As the authors of the forthcoming book "Red Families v. Blue Families," we often give talks about the recent rise in what's called the "nonmarital birthrate." More than 40 percent of children are now born to women who aren't married. Sometimes someone will come up to us and say something like: "When my daughter got pregnant and decided to keep the child, we were OK with that because we are Christians. When she decided not to marry the father, we were relieved because we knew the marriage would never work."

They express these two beliefs -- that they are Christian and thus uncomfortable with abortion and that they are relieved their daughter decided to raise the child alone -- as if they are not connected. But in fact this may be one of the stranger, more unexpected legacies of the pro-life movement that has arisen in the 40 years since Roe v. Wade: In conservative communities, the hardening of anti-abortion attitudes may have increased the acceptance of single-parent families.

Researchers have considered many reasons for the rise in the nonmarital birthrate -- the welfare state, the decline of morals, the increasing independence of women, even gay marriage. But one that people on neither the left nor the right talk about much is how it's connected to abortion.

The working class had long dealt with the inconvenient fact of an accidental pregnancy through the shotgun marriage. As blue-collar jobs paying a family wage have disappeared, however, so has early marriage. Women are left with two choices: They can delay childbearing (which might entail getting an abortion at some point) until the right man comes along, or get more comfortable with the idea of becoming single mothers.

College-educated elites have endorsed the first option, but everyone else is drifting toward the second. In regions and social classes where the stigma of having an abortion is high, the nonmarital birthrate is also high. This is a legacy the pro-life movement has not really grappled with.

Blue states, despite their relatively progressive politics, have lower divorce and teen birthrates than red states. In fact, the college-educated middle class -- partly by postponing having children -- has managed to better embody the traditional ideal: a greater percentage of children being raised in two-parent families.

Although college-educated women have a relatively low number of abortions, a higher percentage of their unplanned pregnancies end in abortion than for any other group. The college-educated almost certainly think of themselves as resorting to abortion only in the relatively rare cases where contraception fails. Yet the bottom line is that the willingness to abort -- however infrequently it occurs -- makes it possible to reinforce the norm against having a child outside of marriage.

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The Christian right preaches that contraception is not perfect, sex inevitably risks pregnancy, and abstinence provides the only solution. Indeed, as the number of abortions has dropped, the rate of unmarried women giving birth has increased. These young women often give their opposition to abortion as an explanation for why they went ahead and had the child, even if in other ways religion has not influenced them much. If abortion is not an option, then more single-parent births are pretty inevitable.

Think of it as the Bristol Palin effect. Sarah Palin's daughter, 17 at the time, announced her pregnancy shortly after her mother's selection in 2008 as the Republican vice-presidential candidate. Republican women applauded the Palins' choice to support their daughter's decision to have the child. They wrote that unlike other Republican leaders, the Palins were sticking to their values rather than doing what others had done and quietly arranging an abortion. Democratic women were appalled -- mystified why anyone thought having a 17-year-old raise a child was a good idea. Liberal and conservative women did agree on one thing, however: Neither group thought there was any point in having Bristol marry Levi Johnston, the father of the child.

Naomi Cahn is a law professor at George Washington University. June Carbone is a law professor at the University of Missouri-Kansas City. They are authors of "Red Families v. Blue Families and Family Classes." This is from Slate.