Anne Bancroft and Dustin Hoffman in "The Graduate" Take that, monogamy! We're actually hard-wired for polygamy, which helps explain why so many cheat Monogamy is mandated throughout the Western world, infidelity is universal. A leading biologist explains why

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 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. 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. 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.