Don’t deny it. There is a high likelihood you have thought about cheating on your partner.

If you are a woman, the odds are 4 to 1 that you have had a fantasy about an extradyadic partner during the last two months alone. If you are a man, the odds are much higher (fully 49 to 1) that you’ve recently thought about cheating.

Since it’s pretty likely that you have, like former President Jimmy Carter, lusted in your heart, you’re probably inclined to give your partner a little consideration when it comes to his or her lustful desires. Right?

Probably not, as it turns out. In a soon-to-be published chapter on “the universal threat and temptation of extradyadic affairs” my colleague Bram Buunk of the University of Groningen reviews evidence that most of us are hypocrites in this regard. Along with Pieternel Dijkstra, and Karlijn Massar, Buunk reviews findings suggesting that 90 percent of American men and women believe that extramarital sex is always or almost always wrong (despite the near certainty that they have dreamed about it themselves). Even in the relatively libertine Netherlands, where prostitution is legal and your sexual orientation is your own business, 78 percent of the population considers extradyadic sex just wrong.

Another recent review of the research on infidelity by Florida State University researchers Frank Fincham and Ross May comes to similar conclusions; they note that a Gallup poll suggests that 9 out of 10 people view infidelity as immoral, and almost 7 out of 10 view it as unforgivable. Nevertheless, at least 25 percent of men and 15 percent of women, by one estimate, will cheat on their partners. Other studies indicate lifetime incidences even higher, closer to 50 percent, according to David Buss and Todd Shackelford. And, according to Fincham and Ross, more recent studies seem to suggest that younger women are catching up to men (although there seems to be a recent boost among men over 65 as well, which Fincham and Ross attribute to the ready availability of drugs for erectile dysfunction).

What predicts infidelity?

Before I dove into the actual literature, I pondered some of the people I know who have been involved in extramarital liaisons (don't worry, you won't be named). I came up with my own off-the-top-of-the-head list of likely infidelity signals:

1. Your partner is becoming increasingly obsessed with physical fitness, and not in the good-for-your-cardiovascular system way, but in the muscle-toning, good-for-your-butt-definition way.

2. Your partner is spending an inordinate amount of money on clothing, especially of the form-fitting, hugs-that-well-defined-butt variety. If your partner is a male, maybe he spends his money on a fast and flashy sports car instead.

3. Your partner is spending extra hours at social events with coworkers, or volunteering to travel out of town on assignments.

4. Your partner, despite looking increasingly sexy, is showing less erotic interest in you.

5. Your partner seems to get easily irritated at you of late.

6. Your partner is drinking a lot.

7. Your partner is spending a lot of time on his or her iDevice, and quickly turns it off when you enter the room.

8. Your partner has a history of sexual unrestrictedness.

9. Your partner is extraverted.

10. Your partner is socially unconventional.

The last two aren’t completely blind guesses, because I teach social psychology and know there is research connecting extraversion and unconventionality to divorce. Number 8 is kind of circular, since sexually unrestricted people are, well, sexually unrestricted. Past behavior is the best predictor of future behavior, and sowing your wild oats isn’t likely to get the urge out of your system, any more than a few years of eating in high-end restaurants is likely to quell your appetite for rich foods.

But then, like any scientifically minded person should do, I looked to see what the actual research has to say. There are some nice reviews of the literature on this topic, and I’ll summarize a few patterns that seem to have reasonably solid support.

The research findings

Twenty years ago, Buss and Shackelford did a study of newlywed couples and asked what personality factors predicted people’s own inclinations to cheat. They found that infidelity susceptibility was relatively high in people who scored low on Conscientiousness (meaning they were laid back and careless in their work habits rather than hard-working, neat, and orderly, and thus less likely to worry about stepping outside the lines). Those who scored high in Narcissism were also at risk. Narcissism as measured here involved talking about yourself without listening to others, and being exhibitionist, self-aggrandizing, and self-absorbed. The potentially unfaithful also scored relatively high on Eysenck’s scale of “Psychoticism,” which has little or nothing to do with schizophrenia and actually measures impulsiveness and psychopathy. Buss and Shackelford also found that the proneness to be unfaithful was higher in people who were sexually dissatisfied with their relationship, and who had conflicts with their partners—often over their partner’s jealousy, which may not have been misplaced.

In their more recent review, Fincham and May report that actual infidelity is higher in people who are neurotically anxious, who have a history of numerous partners before marriage, and who are insecurely attached to their partner.

Problem drinking is indeed associated with infidelity. More generally, Jason Weeden and his colleagues have shown that recreational use of drugs—from alcohol to marijuana—is associated with unrestricted sexual behavior. It may partly be a signal that you’re a party animal, which may loosen you up to violate social conventions.

Interestingly, social class, education, and economic wealth are not reliably linked to infidelity. Lusting in your heart (and acting on it) is as likely to happen in the White House as in the West Virginia hollows, among Ph.D.s as well as high school dropouts.

Several studies have replicated the link between relationship problems and marital infidelity. This is an area where it is critically important to keep in mind that correlations do not establish cause and effect. If you’re running around, having fun sex with someone else on the side, and then come home to a house full of dishes, overdue bills, children screaming for some attention, and an exhausted grumpy spouse, your extramarital affair is likely to look increasingly good—and your married life increasingly bad—by simple contrast. And as you bicker more with your overwrought partner, your “understanding” outside lover starts looking even better. In fact, a 17-year longitudinal study of married couples suggested that marital infidelity is both a cause and a consequence of marital conflict (Previti & Amato, 2004).

The other consequences of infidelity

Though we may snicker when we hear about politicians and religious leaders cheating, and though we may ourselves fantasize about it, infidelity is no laughing matter. All around the world, it is a leading cause of divorce, and one of the leading causes of spousal homicide (Betzig, 1989; Daly & Wilson, 1988). Many couples divorce without beating each other up physically, but the experience is psychologically damaging for them and their children (Amato, 2010). And even if you avoid divorce, sex outside marriage is often had without condoms (we’re talking about impulsive self-indulgence after all). Hence infidelity is associated with an increase in sexually transmitted diseases, including HIV, which is doubly unfair to the faithful and trusting spouse at home. And Fincham and May note that it is a public health problem as well.

So if you’re thinking about it, think twice. And if you’re already running around, stop. Howard Markman (2005), who is a leader in research on relationship improvement, has offered some insights about how to make your relationship more resistant to extramarital affairs and how to reconcile if there has been infidelity. If you’re the victim rather than the offender, this is a crucial time to study the research on forgiveness.

I come at all this not as a conservative and self-righteous old man who doesn’t want young people to have fun (see Kenrick, 2011). Instead, I am someone whose youthful infidelities broke up more than one relationship. One of those break-ups totally disrupted the life of my 2-year-old son, and decades later, I still regret my self-indulgent decisions. Some of the worst and most short-sighted advice I got at that time was this: “You’ve got to do what’s right for you.” What seemed right for me at the time turned out to be very wrong not only for those I loved, but even for myself. So, do the right thing. You’ll not only be less likely to get battered with a rolling pin or thrown out of your happy home, you’ll feel better about yourself in the long run.

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