Why We Cheat (cont.)

Other animals, including humans, are motivated to ensure their reproductive success not only by picking the highest quality mate they can get but also by taking others on the side.

"The examples where monogamy is perceived to be the norm are generally facades when you actually do DNA testing and see who's sleeping with whom," Lipton says. She and Barash make a distinction between sexual fidelity and what they call "social monogamy." Even in animals that mate for life, like many birds do, DNA tests reveal that the offspring are often not related to the male of the pair.

That is the case with people, too. Lipton says she was once contacted by a Canadian hospital, where doctors were running genetic tests to find out children's risks for inherited diseases. In about 10% of the samples, the children were not genetically related to the supposed father.

But make no mistake: Lipton and Barash, who have been married to each other for 28 years, don't say that sexual fidelity is impossible or wrong because it is not natural, only that it takes some effort. "We human beings spend a large part of our lives learning to do unnatural things, like play the violin or type on a computer," Lipton says.

The Flawed and the Faithful

If fidelity is a matter of skill, then why are some talented and others terribly clumsy?

People who enter into long-term monogamous relationships, and who really keep their promises, "tend to be very healthy mentally," Peter Kramer, MD, tells WebMD. Kramer, a psychiatrist, is the host of The Infinite Mind on NPR and author of Listening to Prozac, Should You Leave? and most recently, Against Depression.

"There are lots of things that they're not, and that makes it possible for them to do this thing that may be in some ways difficult," he says.

Don-David Lusterman, PhD, a marriage and family therapist and author of Infidelity: A Survival Guide, says he thinks some people who cheat are what he calls "pursuers," who are also called womanizers when they are men. "They tend to require great numbers of conquests and they perceive them as conquests," Lusterman tells WebMD. "I see that as a developmental flaw in an individual, as opposed to an affair frequently being a function of some disruption in the couplehood. They're very different things."

In clinical terms, he says, pursuers often have a narcissistic personality disorder. They crave and demand affection and attention but are not able to return it in kind.

Those who aren't pursuers may be susceptible to an affair because they are not aware that something is amiss or lacking in the relationship. Given the attention of another man or woman, "they just suddenly feel more special," says Luanne Cole Weston, PhD, a psychologist and expert moderator of WebMD's Sex Matters® message boards. "They ceased to feel as special in their own first relationship."