"I grew up in a neighborhood where there was a case of husband A running off with wife B, and it was a talked-about scandal for years afterward," says Tom W. Smith, director of the University of Chicago's National Opinion Research Center, which has researched adult sexual behavior. "It's just not shocking anymore. Our TV images have gone from 'Ozzie and Harriett' to 'Desperate Housewives.' "

Yet 91 percent of those questioned in a Gallup Poll last year said affairs are morally wrong.

Theories on who cheats and why abound among social scientists and jilted lovers, but those who have studied the issue are hard-pressed to come up with a one-size-fits-all answer.

Academics can't even agree on the extent to which adultery is happening. Various studies have found that anywhere from 15 percent to 70 percent of people have had sex with someone other than their spouse while married.

There is, however, consensus that men have been more likely to be unfaithful than women, although the gap is closing. Couples in large urban areas are more shaky than rural ones. And younger spouses are more likely to wander than those 50 and older.

In "Not Just Friends: Rebuilding Trust and Recovering Your Sanity After Infidelity," author Shirley Glass identifies five motivators — emotional intimacy, love, sex, ego and revenge.

Generally, men and women cheat for different reasons, according to Ruth Houston, author of "Is He Cheating on You? 829 Telltale Signs" (Lifestyle Publications, $29.95).

"Women are usually looking for emotional fulfillment, and men are looking for sex," she says.

Houston became interested in the topic after a failed marriage to a man with a wandering eye. Her book is based on years of interviews with cheaters, divorce lawyers and private detectives.

Be cautious about gender stereotypes, warns San Diego psychologist David Wexler.

Sure, some guys are just selfish, lusty jerks, he says, but plenty of fundamentally decent men go astray, too, and blaming libido oversimplifies the issue.

In his book "When Good Men Behave Badly" (New Harbinger Publications, $15.95), Wexler talks about "broken mirrors."

Men feel alive and worthy when they look into the eyes of a partner and see love, delight and respect mirrored back.

A "broken mirror," then, is a partner's negative view.

"That makes him feel crummy, and many men don't react well to those broken-mirror experiences," Wexler says. "They can withdraw or get critical or sarcastic or even ... abusive or violent.

"Or some men will choose to look for positive validation elsewhere. It's a dysfunctional and emotionally immature response, but it happens."

"Men will boast about their affairs, and women underreport," says Diane Shader Smith, author of "Undressing Infidelity: Why More Wives are Unfaithful" (Adams Media, $14.95). The book profiles 14 women who've had one or more affairs, as well as the author's own flirtation with temptation.

Many societies treat adulterous women more harshly than men. All the way back to ancient times, men could have multiple wives, but a woman who cheated was scorned or could even be killed. It's no coincidence that infidelity rates among women have been creeping up since the 1960s, when women began entering the work force in large numbers for the first time, says Shader Smith. Now that many women are able to support themselves, and in some cases can avail themselves of no-fault divorce laws, they're more willing to take a chance.