“Why is there so much cheating? All of the scholars I spoke with point to the higher median age at which young people get married as the most likely explanation. Since 1950, the age of first marriage has risen to 25 from 20 for women and to 27 from 22 for men. “It’s more common for people to be hooking up or having relationships with multiple partners” before marriage, says Prof. Laumann.

Even young people who engage in monogamous relationships before marriage may be hurting their prospects for a faithful married life. The habits they form in those premarital relationships are likely to affect their marriages, according to Barry McCarthy, a psychologist and professor at American University in Washington. “The most common way that dating couples end a relationship is by starting another” — that is, by cheating on their current partner. Moreover, once people have gone through a couple of breakups of long-term relationships, they may not be as worried about what will happen at the end of their marriage. “The costs of exiting have changed,” says Mr. Laumann.”