Infants have their infancy, and adults? Adultery. Even though monogamy is mandated throughout the Western world, infidelity is universal. Revelations of marital infidelity occur regularly, often among the most prominent individuals—most of them men—who have the most to lose: Bill Clinton, Newt Gingrich, Jesse Jackson, Mark Sanford, Elliot Spitzer, Tiger Woods. The list is enormous and is “updated” almost daily. In this chapter, I will examine the somewhat divergent motivations of men and women when it comes to infidelity—pointing out that in both cases, the underlying causes derive from internal whisperings of fitness maximization, underpinned by the fundamental biology expressed in our polygamous heritage.

In short, when adultery happens—and it happens quite often—what’s going on is that people are behaving as polygynists (if men) or polyandrists (if women), in a culturally defined context of ostensible monogamy. Adultery, infidelity, or “cheating” are only meaningful given a relationship that is otherwise supposed to be monogamous. A polygynously married man—in any of the numerous cultures that permit such an arrangement—wasn’t an adulterer when he had sex with more than one of his wives. (As candidate Barack Obama explained in a somewhat different context, “That was the point.”) By the same token, a polyandrously married Tre-ba woman from Tibet isn’t an adulteress when she has sex with her multiple husbands. Another way of looking at this: when people of either gender act on their polygamous inclinations while living in a monogamous tradition, they are being unfaithful to their sociocultural commitment, but not to their biology.

“Variability,” wrote Christian apologist G. K. Chesterton, “is one of the virtues of a woman. It avoids the crude requirement of polygamy. So long as you have one good wife you are sure to have a spiritual harem.” The problem—for Chesterton and others—is that many (perhaps most) men w...

When describing the basic biology of male–female differences, we considered the Coolidge Effect. There is a large body of literature commenting on it, and on the tendency for men in particular to equate monogamy with monotony. Lord Byron wondered “how the devil is it that fresh features/Have such a charm for us poor human creatures?” Speaking more delicately, W. S. Gilbert, in "Trial by Jury," alluded to the flip side of the male fondness for variety with the knowing line, “Love unchanged will cloy.” Three hundred years earlier, Shakespeare had described Cleopatra as follows: “Age cannot wither her, nor custom stale her infinite variety.”

But then, Cleopatra was supposed to have been remarkable precisely because by contrast, “other women cloy the appetites they feed.”

Once more, the ultimate mechanism of all this “cloying” is likely to be found in the adaptive advantage gained by turning some proportion of male sexual energy toward new exploits and thus, more potential evolutionary success. As to its proximate mechanism, we can only guess. At the level of brain cells and neurochemicals, we do know that repeated stimulation can result in a degree of insensitivity—the flip side of attachment. It is therefore possible that something happens with respect to sexual enthusiasm analogous to the habituation that occurs with, say, the constant hum of a refrigerator motor: after a time, people habituate to the noise and only notice it when it stops! Thus, perhaps after a prolonged sexual association (perhaps weeks, months, even years) brain cells—and male brain cells in particular—might simply become habituated: that is, saturated with neurotransmitters, or refractory to them.

Here is yet another—and, I announce with regret, the last—extended quotation from the estimable and curmudgeonly Mr. Mencken, this time elaborating on the boredom that can accompany monogamy, and how it might be assuaged:

Monogamous marriage, by its very conditions . . . forces the two contracting parties into an intimacy that is too persistent and unmitigated; they are in contact at too many points, and too steadily. By and by all the mystery of the relation is gone, and they stand in the unsexed position of brother and sister.

. . . A husband begins by kissing a pretty girl, his wife; it is pleasant to have her so handy and so willing. He ends by making Machiavellian efforts to avoid kissing the every day sharer of his meals, books, bath towels, pocketbook, relatives, ambitions, secrets, malaises and business: a proceeding about as romantic as having his boots blacked. The thing is too horribly dismal for words. Not all the native sentimentalism of man can overcome the distaste and boredom that get into it. Not all the histrionic capacity of woman can attach any appearance of gusto and spontaneity to it.

[O]nce the adventurous descends to the habitual, it takes on an offensive and degrading character. The intimate approach, to give genuine joy, must be a concession, a feat of persuasion, a victory; once it loses that character it loses everything. Such a destructive conversion is effected by the average monogamous marriage. It breaks down all mystery and reserve, for how can mystery and reserve survive the use of the same hot water bag and a joint concern about butter and egg bills? What remains, at least on the husband’s side, is esteem—the feeling one has for an amiable aunt. And confidence—the emotion evoked by a lawyer, a dentist or a fortune-teller. And habit—the thing which makes it possible to eat the same breakfast every day, and to windup one’s watch regularly, and to earn a living.

