Men have always been more likely than women to cheat, or at least to report having done so, but researchers have noticed a shift in recent years: 16 percent of adults—about 20 percent of men and 13 percent of women—report that they’ve had sex with someone other than their spouse while married. But among adults under 30 who have ever been married, 11 percent of women report having committed infidelity, as opposed to 10 percent of men.