Researchers at Ohio State University garnered little attention in February when they found that youngsters who lose their virginity earlier than their peers are more likely to become juvenile delinquents. So obvious and well established was that cause-and-effect connection that it was already part of the required curriculum for federal "abstinence only" programs.

There was just one problem: It is probably not true. Other things being equal, a more probing study has found, youngsters who have consensual sex in their early-teen or even pre-teen years are if anything less likely to engage in delinquent behavior later on.

That new analysis, a reworking of the same data the Ohio team used, is one of several recent instances in which a more-precise parsing of data has begun to turn long-standing societal presumptions on their head. By bringing evidence to bear on complex social issues, these studies are forcing individuals and policymakers to rethink such hot-button topics as the benefits of breast-feeding, the risks of teen child-bearing and, in the latest example, the harms long presumed to result from teen sex.

Like many of the newer studies, the latest one - led by Paige Harden, a doctoral candidate in psychology at the University of Virginia in Charlottesville - uses the powerful techniques of behavioral genetics. The field specializes in studies on twins, which can help tell whether behavioral traits are the result of genes or the social environment and which have periodically stirred controversy when they focused on the genetic underpinnings of criminality and intelligence.

But the specialty's analytic methods can also help tell whether one behavior, such as early sex, is merely correlated with or actually causes a second behavior that is often found with it, such as delinquency. If two behaviors often exist in the same people but are found not to be connected by cause and effect, then a third factor is likely to be causing both.

That kind of finding can help identify better targets for prevention efforts, experts say.

"Behavioral geneticists have long sought to establish causal links between genes and complex behaviors. So it's fascinating to see them use the tools of their trade to dispute widely held beliefs" about the social roots of some of those behaviors, said Erik Parens, a senior research scholar who has tracked the field intensively at the Hastings Center, a Garrison, N.Y., science and ethics think tank.

The latest example started when Dana Haynie, a professor of sociology at Ohio State, and her then-graduate student, Stacey Armour, published a study in February in the Journal of Youth and Adolescence. They analyzed data collected from more than 7,000 children as part of the National Longitudinal Study of Adolescent Health, a federally funded survey that in 1994 began gathering information about the health-related behavior of U.S. school children who were then in grades 7 through 12.

Haynie and Armour divided the children into three groups: Those who had sex for the first time when they were either younger, about the same age, or older than the age at which most of their local peers lost their virginity. They also compiled information on graffiti painting, shoplifting, drug selling and other "problem behaviors" by those youngsters in later years.

Their conclusion: One year after losing their virginity, children in the "early" category were 20 percent more likely than those who started sex at the average age to engage in delinquent behavior, even when several other relevant factors such as wealth, race, parental involvement and physical development were taken into account.

Those findings supported the widely held notion that loss of virginity at a relatively young age appears to, as Haynie and Armour wrote, "open the doorway to problem behaviors."

Harden didn't believe it.

Looked at from a similarly high altitude, people might conclude that red meat is a health food, she said, because people live longer in countries where more is eaten. Only when the issue is studied within one country does red meat's link to chronic diseases appear.

Suspecting such an error in the Haynie study, Harden and three colleagues, including her adviser Eric Turkheimer, an expert in behavioral genetics, studied more than 500 pairs of twins in the same national survey analyzed by the Ohio team. Because twin pairs share similar or identical genetic inheritances (depending on whether they are fraternal or identical) and the same home environment, they are useful for seeing through false cause-and-effect relationships.

The team looked at identical twin pairs in which one twin initiated sex younger than the other, then tallied subsequent problem behaviors. If sex really adds to the chances of delinquency, then early sex teens should end up delinquent more than their later-sex twins.

"It turns out that there was no positive relationship between age of first sex and delinquency," Harden said.

The way to reconcile that with the previous evidence of a link is to conclude that some other factors are promoting both early sex and delinquency, she said. In an e-mail, Haynie agreed. And the Virginia study offers some clues.

It found that identical twins, who have the same DNA, were more similar to one another in the ages at which they lost their virginity than were fraternal twins, whose DNA patterns are only 50 percent the same - an indication that genes influence the age at which a person will first have sex. Other twin studies have found the same pattern for delinquency.

Together, those findings suggest that some genes - perhaps, for example, those that increase impulsivity and risk-taking - may underlie both behaviors.

"You need to have some appetite for risk-taking to be a delinquent. And the same if you're 13 and going to have sex for the first time," Harden said.

Efforts to prevent delinquency can hardly take aim at people's genes. But the Virginia study also indicates that social factors - as yet unidentified but perhaps involving relationships with family and friends - have an even bigger impact than genes on whether a child will become delinquent. Those are the things that should be identified and targeted by delinquency prevention programs, said Jeanne Brooks-Gunn, co-director of Columbia University's National Center for Children and Families.

Perhaps most surprising, the Virginia study found that adolescents who had sex at younger ages were less likely to end up delinquent than those who lost their virginity later. Many factors play into a person's readiness for sex, but in at least some cases sexual relationships may offer an alternative to trouble, the researchers say.

Even then, there are emotional and physical risks. Young adolescents, in particular, are less likely to use condoms and so are vulnerable to sexually transmitted diseases and pregnancy.

But those are risks that other nations have mitigated with education, Harden and Turkheimer said, while U.S. educators wanting a piece of the nation's $200 million "abstinence only" budget must adhere to a curriculum that links sex to delinquency and explicitly precludes discussion of contraception.

The new study "really calls into question the usefulness of abstinence education for preventing behavior problems," Harden said, "and questions the bigger underlying assumption that all adolescent sex is always bad."