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...