[One might] prevent this stodgy dephlogistication of marriage by interrupting its course—that is, by separating the parties now and then, so that neither will become too familiar and commonplace to the other. By this means, . . . curiosity will be periodically revived, and there will be a chance for personality to expand a cappella, and so each reunion will have in it something of the surprise, the adventure and the virtuous satanry of the honeymoon. The husband will not come back to precisely the same wife that he parted from, and the wife will not welcome precisely the same husband. Even supposing them to have gone on substantially as if together, they will have gone on out of sight and hearing of each other. Thus each will find the other, to some extent at least, a stranger, and hence a bit challenging, and hence a bit charming. The scheme . . . has been tried often, and with success. It is, indeed, a familiar observation that the happiest couples are those who are occasionally separated, and the fact has been embalmed in the trite maxim that absence makes the heart grow fonder. Perhaps not actually fonder, but at any rate more tolerant, more curious, more eager. Two difficulties, however, stand in the way of the widespread adoption of the remedy. One lies in its costliness: the average couple cannot afford a double establishment, even temporarily. The other lies in the fact that it inevitably arouses the envy and ill-nature of those who cannot adopt it, and so causes a gabbling of scandal. The world invariably suspects the worst. Let man and wife separate to save their happiness from suffocation in the kitchen, the dining room and the connubial chamber, and it will immediately conclude that the corpse is already laid out in the drawing-room.

Obviously, a fondness for sexual variety can lead to adultery. Just as obviously, however, it doesn’t have to. But as the famous team of sex researchers led by Dr. Alfred Kinsey pointed out

Most males can immediately understand why most males want extramarital coitus. Although many of them refrain from engaging in such activity because they consider it morally unacceptable or socially undesirable, even such abstinent individuals can usually understand that sexual variety, new situations, and new partners might provide satisfactions which are no longer found in coitus which has been confined for some period of years to a single sexual partner. . . . On the other hand, many females find it difficult to understand why any male who is happily married should want to have coitus with any female other than his wife.

Recall the comical dialog between Anna and the King of Siam. This man–woman disparity is not simply because society has typically sought to repress female sexual desire (although it has, and for reasons that are also consistent with biology) but because most women do not experience heightened lust simply upon being presented with a new, anonymous partner. Again, at the ultimate, evolutionary level this is almost certainly because a new partner as such is unlikely to enhance a woman’s reproductive success; and so, women have not been outfitted with a comparable “Mrs. Coolidge effect.” Certainly, a woman is capable of sexual intercourse with new and different men—sometimes, as in the case of prostitutes, many different men in succession—but this is quite different from being inspired to do so by the very newness of the partner. Indeed, the fact of sexual variety is itself cited by prostitutes as one of the emotionally deadening aspects of their work.

Nevertheless, men have not cornered the market on infidelity. If nothing else, for every adulterous man there must be at least one sexually willing woman—although this individual need not be married herself. We know that women’s sexual inclinations are not simply the converse of men’s, with utter monogamous fidelity the opposed mirror image of randy male infidelity. But we also know that because of the greater physical size, strength, and potential violence of men, women are inclined to be especially secretive when it comes to their philandering.

Biologists have long known that monogamy is rare in the animal world, especially among our fellow mammals. But we didn’t have any idea how truly rare it is until DNA fingerprinting arrived on the scene and was applied to animals in the mid-1990s. Previously there had been hints, but these were largely ignored. For example, in an attempt during the 1970s to reduce excessive numbers of blackbirds without killing them, a substantial number of territorial males were surgically sterilized. To the surprise of the researchers, many female blackbirds—mated to these vasectomized males—produced perfectly normal offspring.  Evidently, there was hanky-panky going on within seemingly sedate blackbird society. Nonetheless, for decades it has been the received wisdom among ornithologists that 92% of bird species were monogamous.

Over time, however, a new realization dawned: social monogamy—in which a male and a female court, spend time together, and set up joint housekeeping—is not the same as sexual monogamy, that is, limiting copulation to one’s social partner. Not that members of a socially monogamous pair don’t have intercourse with each other, it’s just that often they also do so with others. Hence the term Extra-Pair Copulations (EPCs) was born, now a standard concept in animal behavior research, as studies employing DNA fingerprinting have revealed, time and again, that even those species that seemed devotedly monogamous were only socially so, and sexually? Not so much. Depending on the species, it is common to find that from 10% to 60% of avian offspring are not fathered by the mother’s social partner.

