Do men and women cheat for different reasons?

On AMC's "Mad Men," Jon Hamm plays a character who frequently cheats on his wife. See more pictures of kissing.
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If you're exposed to any form of American media, you may think that everyone is having an affair. The news is full of celebrities, athletes and politicians caught with someone other than their spouses. In prime time, you can watch the Desperate Housewives commit adultery, then flip the channel and find the Mad Men doing the same thing. Romance novels feature illicit trysts, and much of country music would have gone unwritten without someone's cheatin' heart.

Is infidelity the norm? Scientists know of only a few species that are completely sexually monogamous; one such species is the Diplozoon paradoxum, a flatworm that fuses to its partner until its death [source: Angier]. Even in animal species that practice social monogamy, such as birds that raise their broods in pairs, sexual monogamy is not the norm. A male bird may be raising a brood in which 10 to 30 percent of the offspring aren't his own [source: Barash].

But here's one case in which humans aren't emulating the birds and the (assumedly frisky) bees. Particularly in the United States, complete and utter monogamy is the golden standard, no matter what the media would have you believe. According to the General Social Survey, which has tracked Americans' positions on a wide variety of issues since 1972, the United States is becoming less indulgent of infidelity. In the swinging 1970s, 63 percent of men and 73 percent of women believed marital infidelity to be always wrong; in the 2000s, 78 percent of men and 84 percent of women believed it was always wrong [source: Jayson]. Though we live in a time when many people are open-minded about most things that go on in the bedroom, Americans ranked adultery more morally disturbing than polygamy and human cloning in a 2006 Gallup poll [source: Kohen].

So if everyone believes this act to be so horrendous, why does it happen at all? Though betrayed partners have grappled with this question for centuries, researchers have taken up the conundrum in recent years as well. In this article, we'll take a look at some of their findings, and how men and women differ when it comes to breaking their marital vows.