Why is marriage thriving among (and only among) the affluent? Experts struggle to explain one of the biggest drivers of inequality.

• Author: Kim, Anne

In 1950, the typical man married for the first time at about age twenty-three, while women married at a tender median of twenty. In this heyday of America's Leave It to Beaver period, few children were born outside marriage==just 7.9 percent of births were "premarital" in 1950, according to census data--and divorce was equally rare.

Today, however, marriage in America seems to be dying.

In 2010, according to the Pew Research Center, only about half of all Americans over age eighteen were married, compared to nearly three out of four in 1960. Americans today are marrying later, if at all, and the share of Americans who've never married has climbed to record highs. As one result, the share of children growing up with single moms is also skyrocketing; in 2013, 41 percent of all births were to unmarried women.

But the seeming decline of marriage includes one major caveat: educated elites. When it comes to marriage, divorce, and single motherhood, the 1950s never ended for college-educated Americans, and for college-educated women in particular. According to the researchers Shelly Lundberg, of the University of California, Santa Barbara, and Robert Poliak, of Washington University in St. Louis, the share of young college-graduate white women who were married in 2010 was a little over 70 percent-- almost exactly the same as it was in 1950. College-educated white women are, moreover, half as likely as other women to be divorced, according to Steven Martin of the University of Maryland, and they are also refusing single motherhood. Fewer than 9 percent of women with a bachelor's degree or more had an unwed birth in 2011--a level barely higher than what it was for all women in 1950.

It's also seemingly only Americans with four-year degrees or better who appear immune to the broader cultural and social forces eroding marriage. In 1950, white women with "some college," such as an associate's degree, were actually more likely to be married than their better-educated sisters. Today, it's the opposite. Though women with a high school diploma or less have seen the sharpest drop in marriage rates, the decline has been almost as severe-- and ongoing--for women just one short rung down the education ladder, regardless of race.

The endurance of marriage among elites--and, it seems, elites alone--is important not just as a cultural anomaly. As the class divide in marriage grows, elites are compounding the advantages of their status, especially for their kids. Since the release of Daniel Patrick Moynihan's now-famed report on the breakdown of black families in 1965, researchers have amassed a growing mountain of evidence that family structure and marriage matter. Compared to children living with single parents, or even with parents who are cohabiting, kids raised in...