Biologists were not surprised to find evidence that males are prone to sexual gallivanting. After all, even though the basic biology of sperm makers doesn’t quite mandate searching for and when possible indulging in multiple sexual partners, it clearly inclines males of most species in that direction. What was perplexing, however, was the finding that females—even those in apparently stable domestic unions—were similarly disposed. It’s just that they are more secretive about it. As a result, biologists (such as me) would spend hundreds of hours carefully watching the behavior of a mated pair of birds without seeing any indication of sexual infidelity by the female— not necessarily because she was sexually faithful to her mate but because she was hiding her EPCs: not from researchers, but from her social partner. Why was she doing this? It was for reasons not dissimilar from why socially monogamous human beings are comparably secretive when it comes to their own EPCs.

Evidence has accumulated from a variety of animal species that if and when the male finds evidence that “his” female has been associating with other males, he is prone to do the animal equivalent of refusing to provide child support: no longer provisioning or defending the offspring, presumably because they might not be genetically his. We have already considered male–male violence, especially in the context of a man encountering his wife’s paramour. Although male–male violence is also found in animals, it is rare for nonhuman males to attack their mate upon discovering her infidelity. Most commonly, he punishes her—biologically, he defends his own fitness—by abandoning the “bastard” offspring, a response that can be devastating for the success of those offspring, and hence, for the “unfaithful spouse.” This pattern has been found in mammals  as well as birds. And although there do not seem to be any data dealing explicitly with the effect of suspected adultery on divorce and disputed child support in human beings, common sense suggests a close connection.

Not all organisms are equally prone to EPCs, if only because some species don’t form pair bonds in the first place. Others, such as the flatworm parasite, Diplozoon paradoxum, found in freshwater fish, are strictly monogamous: male and female encounter each other while adolescents, after which their bodies literally fuse together, until death do they not part. (Hence the genus name, Diplozoon, indicating two animals, while paradoxum is self-explanatory.) Other animals are only sexually receptive for very brief intervals, greatly reducing the opportunity for sexual exploration; female giant pandas, for example, are only in estrus for two to three days per year. Human beings, on the other hand—women no less than men—are endowed with 24/7/365 sexual potential, providing immense opportunities: for fitness enhancement or diminution, exciting adventure as well as miserable heartbreak.

There are remarkably few reliable DNA data on the actual frequency of human extrapair paternity, despite the fact that such testing is now widely available. Maybe this paucity shouldn’t be surprising because of possible reluctance on the part of most men to question their wife’s fidelity. As a result, the available information tends to be biased toward circumstances in which there is liable to be higher than average extrapair paternity, such as divorce proceedings, when child support is in dispute. In any event, the frequency of human marriages in which the husband is not the genetic father of the mother’s children range from as low as 0.03 to 11.8%.

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It is one thing to understand why females—of pretty much all species—hide their EPCs from their social partners, especially if biparental cooperation is expected when it comes to rearing offspring. More mysterious is this: given the potential costs of being caught, why do females engage in any EPCs at all? Bear in mind that because eggs are produced in very small numbers, whereas sperm are astoundingly abundant, females very rarely need to copulate with more than one male to be fertilized. It turns out that biologists have identified a number of potential benefits accruing to a “cheating” female, with the specifics varying with the species and sometimes with individual circumstances.

Here are a few of the major fitness payoffs thus far recognized:

• Increase the genetic variety of their offspring
• Obtain more desirable (i.e., fitness-enhancing) genes for their offspring than can be provided by their social mate
• Obtain additional resources, notably food, from their “lovers”
• Enhance their social status by affiliating with a male who is more dominant than their current partner
• Purchase “infanticide insurance” by inducing other males to behave parentally toward the females’ offspring
• Explore the potential of switching from a less desirable to a more desirable partner

One of the enduring mysteries of animal behavior and evolution has been why some species show considerable sexual dimorphism even though their primary mating system is socially monogamous. For example, a neotropical bird known as the resplendent quetzal is truly resplendent, so much so that it is the national bird of Guatemala, with its image on that country’s flag and coat of arms. But only the male is endowed with resplendently shimmering feathers and a dramatically elongated tail; female quetzals are relatively drab. Now that even social monogamy is revealed to be rife with polygynous and polyandrous departures, we can speculate as to the basis of such dimorphism, which might well owe its existence to the fact that the male quetzal’s resplendence, for example, likely enables him, at least on occasion, to achieve additional matings outside his seemingly monogamous union.

