All of this has made marriage rarer, something that not everyone needs--or wants. Yet, paradoxically, it has become all the more important. At a time when two incomes are often necessary for a middle-class lifestyle, marriage can matter a lot. Years ago, most men, even those with only a high school degree, could settle down, buy a home, support a family, and lead some semblance of a middle-class life. Such economic security is gone for lower-income, less-educated, or working-class Americans, especially if they lack a college education. It's tough for any single person, male or female, regardless of educational attainment, to be assured of supporting a family.

"If marriage goes well now, it can help people out economically," said Stephanie Coontz, a professor of history and family studies at Evergreen State College. "But it's also a riskier investment. Are you going to get saddled with a guy who can't hold down a job?"

Even as fewer people marry overall, a demographic divergence has emerged. Increasingly, marriage is more common among college graduates than among Americans with less education. Roughly 69 percent of adults who finished college are married, according to Pew, compared with 56 percent of those without a college degree.

For educated women, the prospects for marriage have improved considerably. They now marry with greater frequency, while feeling less pressure to conform to a 1950s domestic ideal. In 1960, according to economists Betsey Stevenson and Justin Wolfers, 29 percent of college-educated women in their sixties had never married; now, only 8 percent of them stay single. Well-educated women tend to marry later than others do, but they stay married longer and report higher levels of marital satisfaction.

College grads may marry later than the rest of the population, yet more of them wind up settling down--and with one another. Increasingly, Americans are choosing spouses like themselves. In 1970, according to the Pew Center's data, only 37 percent of married, college-educated men had a wife with a bachelor's degree; by 2007, 71 percent of them did. A middle-class fellow with a typical salary and a 401(k) plan seeks out a woman with a similar economic profile. Same goes for the working-class or poorer couples. Gone are the days when the Harvard grad marries the girl with the high school degree simply because, well, she's pretty.

Marriage, as a result, now offers fewer people a boost up the economic ladder. Stop and think what this means for the growing inequality in Americans' incomes over the next decade or more. If well-educated people with good jobs marry one another, they'll have a better shot at saving money and accumulating wealth. Less-educated, lower-income couples may stick together, but their lack of schooling means they're both more likely to struggle to find work, and they'll have sparser resources to fall back on if one of them loses a job.