So if not birth control and not lack of access, what does explain it?

This chart offers a clue:

Births to Unmarried Women, 1940-2007

CDC/NCHS, National Vital Statistics System

When marriage was the near-universal norm in American society, a pregnancy out of wedlock pushed a couple toward one of four choices: shotgun wedding; adoption; abortion; or single motherhood, in that order of social acceptability. The result was a society in which both abortion and single motherhood were rare.

In the decade after 1965, both women and men claimed greater sexual autonomy for themselves. The shotgun marriage seemed an increasingly outrageous imposition to meet increasingly irrelevant social expectations. After 1970, adoption of native-born American children by non-related parents rapidly dwindled. Yet outright single motherhood remained comparatively unusual for middle-class Americans, and especially for white middle-class Americans.

The abortion spike between 1975 and 1990 reflected a new ranking of acceptable responses to an unmarried pregnancy: abortion, single parenthood, shotgun wedding, and adoption, in that order.

Since 1990, two other things seem to have changed. First, the pro-life movement really does seem to have changed American minds about the morality of abortion. Only about one-fifth of Americans wish to see abortion outlawed—a proportion that has remained steady since the mid-1970s. But the proportion that thinks abortion is wrong has edged up over the past 15 years: Only 38 percent of Americans now describe abortion as “morally acceptable.”

Meanwhile, marriage has receded further from the cultural experience of the less affluent two-thirds of American society. As the wages of non-college-educated men have tumbled, marriage has looked like an increasingly pointless and even dangerous choice for poorer women. As marriage fades, unwed motherhood has evolved from an acceptable outcome to something close to an inevitability. The order of choices in the face of an unexpected pregnancy has thus shifted again: single parenthood, abortion, shotgun wedding, and adoption.

Women who already have one or two children outside marriage may continue to choose abortion as a way to avoid a third or fourth. As the Guttmacher Institute notes, 61 percent of women who have abortions are already mothers. But the urgency of having an abortion to terminate a first pregnancy has clearly faded, as single parenthood has become the norm for non-affluent Americans of all races.

This is the fascinating irony of the pro-life movement. The cause originated as a profoundly socially conservative movement. Yet as it grew, it became less sectarian. Women came to the fore as leaders. It found a new language of concern and compassion, rather than condemnation and control. Most radically and decisively, the movement made its peace with unwed parenthood as the inescapable real-world alternative to abortion.