There is no reason why similar considerations (obtaining additional resources, better genes, etc.) couldn’t motivate human females, too, although in the case of women, other factors could be involved as well—which typically are not assumed to operate among animals. It is worth noting that these causes are all “proximate,” although they each have straightforward ultimate underpinnings. Moreover, although biologists have had less reason to seek explanations for EPCs by men than by women (because the biology of sperm making provides more than enough evolutionary rationale), the following considerations could apply to both sexes:

• Retaliating for infidelity by one’s partner
• Responding to other sources of anger with one’s partner
• Short- or long-term fascination or interest in a particular lover
• Seeking sexual or social gratification not otherwise available in the primary relationship

A bottom-line, take-home message is that when sexual infidelity occurs among human beings—and whether the “infidel” is a woman or man—it is because a fundamental, biologically generated, polygamous inclination (polygyny in the case of men, polyandry in the case of women) has broken through the existing monogamous social structure. Not surprisingly, men consistently report higher levels of sexual infidelity in marriage than do women. This, in turn, conforms to the biology of maleness versus femaleness discussed earlier, and with such findings as a study that encompassed 52 different countries and about 16,000 respondents, and found that men consistently expressed interest in having more sexual partners than did women. But a male–female difference in adultery could also be due, at least in part, to the near-universal double standard in which men are socially encouraged to be more sexually adventurous and to seek multiple partners so as to be seen as “real men,” whereas women who are acknowledged to be similarly inclined are often denigrated as “loose,” “easy,” or “sluts.” Some men, as well, may be liable to exaggerate their number of infidelities, just as some women may be inclined to understate theirs. It is pretty much a cross-cultural universal that men intimidate their spouses to refrain from extramarital sex, punishing them—often severely and not uncommonly, lethally—should they do so. Aside from their internal motivations, women may be most prone to “cheating” under the following circumstances:

1. When they have the support of their own relatives, which is especially likely in societies that are “matrilocal”—that is, when women live near their extended families—as opposed to “patrilocal” societies in which following marriage, the bride moves in with her husband’s family. Most human societies are patrilocal, which means that a wife is surrounded by her husband’s kin. This makes it easier for a man to keep tabs on her, and itself facilitates a double standard. It is quite possible that patrilocality became the most common marital living arrangement precisely because it serves to discourage a wife’s infidelity, thereby reassuring husbands.
2. When wives don’t rely very heavily on their husbands for material support, protection, and assistance in childrearing. Cross-cultural studies have found that among societies in which women get most of their sustenance from relatives, rather than from their husbands, they are more likely to have extramarital affairs and are also more prone to divorce.
3. When their husband is less “desirable” than others who are sexually available, with “desirability” assessed either biologically or socioeconomically. Interestingly, something comparable occurs in at least one bird species. Research on EPCs in black-capped chickadees found that females were more likely to “cheat” with males who are socially dominant, and especially when their current mate is relatively subordinate. Where females have the opportunity to learn the relative rank of all neighboring males with respect to her own mate, females regularly pursue the strategy of seeking out EPCs with superior partners. Although lower-ranked males may suffer temporary losses through the EPCs of their mates, each male has some chance of attaining alpha rank if he lives long enough. Once at alpha rank, a male will likely engage in more EPCs himself, while having a mate that will no longer seek EPCs elsewhere.

Not just chickadees cheat: a study of sexual behavior in modern China found that women whose husbands’ income was lower than the median were more likely to engage in extramarital sex.

Even in societies that explicitly permit extramarital sex (and there are a few), infidelity is nearly always carefully circumscribed and is not simply permitted willy-nilly. A now-classic review of human sexuality from a cross-cultural perspective concluded that

With few exceptions . . . every society that approves extra-mateship liaisons specifies and delimits them in one way or another. There are some peoples, for example, who generally forbid extra-mateship liaisons except in the case of siblings-in-law. This is true among the Siriono, where a man may have liaisons with his mate’s sisters and with his brother’s wives and their sisters. Similarly, a woman has sexual access to her husband’s brothers and the husbands of her sisters. . . . In some societies extra-mateship liaisons take the form of “wife lending” or wife exchange. Generally, the situation is one in which a man is granted sexual access to the mate of another man only on special occasions.

