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 blacks, were rare in almost every region and community.
That was a long time ago, and the intact two-parent family has been in eclipse for decades now.
The Pew Research Center recently 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.
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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 Latino alike), intact families are 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.
When it comes to drawing lessons from this story, though, the agreement between liberals and conservatives ends.
The right tends to emphasize what's been lost, arguing that most Americans -- especially the poor and working-class -- would benefit from a stronger link between sex, marriage and procreation.
The left argues that the revolution just hasn't been completed yet: It's the right-wing backlash against abortion, contraception and sex education that's preventing downscale Americans from attaining the new upper-middle-class stability, and reaping its social and economic benefits.
This is one of the themes of "Red Families v. Blue Families," a provocative new book by two law professors, Naomi Cahn and June Carbone. The authors depict a culturally conservative "red America" that's stuck trying to sustain an outdated social model.
By insisting on chastity before marriage, Cahn and Carbone argue, social conservatives guarantee that their children will get pregnant early and often (see Palin, Bristol), leading to teen childbirth, shotgun marriages and high divorce rates.
This self-defeating cycle could explain why socially conservative states have more family instability than, say, the culturally liberal Northeast.
To Cahn and Carbone's credit, their book is nuanced enough to complicate this liberal-friendly thesis.
They acknowledge, for instance, that there are actually multiple "red family" models, from the Mormon west to the Sun Belt suburbs to the rural South.
More important, Cahn and Carbone also acknowledge one of the more polarizing aspects of the "blue family" model. Liberal states have many more abortions.
Liberals sometimes argue that their preferred approach to family life reduces the need for abortion. But in reality, it may depend on abortion to succeed.
The teen pregnancy rate in blue Connecticut, for instance, is roughly identical to the teen pregnancy rate in red Montana. But in Connecticut, those pregnancies are half as likely to be carried to term. Overall, the abortion rate is twice as high in New York as in Texas and three times as high in Massachusetts as in Utah.
So it isn't just contraception that delays childbearing in liberal states, and it isn't just a devotion to abstinence education that leads to teen births and hasty marriages in conservative America.
It's also a matter of how plausible an option abortion seems, both morally and practically, depending on who and where you are. Whether it's attainable for most Americans or not, the "blue family" model clearly works: It leads to marital success and material prosperity, and it's well suited to our mobile, globalized society.
By comparison, the "red family" model can look dysfunctional -- an uneasy mix of rigor and permissiveness, whose ideals don't always match up with the facts of contemporary life. But it reflects something else as well: an attempt to navigate post-sexual revolution America without relying on abortion.
Douthat is a columnist for The New York Times.
THE NEW YORK TIMES