In early September, when Sarah Palin, the Republican candidate for Vice-President, announced that her unwed seventeen-year-old daughter, Bristol, was pregnant, many liberals were shocked, not by the revelation but by the reaction to it. They expected the news to dismay the evangelical voters that John McCain was courting with his choice of Palin. Yet reports from the floor of the Republican Convention, in St. Paul, quoted dozens of delegates who seemed unfazed, or even buoyed, by the news. A delegate from Louisiana told CBS News, “Like so many other American families who are in the same situation, I think it’s great that she instilled in her daughter the values to have the child and not to sneak off someplace and have an abortion.” A Mississippi delegate claimed that “even though young children are making that decision to become pregnant, they’ve also decided to take responsibility for their actions and decided to follow up with that and get married and raise this child.” Palin’s family drama, delegates said, was similar to the experience of many socially conservative Christian families. As Marlys Popma, the head of evangelical outreach for the McCain campaign, told National Review, “There hasn’t been one evangelical family that hasn’t gone through some sort of situation.” In fact, it was Popma’s own “crisis pregnancy” that had brought her into the movement in the first place.

During the campaign, the media has largely respected calls to treat Bristol Palin’s pregnancy as a private matter. But the reactions to it have exposed a cultural rift that mirrors America’s dominant political divide. Social liberals in the country’s “blue states” tend to support sex education and are not particularly troubled by the idea that many teen-agers have sex before marriage, but would regard a teen-age daughter’s pregnancy as devastating news. And the social conservatives in “red states” generally advocate abstinence-only education and denounce sex before marriage, but are relatively unruffled if a teen-ager becomes pregnant, as long as she doesn’t choose to have an abortion.

A handful of social scientists and family-law scholars have recently begun looking closely at this split. Last year, Mark Regnerus, a sociologist at the University of Texas at Austin, published a startling book called “Forbidden Fruit: Sex and Religion in the Lives of American Teenagers,” and he is working on a follow-up that includes a section titled “Red Sex, Blue Sex.” His findings are drawn from a national survey that Regnerus and his colleagues conducted of some thirty-four hundred thirteen-to-seventeen-year-olds, and from a comprehensive government study of adolescent health known as Add Health. Regnerus argues that religion is a good indicator of attitudes toward sex, but a poor one of sexual behavior, and that this gap is especially wide among teen-agers who identify themselves as evangelical. The vast majority of white evangelical adolescents—seventy-four per cent—say that they believe in abstaining from sex before marriage. (Only half of mainline Protestants, and a quarter of Jews, say that they believe in abstinence.) Moreover, among the major religious groups, evangelical virgins are the least likely to anticipate that sex will be pleasurable, and the most likely to believe that having sex will cause their partners to lose respect for them. (Jews most often cite pleasure as a reason to have sex, and say that an unplanned pregnancy would be an embarrassment.) But, according to Add Health data, evangelical teen-agers are more sexually active than Mormons, mainline Protestants, and Jews. On average, white evangelical Protestants make their “sexual début”—to use the festive term of social-science researchers—shortly after turning sixteen. Among major religious groups, only black Protestants begin having sex earlier.

Another key difference in behavior, Regnerus reports, is that evangelical Protestant teen-agers are significantly less likely than other groups to use contraception. This could be because evangelicals are also among the most likely to believe that using contraception will send the message that they are looking for sex. It could also be because many evangelicals are steeped in the abstinence movement’s warnings that condoms won’t actually protect them from pregnancy or venereal disease. More provocatively, Regnerus found that only half of sexually active teen-agers who say that they seek guidance from God or the Scriptures when making a tough decision report using contraception every time. By contrast, sixty-nine per cent of sexually active youth who say that they most often follow the counsel of a parent or another trusted adult consistently use protection.

The gulf between sexual belief and sexual behavior becomes apparent, too, when you look at the outcomes of abstinence-pledge movements. Nationwide, according to a 2001 estimate, some two and a half million people have taken a pledge to remain celibate until marriage. Usually, they do so under the auspices of movements such as True Love Waits or the Silver Ring Thing. Sometimes, they make their vows at big rallies featuring Christian pop stars and laser light shows, or at purity balls, where girls in frothy dresses exchange rings with their fathers, who vow to help them remain virgins until the day they marry. More than half of those who take such pledges—which, unlike abstinence-only classes in public schools, are explicitly Christian—end up having sex before marriage, and not usually with their future spouse. The movement is not the complete washout its critics portray it as: pledgers delay sex eighteen months longer than non-pledgers, and have fewer partners. Yet, according to the sociologists Peter Bearman, of Columbia University, and Hannah Brückner, of Yale, communities with high rates of pledging also have high rates of S.T.D.s. This could be because more teens pledge in communities where they perceive more danger from sex (in which case the pledge is doing some good); or it could be because fewer people in these communities use condoms when they break the pledge.

Bearman and Brückner have also identified a peculiar dilemma: in some schools, if too many teens pledge, the effort basically collapses. Pledgers apparently gather strength from the sense that they are an embattled minority; once their numbers exceed thirty per cent, and proclaimed chastity becomes the norm, that special identity is lost. With such a fragile formula, it’s hard to imagine how educators can ever get it right: once the self-proclaimed virgin clique hits the thirty-one-per-cent mark, suddenly it’s Sodom and Gomorrah.

Religious belief apparently does make a potent difference in behavior for one group of evangelical teen-agers: those who score highest on measures of religiosity—such as how often they go to church, or how often they pray at home. But many Americans who identify themselves as evangelicals, and who hold socially conservative beliefs, aren’t deeply observant.

Even more important than religious conviction, Regnerus argues, is how “embedded” a teen-ager is in a network of friends, family, and institutions that reinforce his or her goal of delaying sex, and that offer a plausible alternative to America’s sexed-up consumer culture. A church, of course, isn’t the only way to provide a cohesive sense of community. Close-knit families make a difference. Teen-agers who live with both biological parents are more likely to be virgins than those who do not. And adolescents who say that their families understand them, pay attention to their concerns, and have fun with them are more likely to delay intercourse, regardless of religiosity.