Some 60 years ago, Alfred Kinsey delivered a shock to midcentury sexual sensibilities when he reported that at some point in their marriages, half of the men and a quarter of the women in the U.S. had an extramarital affair. No one puts much stock in Dr. Kinsey’s high numbers any more—his sampling methods suffered from a raging case of selection bias—but his results fit the long-standing assumption that men are much more likely to cheat than women.

Lately, however, researchers have been raising doubts about this view: They believe that the incidence of unfaithfulness among wives may be approaching that of husbands. The lasting costs of these betrayals will be familiar to the many Americans who have experienced divorce as spouses or children.

Among the most reliable studies on this issue is the General Social Survey, sponsored by the National Science Foundation, which has been asking Americans the same questions since 1972. In the 2010 survey, 19% of men said that they had been unfaithful at some point during their marriages, down from 21% in 1991. Women who reported having an affair increased from 11% in 1991 to 14% in 2010.

A 2011 study conducted by Indiana University, the Kinsey Institute and the University of Guelph found much less of a divide: 23% for men and 19% for women. Such numbers suggest the disappearance of the infidelity gender gap, but some caution is in order.

An enduring problem for researchers—even those who sample with meticulous care—is that any such survey is asking for confessions from people who are presumably lying to their spouses. Researchers generally believe that actual infidelity numbers are higher than the results indicate.

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