Sexual Infidelity: Nature or Nurture?

If Elizabeth Edwards were behaving as evolutionary psychology says she should, she would not be separating from her philandering husband, former senator John Edwards. He, after all, merely slept with the help; he never pulled a Mark Sanford, who called his mistress his “soulmate." Women are supposed to find only emotional betrayal upsetting; they're not supposed to care if their mate shtups anything in a skirt (Elin Woods is therefore conforming to the stereotype of women being forgiving of sexual but not emotional infidelity if she, as reported, stays with Tiger; the very fact that his mistresses numbered in double digits suggests there wasn't exactly a deep emotional commitment there).

Of all the ways men are from Mars and women from Venus, this supposed sex difference in jealousy is one of the most amusing. But an intriguing new study suggests that the gender gap in jealousy may be the result of something that is not at all hard-wired: the different ways boys and girls are raised.

First the backstory. According to evolutionary psychology (the field that seeks the roots of human behavior in our Paleolithic past), men's brains are wired to become more upset by sexual infidelity, and women's to become more upset by emotional infidelity. This difference is the result of "caveman DNA" that everyone alive today inherited from our Stone Age ancestors of 100,000 years ago, as I explained in a story last year, and so is as hard to flout as DNA specifying that we grow two arms.

The evolutionary reason for the men-women split is that a partner's sexual infidelity is more damaging to a man, but a partner's emotional infidelity is more damaging to a woman, and people care more about what is most harmful to them. To wit: if a woman sleeps around, then her partner might (unknowingly) be deprived of her reproductive services for at least nine months, and could wind up raising another man's child—both of which hurt his own chances of reproducing, which is the currency of evolutionary success. A man should therefore become much more upset by his partner's sexual infidelity than by her emotional infidelity (developing a crush, for instance, but not acting on it). In contrast, if a man falls in love with another woman, he might abandon his wife and children, putting them at risk, but meaningless extramarital sex is unlikely to lead to such a drastic outcome. A woman should therefore care more about her partner's emotional infidelity than his casual hookups. This is supposedly why Hillary Clinton stuck by her man despite Monica, Gennifer, and who-knows-who-else.

The main support for the evolutionary-based Mars-Venus divide in jealousy comes from David Buss of the University of Texas. In studies of American college students, he has found that about twice as many men as women say they'd be upset more by sexual infidelity than by emotional infidelity. But psychology researchers Kenneth Levy and Kristen Kelly of Pennsylvania State University had doubts about the evolutionary explanation. For one thing, many men find emotional betrayal more upsetting than the sexual kind, Levy told me. "I noticed that although studies found that more men than women indicated that sexual infidelity was more bothersome than emotional infidelity—30 to 50 percent of men say that, compared to only 20 percent of women—an awful lot of men weren't behaving as evolutionary psychology says they are supposed to." That is, 50 to 70 percent of men agree with women that emotional infidelity is more distressing than sexual betrayal, says Levy.

As I wrote in that 2009 story:

"Men are evenly split on which kind of infidelity upsets them more: half find it more upsetting to think of their mate falling in love with someone else; half find it more upsetting to think of her sleeping with someone else … And in some countries, notably Germany and the Netherlands, the percentage of men who say they find sexual infidelity more upsetting than the emotional kind is only 28 percent and 23 percent … New data on what triggers jealousy in women also undercut the simplistic evo-psych story. Asked which upsets them more—imagining their partner having acrobatic sex with another woman or falling in love with her—only 13 percent of U.S. women, 12 percent of Dutch women, and 8 percent of German women chose falling in love, as evolutionary theory says they should."

Levy and Kelly (husband and wife) therefore suspected that the differences between men and women might reflect not a deep-seated evolutionary bent—in which case there should be no cultural differences, and more than half of all men should not deviate from the prediction—but something else. That something might be how secure people feel in their emotional attachments to others.

According to "attachment theory," how you are raised leaves a lasting impression on how trusting you are in intimate relationships. (Some of the most interesting work on this is by Phillip Shaver of the University of California, Davis.) In a nutshell, people whose parents were warm and loving and reliable sources of emotional support tend to be "securely" attached, forming successful adult relationships that are not marred by excessive clinginess or jealousy. But people whose parents were distant or cold tend to be "avoidant": they are either dismissive of close relationships (and therefore prefer autonomy to commitment, and are often promiscuous) or afraid of them. (That is, the "avoidant" attachment style comes in two forms, fearful or dismissive. The former is often of the "once bitten, twice shy" variety, in which someone is afraid of being hurt in a relationship. But dismissive people actively scorn relationships.) The Penn State duo hypothesized that people who are dismissive of relationships would be more distressed by sexual than emotional infidelity.

As they will report in a study to be published in the February issue of the journal Psychological Science, securely attached people were, as predicted, much more upset about emotional infidelity than sexual infidelity: 77 percent said they are much more likely to find emotional infidelity more upsetting than sexual infidelity. That held for men as well as women—no sex difference. They also found that men and women who are fearful of relationships are more upset by emotional infidelity; again, no sex difference. Only men and women who are dismissive of relationships, the scientists found, are more upset by sexual straying than by a mate's finding his or her soul mate in someone else. Because "more men than women are dismissive of relationships, and because such people are concerned more about sexual infidelity," they write, "what looks like a gender difference is in fact an attachment effect"—that is, a product of how people feel about forming close relationships. Conclusion: Mars-Venus differences in jealousy are the result of attachment style and not of our caveman genes.

Now for the speculative part. Because of how American culture raises little boys ("Be a man!" "Don't cry!"), boys are more likely to grow up to be insecurely attached. "More men than women have an insecure, dismissing avoidant attachment style," the scientists note. Yes, ladies, you can blame his fear of commitment on his parents. It has nothing to do with caveman DNA, which I must say is welcome news: it's so much easier, after all, to change how we treat our children than to alter our DNA. So Elizabeth Edwards is acting perfectly rationally.

In addition to undercutting another of the popular Mars-Venus theories, the new work has another, somber implication. Sexual jealousy is a cause of some of the worst domestic abuse, up to and including murder. If men's jealous rage is not wired into their DNA, there should be ways to defuse it—and not to chalk it up to some inalterable evolutionary legacy.

Sharon Begley is NEWSWEEK's science editor and author of  The Plastic Mind: New Science Reveals Our Extraordinary Potential to Transform Ourselves and  Train Your Mind, Change Your Brain: How a New Science Reveals Our Extraordinary Potential to Transform Ourselves.

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