Why We Cheat

Sexual infidelity is one of humanity's great obsessions, perhaps second only to violence. We abhor it, yet we want to hear all about it, and some can't resist it. It's what has kept Jerry Springer on TV for the past 14 years and Greek mythology alive in the retelling for the past 3,000.

In one story after another, mundane and epic, we are reminded of the emotional and social fallout of messing around. That's in addition to the scowls it gets from the world's biggest religions. Why, then, is monogamy so hard for so many?

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Perhaps for humans, monogamy does not come naturally, and biology predisposes us to seek multiple sex partners. That's what zoologist David Barash, PhD, and psychiatrist Judith Lipton, MD, argue in their book, The Myth of Monogamy: Fidelity and Infidelity in Animals and People. Virtually all animals, they say, are far from being 100% monogamous 100% of the time.

"The only completely, fatalistically monogamous animal we've been able to identify is a tapeworm found in the intestines of fish," Lipton tells WebMD. That's because the male and female worms fuse together at the abdomen and never separate afterward.

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.

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.