had become accustomed. Maybe we should, in short, permit polygyny. A lot of presently divorced women, and their children, might be better off.

The only way to intelligently address this option is to first ask a simple question (one that turns out to have a counterintuitive answer): How did a strict cultural insistence on monogamy, which seems to go against the grain of human nature, and several millennia ago was almost unheard of, ever come to be? {92}