. . . Another type of permission in respect to extra-mateship liaisons appears in some societies in the form of ceremonial or festive license . . . [ranging] from harvest festivals to mortuary feasts.

Human sexual practices are notably diverse, but mostly with regard to rules about which partners are suitable, permissible, desirable, recommended, or prohibited, sometimes including details as to frequency of coitus and potential physical positions. But hopes, or—in some cases—fears that the primordial human condition is one of bonobo-like sexual promiscuity are simply not justified by anything ever theorized by biologists or found by anthropologists.

On the other hand, there is a growing body of experimental evidence to suggest that women partake of a “dual-mating strategy,” consisting of both long-term and short-term tactics. The former involves establishing a bonded relationship with a consistent partner, typically someone able to invest sufficiently in offspring and also predisposed to do so, whereas the latter calls for responding positively—especially when ovulating—to men who are literally perceived as “sexy,” which is to say, possessing good genes.

Evidence for this dual strategy comes largely from a diversity of studies showing that when they are most fertile, women are especially predisposed to prefer images, sounds, even smells arising from men who are higher in testosterone, who possess greater body symmetry—in short, who are likely to offer “good genes.”

At the same time, it must be emphasized that many of these findings involve laboratory assessments, and their highly artificial conditions may or may not reflect what people really do. Technical questions about the details of these studies go beyond the scope of this book; suffice it to say that they appear to be effectively resolved. To put it baldly and admittedly with some oversimplification, there appears to be at least a faint female predisposition to marry the more androgynous, “good father” type but dally with the bad boy stud.

It is nonetheless possible that human physiology gives a positive payoff to couples remaining together, not just long enough to produce a child, but to have achieved a level of physical intimacy along the way, which would predispose against the short-term mating strategy just described. Preeclampsia, a form of hypertension, results from immunological disparity between mother and fetus; it can be a serious complication of pregnancy and occurs roughly 10% of the time. The risk of preeclampsia decreases with increased duration of a woman’s sexual relationship with a given partner, evidently because the woman’s immune system becomes increasingly habituated to the seminal products of a given man and therefore less liable to a potentially dangerous immune response when she is carrying an embryo that contains 50% of his genes. If this scenario is valid, then mating with a new, short-term partner and promptly producing a child would increase the risk of this complication. This, in turn, would tend to mitigate against the adaptiveness of a “dual mating strategy.”

As already described, among some Amazonian peoples in particular, polyandry is facilitated by a belief in “partible paternity,” the biologically inaccurate but superficially logical notion that a child can have multiple fathers, consisting of the men who had intercourse with the mother during her pregnancy. It is probably significant that the majority of these societies are matrilocal (husbands reside with the wife’s relatives), so the women have social support. This is important because even when paternity is thought to be partible, human sexual jealousy is such that the woman’s designated husband is typically unenthusiastic about sharing his paternity as well as his wife.

When it comes to the Seven Deadly Sins, cavorting along with anger, greed, sloth, pride, lust, and gluttony, we find “envy” (in Latin, invidia). Not “ jealousy,” although in fact jealousy is a whole lot more deadly than envy. Jealousy is also a whole lot more biologically motivated. Jealousy and envy are close, but not identical. A useful distinction is that “Envy concerns what you would like to have but don’t possess, whereas jealousy concerns what you have and do not want to lose.”  You might be envious of someone who has a rich, attractive spouse, but jealous if your own partner seems to be interested in her or him. You could envy the person who “has” this spouse, but at the same time, you don’t want to lose the partner you currently have, and would be jealous if he or she were unfaithful.

An evolutionary perspective shows that this anxiety about possible loss is, at balance, worry about losing fitness, something that is particularly acute if it involves loss of an otherwise reproductively enhancing relationship. It doesn’t matter, by the way, if you and your partner have firmly decided not to have children, or if secure birth control measures were followed during any EPCs; just as people are to some degree biologically inclined toward polygamy—for themselves—they are equally predisposed against similar inclinations by their partners. Our biology operates largely independent of our cognitive intent, just as women ovulate and men produce sperm, whether or not they intend to become parents.

Sinful or not, sexual jealousy is certainly real, and is particularly evoked in the aftermath of real or imagined adultery. It is also found in women no less than men, although for perfectly “good” biological reason, the male version tends to be more cross-culturally prevalent as well as more violent. In his recent book, Jealousy, classicist Peter Toohey has unearthed a number of ancient imprecations reflecting male jealousy, including this one from second-century Egypt in which a betrayed husband begs the gods to “let burning heat consume the sexual parts of [his wife], including her vulva, her members until she leaves the household.”

