Fifty years ago, American family structures were remarkably uniform. The rich married at roughly the same rate as the poor and middle class. Divorce rates were low for the college educated and high school graduates alike. Out-of-wedlock births, while more common among African-Americans, were rare in almost every region and community.

That was a long time ago. The intact two-parent family has been in eclipse for decades now: last week, the Pew Research Center reported that in 2008, 41 percent of American births occurred outside of marriage, the highest figure yet recorded. And from divorce rates to teen births, nearly every indicator of family life now varies dramatically by education, race, geography and income.

In a rare convergence, conservatives and liberals basically agree on how this happened. First, the sexual revolution overturned the old order of single-earner households, early marriages, and strong stigmas against divorce and unwed motherhood. In its aftermath, the professional classes found a new equilibrium. Today, couples with college and (especially) graduate degrees tend to cohabit early and marry late, delaying childbirth and raising smaller families than their parents, while enjoying low divorce rates and bearing relatively few children out of wedlock.

For the rest of the country, this comfortable equilibrium remains out of reach. In the underclass (black, white and Hispanic alike), intact families are now an endangered species. For middle America, the ideal of the two-parent family endures, but the reality is much more chaotic: early marriages coexist with frequent divorces, and the out-of-wedlock birth rate keeps inching upward.