For a long time, the contours of America’s culture war seemed relatively straightforward. On one side was the country’s growing educated class, who tended to be secular, permissive and favorably disposed to the sexual revolution. On the other side were the social conservatives of middle America — benighted yahoos or virtuous yeomen, depending on your point of view, but either way a less-educated and more pious demographic, with more traditional attitudes on sexuality and family.

Decades of punditry, pop sociology and prejudice have been premised on this neat division — from the religious right’s Reagan-era claim to be a “Moral Majority” oppressed by a secular elite, to Barack Obama’s unfortunate description of heartland America “clinging” to religion. Like any binary, it oversimplified a complicated picture. But as a beginner’s guide to the culture war, the vision of white-collar social liberals and blue-collar cultural conservatives was, for a substantial period, more accurate than not.

That may no longer be the case. This week, the National Marriage Project is releasing a study charting the decline of the two-parent family among what it calls the “moderately educated middle” — the 58 percent of Americans with high school diplomas and often some college education, but no four-year degree.

This decline is depressing, but it isn’t surprising. We’ve known for a while that America has a marriage gap: college graduates divorce infrequently and bear few children out of wedlock, while in the rest of the country unwed parenthood and family breakdown are becoming a new normal. This gap has been one of the paradoxes of the culture war: highly educated Americans live like Ozzie and Harriet despite being cultural liberals, while middle America hews to traditional values but has trouble living up to them.