By David Barash: We now know scientifically what most people have long known privately: that social monogamy does not necessarily imply sexual monogamy.

Right-wing pro-marriage advocates are correct: Monogamy is definitely under siege. But not from uncloseted polyamorists, adolescent "hook-up" advocates, radical feminists, Godless communists or some vast homosexual conspiracy. The culprit is our own biology.

Researchers in animal behavior have long known that monogamy is uncommon in the natural world, but only with the advent of DNA "fingerprinting" have we come to appreciate how truly rare it is. Genetic testing has recently shown that even among many bird species — long touted as the epitome of monogamous fidelity — it is not uncommon for 6 percent to 60 percent of the young to be fathered by someone other than the mother's social partner. As a result, we now know scientifically what most people have long known privately: that social monogamy does not necessarily imply sexual monogamy.

In the movie "Heartburn," the lead character complains about her husband's philandering and gets this response: "You want monogamy? Marry a swan!" But now, scientists have found that even swans aren't monogamous. (Nor are those widely admired emperor penguins, whose supposed march to monogamy was misconstrued from another popular movie; their domesticity lasts only for the current breeding season — next year, they'll find new mates.)

For some, findings of this sort may mitigate a bit of the outrage visited on the current and future crop of adulterers du jour, recently including but assuredly not limited to Eliot Spitzer, Mark Sanford, John Ensign and John Edwards. For others, it simply shows that men are clueless, irresponsible oafs. The scientific realty, however, is more nuanced, and more interesting, especially for those looking to their own matrimonial future.

First, there can be no serious debate about whether monogamy is natural for human beings. It isn't. A Martian zoologist visiting planet Earth would have no doubt: Homo sapiens carries all the evolutionary stigmata of a mildly polygamous mammal in which both sexes have a penchant for occasional "extra-pair copulations."

But natural isn't necessarily good. Think about earthquakes, tsunamis, gangrene or pneumonia. Nor is unnatural bad, or beyond human potential. Consider writing a poem, learning a second language or mastering a musical instrument. Few people would argue that learning to play the violin is natural; after all, it takes years of dedication and hard work. A case can be made, in fact, that people are being maximally human when they do things that contradict their biology. "Doing what comes naturally" is easy. It's what nonhuman animals do. Perhaps only human beings can will themselves to do things that go against their "nature."

And finally, even though anyone aspiring to genuine monogamy will, on balance, have to swim upstream against the current of his or her evolutionarily bequeathed inclinations, there are also considerable biological forces supporting such efforts. Some animals manage to be monogamous. California mice (Peromyscus californicus), for example, pair up and remain paired, forsaking all others, largely because of the payoff derived from having two parents to care for offspring. Beavers establish lasting pair-bonds that enable them to cooperate in building a valuable, complex home site. The Malagasy giant jumping rat has evidently made the jump to monogamy because of the predator-fighting benefits thereby provided. And among pygmy marmosets, monogamy gives males unconscious confidence of their paternity, which in turn supports their inclination to be unusually paternal.

And human beings? Our species benefits greatly from bi-parental care. We can profit from shared, reciprocated effort, especially when we're confident both partners will be around for the long term. In addition, human beings are endowed with an array of hard-wired traits that can be used to strengthen monogamy, among them a penchant (perhaps even a need) to attach and connect so-called mirror neurons that underlie empathy; hormonal systems, such as those involving oxytocin and vasopressin, that relate sexual satisfaction to pair-bonding; and neural plasticity that promotes the strengthening of brain circuits associated with repeated reward mechanisms — including, in all likelihood, those activated via interactions with the same individual.

Add to this the fact that people have big brains, and hence, an ability to rescue monogamy from monotony, as well as the capacity to imagine the future and a visceral dislike of dishonesty, and the effect of biology on monogamy becomes complex indeed. Not to mention the adaptive significance of that thing called love.

To be sure, monogamy isn't easy; nor is it for everyone. But anyone who claims that he or she simply isn't cut out for monogamy misses the point: No one is. At the same time, no one's biology precludes monogamy either.

As Jean-Paul Sartre famously advised (albeit in a different context): "You are free; choose."

Barash, an evolutionary biologist, is a professor of psychology at the University of Washington; his most recent book — co-written with Judith Eve Lipton — is "Strange Bedfellows: The Surprising Connection Between Sex, Evolution and Monogamy."