A lot of the statistics on infidelity floating around are dubious. Some say that as many as 50 percent of women cheat on their husbands, and 70 percent of men step out on their wives.

More reliable and believable data come from the University of Chicago's National Opinion Research Center. About 15 percent of women surveyed in 2002 said they'd ever had sex with someone besides their spouse while married, and 22 percent of men had. Roughly 2 percent of women and 4 percent of men had done so in the past year.

It's clear that men are more prone to infidelity, and notably, the longer they live, the more likely they are to cheat. According to the 1992 National Health and Social Life Survey, 37 percent of men aged 50–59 had ever had an extramarital affair, compared with just 7 percent of men aged 18–29. The men's percentages went up steadily in each age range, whereas for women, the most perfidious were the baby boomers, born between 1943 and 1952. About 20 percent of them reported ever having had an affair, but in all other age ranges, infidelity hovered between 11 percent and 15 percent.

What are not taken into account in these surveys are other kinds of infidelity besides having sex. Does a stolen kiss count? What about erotic chats with strangers online? A lap dance?

"Infidelity occurs when one member of a couple secretly violates the commitment to monogamy. That's a very inclusive definition," Lusterman says. If your partner considers it cheating, then it probably is. But what would mortify your partner might be a-okay, or at least tolerated, by mine.

"I think there probably is a bigger range of fidelity than we imagine," Kramer says. Some couples enjoy bringing third parties into their bedroom, yet they would insist that they have never cheated.

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