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.

A terrific 2005 documentary, "The Education of Shelby Knox," tells the story of a teen-ager from a Southern Baptist family in Lubbock, Texas, who has taken a True Love Waits pledge. To the chagrin of her youth pastor, and many of her neighbors, Knox eventually becomes an activist for comprehensive sex education. At her high school, kids receive abstinence-only education, but, Knox says, "maybe twice a week I see a girl walking down the hall pregnant." In the film, Knox seems successful at remaining chaste, but less because she took a pledge than because she has a fearlessly independent mind and the kind of parents who--despite their own conservative leanings--admire her outspokenness. Devout Republicans, her parents end up driving her around town to make speeches that would have curled their hair before their daughter started making them. Her mother even comes to take pride in Shelby's efforts, because while abstinence pledges are lovely in the abstract, they don't acknowledge "reality."

Like other American teens, young evangelicals live in a world of Internet porn, celebrity sex scandals, and raunchy reality TV, and they have the same hormonal urges that their peers have. Yet they come from families and communities in which sexual life is supposed to be forestalled until the first night of a transcendent honeymoon. Regnerus writes, "In such an atmosphere, attitudes about sex may formally remain unchanged (and restrictive) while sexual activity becomes increasingly common. This clash of cultures and norms is felt most poignantly in the so-called Bible Belt." Symbolic commitment to the institution of marriage remains strong there, and politically motivating--hence the drive to outlaw gay marriage--but the actual practice of it is scattershot.

Among blue-state social liberals, commitment to the institution of marriage tends to be unspoken or discreet, but marriage in practice typically works pretty well. Two family-law scholars, Naomi Cahn, of George Washington University, and June Carbone, of the University of Missouri at Kansas City, are writing a book on the subject, and they argue that "red families" and "blue families" are "living different lives, with different moral imperatives." (They emphasize that the Republican-Democrat divide is less important than the higher concentration of "moral-values voters" in red states.) In 2004, the states with the highest divorce rates were Nevada, Arkansas, Wyoming, Idaho, and West Virginia (all red states in the 2004 election); those with the lowest were Illinois, Massachusetts, Iowa, Minnesota, and New Jersey. The highest teen-pregnancy rates were in Nevada, Arizona, Mississippi, New Mexico, and Texas (all red); the lowest were in North Dakota, Vermont, New Hampshire, Minnesota, and Maine (blue except for North Dakota). "The ‘blue states' of the Northeast and Mid-Atlantic have lower teen birthrates, higher use of abortion, and lower percentages of teen births within marriage," Cahn and Carbone observe. They also note that people start families earlier in red states--in part because they are more inclined to deal with an unplanned pregnancy by marrying rather than by seeking an abortion.

Of all variables, the age at marriage may be the pivotal difference between red and blue families. The five states with the lowest median age at marriage are Utah, Oklahoma, Idaho, Arkansas, and Kentucky, all red states, while those with the highest are all blue: Massachusetts, New York, Rhode Island, Connecticut, and New Jersey. The red-state model puts couples at greater risk for divorce; women who marry before their mid-twenties are significantly more likely to divorce than those who marry later. And younger couples are more likely to be contending with two of the biggest stressors on a marriage: financial struggles and the birth of a baby before, or soon after, the wedding.

There are, of course, plenty of exceptions to these rules--messily divorcing professional couples in Boston, high-school sweethearts who stay sweetly together in rural Idaho. Still, Cahn and Carbone conclude, "the paradigmatic red-state couple enters marriage not long after the woman becomes sexually active, has two children by her mid-twenties, and reaches the critical period of marriage at the high point in the life cycle for risk-taking and experimentation. The paradigmatic blue-state couple is more likely to experiment with multiple partners, postpone marriage until after they reach emotional and financial maturity, and have their children (if they have them at all) as their lives are stabilizing."

Some of these differences in sexual behavior come down to class and education. Regnerus and Carbone and Cahn all see a new and distinct "middle-class morality" taking shape among economically and socially advantaged families who are not social conservatives. In Regnerus's survey, the teen-agers who espouse this new morality are tolerant of premarital sex (and of contraception and abortion) but are themselves cautious about pursuing it. Regnerus writes, "They are interested in remaining free from the burden of teenage pregnancy and the sorrows and embarrassments of sexually transmitted diseases. They perceive a bright future for themselves, one with college, advanced degrees, a career, and a family. Simply put, too much seems at stake. Sexual intercourse is not worth the risks." These are the kids who tend to score high on measures of "strategic orientation"--how analytical, methodical, and fact-seeking they are when making decisions. Because these teen-agers see abstinence as unrealistic, they are not opposed in principle to sex before marriage--just careful about it. Accordingly, they might delay intercourse in favor of oral sex, not because they cherish the idea of remaining "technical virgins" but because they assess it as a safer option. "Solidly middle- or upper-middle-class adolescents have considerable socioeconomic and educational expectations, courtesy of their parents and their communities' lifestyles," Regnerus writes. "They are happy with their direction, generally not rebellious, tend to get along with their parents, and have few moral qualms about expressing their nascent sexuality." They might have loved Ellen Page in "Juno," but in real life they'd see having a baby at the wrong time as a tragic derailment of their life plans. For this group, Regnerus says, unprotected sex has become "a moral issue like smoking or driving a car without a seatbelt. It's not just unwise anymore; it's wrong."