We can be quite confident that Homo sapiens did not evolve in a social environment like that of chimpanzees or bonobos in which lots of sperm competition took place. For one thing, our testis size isn’t anything like that of chimps or bonobos. In addition, the anatomical structure of human sperm argues strongly against our species having a multimale, multifemale sexual heritage. On the other hand, however, we are provided with substantial amounts of sexual jealousy, not just in the Judeo-Christian West, but cross-culturally, a behavioral adaptation that presumably wouldn’t exist if it weren’t called for.

The Ten Commandments are clear when it comes to not coveting your neighbor’s wife; and although it is intriguing that no comparable warning proscribes coveting your neighbor’s husband, there is little doubt that sexual covetousness—by either sex—is more risky than the simple material kind. Coveting your neighbor’s lawn mower may be bad, but in the annals of covetousness and its consequences, it could be worse. In the Draconian precepts of Islamic sharia law, an adulterer can readily lose his or her life, whereas a thief will lose only his hand.

There are  some—albeit  rare—human  societies  in  which  married  women  are granted social permission to engage in extramarital sex, mostly with a sibling of their spouse. However, I don’t know of any human groups in which women are granted more sexual freedom than are men. For much of human history, adultery was defined by a pronounced double standard: sexual relations between a married woman and a man other than her husband. Such cases have been—and still are—widely seen as offenses against the woman’s husband. By contrast, if a husband has sexual relations with an unattached woman or with a prostitute, the majority of cultures do not consider this adultery … so long as the woman in question isn’t married to another man. There is a genetic factor—actually, an array of them—that predispose toward marital infidelity in human beings. A version of the dopamine receptor gene (DRD4), occurs on chromosome 11 and is found in all people, although individuals vary in how many times this gene is repeated: from 2 to 11. People with 7 or more repeats of DRD4 turn out to be represented in greater proportion than would be expected due to chance alone among those engaging in extramarital sex.  However, this isn’t an “infidelity gene” but rather a genetic predisposition toward greater sensation seeking. One might expect that individuals with multiple repeats of DRD4 would also be more likely to go skydiving, or to enjoy roller coasters. Nor is it that they necessarily have a higher sex drive, or a genetic proclivity to extramarital sexual exploits as such; rather, they crave novelty.

It is one thing to insist on something new for dinner every night, quite another to insist on a new lover. One way of conceptualizing the problem—without explicitly invoking biology—is that such behavior violates what in the Western tradition is known as “social contract theory.” The idea is fundamental to much political philosophy, including the work of Hugo Grotius, Thomas Hobbes, and especially John Locke, whose Second Treatise of Government (1698) laid out the proposition that people agree to live convivially in a social unit—as large as a nation-state or as small as a domestic family—by forgoing certain individualistic options to gain other benefits, based on cooperation and shared responsibilities. Just as government “derives its just powers from the consent of the governed” (a basic principle employed a century and a half later by the framers of the US Constitution), marriages derive their legitimacy and stability from the consent of the participants. And one of the most important such consensual necessities is sexual fidelity. The husband–wife sociosexual contract is thus the governmental social contract writ small.

Under its terms, and taking a hard-eyed look at its contractual aspects, women provide men with a guarantee of their sexual fidelity as well as a partner for regular intercourse along with other shared domestic payoffs, while men provide women with resources, protection, and assistance in childrearing. Along with a mutual sharing of genes. Although this traditional contract was—and still is—unfair in its implied asymmetry with regard to sexual fidelity, it was in many ways an excellent one. There is an additional problem, however, which is the substance of this book: both men and women carry with them an evolutionarily generated inclination to violate the contract and to consort with other partners—that is, for polyandry as well as polygyny. Biologists have if anything been late to the party when it comes to appreciating the potential of both polygyny and polyandry to assert themselves despite a sociocultural commitment to monogamy. This recognition is part of a new and important realization on the part of evolutionary biologists: men and women often have distinctly different evolutionary interests. This is true despite the fact that human beings are if anything unusual among living things in that their interests are likely to be shared when it comes to caring for their needy offspring. We are stuck with, on one hand, a biological basis for a biparental social contract, and, on the other, a no less biological basis for polygamous yearnings.

Reprinted from "Out of Eden" by David P. Barash with permission from Oxford University Press USA.  Copyright © 2016 Oxford University Press and published by Oxford University Press USA. (www.oup.com/us). All rights reserved.