5 Reasons Men Cheat—and How to Stay Faithful

So you found "The One" and you're deep into relationship-building mode. Nothing's better at derailing "till death do us part" than an "accidental" date or a roll in someone else's hay. We've heard all the excuses: It just happened. I didn't see it coming.

According to a 2008 Gallup poll, 54 percent of Americans know someone personally who has an unfaithful spouse. And the University of Chicago's General Social Survey consistently finds that 20 percent of men cheat in their lifetimes, compared with 12 percent of women. Look around you: attractive colleagues, flirty baristas, hot neighbors. Your day may be coming.

Think it's just about s8x? Not so fast. When marriage therapist M. Gary Neuman interviewed 100 cheating men for his recent book, The Truth about Cheating, only eight cited sex as the main reason for their infidelity. Forty-eight of them said emotional issues drove them to cheat. If sex was a factor, other problems were probably lurking.

Knowing what makes men stray and how to squelch those urges can help keep you and your chosen one happy for the long haul. Tapping new research in genetics, economics, anthropology, and biology, as well as our experts' advice, we've developed this guide to the causes of infidelity—and what you can do to make sure the home fires blaze hot enough to keep you happy.

You're a Dirty Rat
Well, not a rat, exactly, but a vole—a prairie vole. This small rodent is one of the few mammals that actually bond with their mates, and a vole's genetic traits give scientists clues about why humans stray from theirs. It turns out there's a switch inside the brain that controls the desire to form close ties. Emory University psychiatry professor Larry Young, Ph.D., has found that switch in voles: It's a gene that regulates the release of vasopressin, a hormone that activates receptors in the brain to regulate behavior.

In male voles, vasopressin helps keep the mate close and the competition far away. Assuming that what's true for voles is also true for humans, vasopressin activates bonding centers in your brain, making you feel attached and protective. (In women, oxytocin serves the same purpose.)

But according to scientists at Stockholm's Karolinska Institute, if you have a certain variation of the gene coding for one type of vasopressin receptor, you won't be as affectionate and cuddly as your mate would like you to be. The 2008 Karolinska study found that with this version of the gene, you're less likely to commit and twice as likely to report recent relationship problems.

Having the gene variant isn't a get-out-of-jail-free card, however. Your culture, childhood, and other life experiences play a large role in determining your behavior, Young says. "This gene changes the probability of the type of relationship you'll have; it doesn't determine it," he explains. "So you can't ever say you're genetically pre-determined to cheat."

Keep the faith: To ward off any such probabilities in your own relationship, increase your intimacy to boost bonding. "We know that vasopressin is released during sex," says Young, "and it's probably released in other intimate situations as well." Make sure your vasopressin receptors are firing throughout the day by creating what Neuman calls "touch points." Commit to making five small, intimate gestures—a hug, kiss, e-mail, or text, for instance—throughout the day. As she responds with five of her own, you'll ignite the vasopressin bonding centers in your brain at least 10 times each day.

You're Not Risk-Averse Enough
For many men, cheating is simply another decision, one with its own set of costs and benefits. Infidelity has uncertain and individual outcomes—you don't know how guilty you'll feel afterward, whether or not she'll catch you, or exactly what you'll lose in a divorce—so economists Edinaldo Tebaldi, Ph.D., of Bryant University, and Bruce Elmslie, Ph.D., of the University of New Hampshire, have developed a model that identifies some of the factors influencing a person's decision to cheat.

Their study, published last year, reveals that men and women use drastically different decision-making processes. What's she thinking? A lot. Is he worth keeping, or is there someone out there that might be worth the risk of leaving? Just how much (money, love, companionship, security) will I lose if I get caught spending my lunch hour in a seedy motel? It's about her relationship, her future, and the investment she might lose if she's caught.

What are you thinking? Not much: Is there an opportunity, and will I get caught?

Keep the faith: Make an informed decision by figuring out what you'd be losing now and in the future, Neuman says. Once you realize the risks, start sticking around the house. More than half of the men Neuman interviewed spent time away from home before they cheated. "They started working later, scheduling more business travel, or hanging out with friends," he says. While you're at home, do something few men do when they find their eyes roving: Face your spouse and admit to her that something's wrong. "Tell her, 'I'm looking around and I shouldn't be,'" Neuman says. "Then figure out, with her, what can you do to make the relationship better. Once you know what's wrong and how you'll fix it, saving your marriage will replace thoughts of cheating."

You Don't Recognize Threats
When they're surrounded in social situations by what researchers call "attractive alternatives," men tend to let their guard down too much. In a McGill University study, men and women who were presented with a virtual-reality assessment reacted very differently when they were asked to evaluate a group of photographs that included an image of an attractive person of the opposite sex. Women avoided the photo, but men didn't. Asked to imagine an interaction with an attractive classmate of the opposite sex, women increased their thoughts of commitment and threat. Men (drum roll) didn't.

The McGill researchers suspect that women strive to be protectors and gatekeepers of their relationships, or that they view threats to their relationships as personal. (Men, on the other hand, tend to be more individualistic; they don't define themselves by their relationships as much.) Either way, women use if-then contingencies—if he comes over here, then I'll excuse myself to get a drink—that spark automatic defense mechanisms when they see an attractive alternative moving in.