Want a man, or a worm?

As an evolutionary biologist, I look at New York Gov. Eliot Spitzer’s now-public sexual indiscretions and feel justified in saying, “I told you so.”

One of the most startling discoveries of the last 15 years has been the extent of sexual infidelity (scientists call it “extra-pair copulations” or EPCs) among animals long thought to be monogamous. It’s clear that social monogamy -- physical association and child rearing between a male and a female -- and sexual monogamy are very different things. The former is common; the latter is rare.

At one point in the movie “Heartburn,” Nora Ephron’s barely fictionalized account of her marriage to reporter Carl Bernstein, the heroine tearfully tells her father about her husband’s infidelities, only to be advised, “You want monogamy? Marry a swan.” Yet thanks to DNA evidence, we know now that even those famously loyal swans aren’t sexually monogamous.

One species that is, and, significantly, perhaps the only one that could be reliably designated as such, is Diplozöon paradoxum, a parasitic worm that inhabits the intestines of fish. Among these animals, male and female pair up while adolescents; their bodies literally fuse together, whereupon they remain sexually faithful until death does not them part.

One of the most important insights of modern evolutionary biology has been an enhanced understanding of male-female differences, deriving especially from the production of sperm versus eggs. Because sperm are produced in vast numbers, with little if any required parental follow-through, males of most species are aggressive sexual adventurers, inclined to engage in sex with multiple partners when they can. Males who succeed in doing so leave more descendants.

A story is told in New Zealand about the early 19th century visit of an Episcopal bishop to an isolated Maori village. As everyone was about to retire after an evening of high-spirited feasting and dancing, the village headman -- wanting to show sincere hospitality to his honored guest -- called out, “A woman for the bishop.” Seeing a scowl of disapproval on the prelate’s face, the host roared even louder, “Two women for the bishop!”

On balance, the Maori headman had an acute understanding of men. He also reflected a powerful cross-cultural universal: Around the world, high-ranking men have long enjoyed sexual access to comparatively large numbers of women, typically young and attractive. Moreover, women have by and large found such men appealing beyond what may be predicted from their immediate physical traits. “Power,” wrote Henry Kissinger, “is the ultimate aphrodisiac.”

Power-as-pheromone is pretty much the default among mammals. Elk, elephant seal, baboon or chimpanzee, in a wide array of species, females eagerly mate with dominant males while disdaining subordinates. And they do so, more or less, in harems.

Not surprisingly, before the homogenization of cultures that resulted from Western colonialism, more than 85% of human societies unabashedly favored polygamy. In such societies, men who accumulate power, wealth and status gain additional wives and consorts. In avowedly monogamous cultures, successful males accumulate a wife and often additional girlfriends. Even if, thanks to birth control technology, they do not actually reproduce as a result (and thus enhance their evolutionary “fitness”), they are responding to the biological pressures that whisper within men.

Part of being successful, moreover, is a tendency to feel entitled and often to be uninhibited -- in part because one outcome of our species-wide polygamous history is that successful men have been those who took risks, which paid off. The losers were mostly found among the unsuccessful bachelors who, by definition, did not contribute very much to succeeding generations of men, or to their inclinations.

All of which contributes to the apparent sex appeal of such less-than-stunning physical specimens as Kissinger, Woody Allen and Bill Clinton, not to mention the persistence of sex scandals among the popular and powerful across the political and ideological spectrum, including Thomas Jefferson, JFK, Hugh Grant, Newt Gingrich, Larry Craig and a long list, receding almost to the infinite past as well as likely into the indefinite future. For men at the top -- rock stars, successful athletes, politicians, wealthy CEOs, the jet-set glitterati -- such opportunities are exceedingly numerous, not so much because they have insatiable sex drives but because they are dominant males in a biologically randy species.

Some readers may bridle at this characterization of Homo sapiens as EPC-inclined, but the evidence is overwhelming. That doesn’t justify adultery, by either sex, especially because human beings -- even those burdened by a Y chromosome and suffering from testosterone poisoning -- are presumed capable of exercising control over their impulses. Especially if, via wedding vows, they have promised to do so. After all, “doing what comes naturally” is what nonhuman animals do. People, most of us like to think, have the unique capacity to act contrary to their biologically given inclinations. Maybe, in fact, it is what makes us human.

But even a smidgen of evolutionary insight suggests that maleness plus money plus political power isn’t likely to add up to the kind of sexual restraint that the public expects. A concluding word, therefore, to the outraged voters of New York state: You want monogamy? Elect a swan. Or better yet, a Diplozöon paradoxum.

David P. Barash, an evolutionary biologist, is professor of psychology at the University of Washington.