Each of these models of sexual behavior has drawbacks--in the blue-state scheme, people may postpone child-bearing to the point where infertility becomes an issue. And delaying child-bearing is better suited to the more affluent, for whom it yields economic benefits, in the form of educational opportunities and career advancement. But Carbone and Cahn argue that the red-state model is clearly failing on its own terms--producing high rates of teen pregnancy, divorce, sexually transmitted disease, and other dysfunctional outcomes that social conservatives say they abhor. In "Forbidden Fruit," Regnerus offers an "unscientific postscript," in which he advises social conservatives that if they really want to maintain their commitment to chastity and to marriage, they'll need to do more to help young couples stay married longer. As the Reverend Rick Marks, a Southern Baptist minister, recently pointed out in a Florida newspaper, "Evangelicals are fighting gay marriage, saying it will break down traditional marriage, when divorce has already broken it down." Conservatives may need to start talking as much about saving marriages as they do about, say, saving oneself for marriage.

"Having to wait until age twenty-five or thirty to have sex is unreasonable," Regnerus writes. He argues that religious organizations that advocate chastity should "work more creatively to support younger marriages. This is not the 1950s (for which I am glad), where one could bank on social norms, extended (and larger) families, and clear gender roles to negotiate and sustain early family formation."

Evangelicals could start, perhaps, by trying to untangle the contradictory portrayals of sex that they offer to teen-agers. In the Shelby Knox documentary, a youth pastor, addressing an assembly of teens, defines intercourse as "what two dogs do out on the street corner--they just bump and grind awhile, boom boom boom." Yet a typical evangelical text aimed at young people, "Every Young Woman's Battle," by Shannon Ethridge and Stephen Arterburn, portrays sex between two virgins as an ethereal communion of innocent souls: "physical, mental, emotional, and spiritual pleasure beyond description." Neither is the most realistic or helpful view for a young person to take into marriage, as a few advocates of abstinence acknowledge. The savvy young Christian writer Lauren Winner, in her book "Real Sex: The Naked Truth About Chastity," writes, "Rather than spending our unmarried years stewarding and disciplining our desires, we have become ashamed of them. We persuade ourselves that the desires themselves are horrible. This can have real consequences if we do get married." Teenagers and single adults are "told over and over not to have sex, but no one ever encourages" them "to be bodily or sensual in some appropriate way"--getting to know and appreciate what their bodies can do through sports, especially for girls, or even thinking sensually about something like food. Winner goes on, "This doesn't mean, of course, that if only the church sponsored more softball leagues, everyone would stay on the chaste straight and narrow. But it does mean that the church ought to cultivate ways of teaching Christians to live in their bodies well--so that unmarried folks can still be bodily people, even though they're not having sex, and so that married people can give themselves to sex freely."

Too often, though, evangelical literature directed at teen-agers forbids all forms of sexual behavior, even masturbation. "Every Young Woman's Battle," for example, tells teen-agers that "the momentary relief" of "self-gratification" can lead to "shame, low self-esteem, and fear of what others might think or that something is wrong with you." And it won't slake sexual desire: "Once you begin feeding baby monsters, their appetites grow bigger and they want MORE! It's better not to feed such a monster in the first place."

Shelby Knox, who spoke at a congressional hearing on sex education earlier this year, occupies a middle ground. She testified that it's possible to "believe in abstinence in a religious sense," but still understand that abstinence-only education is dangerous "for students who simply are not abstaining." As Knox's approach makes clear, you don't need to break out the sex toys to teach sex ed--you can encourage teen-agers to postpone sex for all kinds of practical, emotional, and moral reasons. A new "abstinence-plus" curriculum, now growing in popularity, urges abstinence while providing accurate information about contraception and reproduction for those who have sex anyway. "Abstinence works," Knox said at the hearing. "Abstinence-only-until-marriage does not."

It might help, too, not to present virginity as the cornerstone of a virtuous life. In certain evangelical circles, the concept is so emphasized that a girl who regrets having been sexually active is encouraged to declare herself a "secondary" or "born-again" virgin. That's not an idea, surely, that helps teen-agers postpone sex or have it responsibly.

The "pro-family" efforts of social conservatives--the campaigns against gay marriage and abortion--do nothing to instill the emotional discipline or the psychological smarts that forsaking all others often involves. Evangelicals are very good at articulating their sexual ideals, but they have little practical advice for their young followers. Social liberals, meanwhile, are not very good at articulating values on marriage and teen sexuality--indeed, they may feel that it's unseemly or judgmental to do so. But in fact the new middle-class morality is squarely pro-family. Maybe these choices weren't originally about values--maybe they were about maximizing education and careers--yet the result is a more stable family system. Not only do couples who marry later stay married longer; children born to older couples fare better on a variety of measures, including educational attainment, regardless of their parents' economic circumstances. The new middle-class culture of intensive parenting has ridiculous aspects, but it's pretty successful at turning out productive, emotionally resilient young adults. And its intensity may be one reason that teen-agers from close families see child-rearing as a project for which they're not yet ready. For too long, the conventional wisdom has been that social conservatives are the upholders of family values, whereas liberals are the proponents of a polymorphous selfishness. This isn't true, and, every once in a while, liberals might point that out.

Some evangelical Christians are starting to reckon with the failings of the preaching-and-pledging approach. In "The Education of Shelby Knox," for example, Shelby's father is uncomfortable, at first, with his daughter's campaign. Lubbock, after all, is a town so conservative that its local youth pastor tells Shelby, "You ask me sometimes why I look at you a little funny. It's because I hear you speak and I hear tolerance." But as her father listens to her arguments he realizes that the no-tolerance ethic simply hasn't worked in their deeply Christian community. Too many girls in town are having sex, and having babies that they can't support. As Shelby's father declares toward the end of the film, teen-age pregnancy "is a problem--a major, major problem that everybody's just shoving under the rug."