(Photo: Bill Davenport/STOCK.XCHNG)

Whether sex education focuses only on abstinence or teaches students about contraception and other topics as well, it all shares one main message: Wait. In abstinence-only, students are exhorted to wait for sex until they're married. In "comprehensive" or "abstinence-plus," the idea is to delay sexual relations until . . . later.

"The underlying assumption is that delay reduces sexual risk-taking"—and with it unwanted pregnancies and sexually transmitted diseases, says University of South Florida psychologist Marina A. Bornovalova. "If they just wait, then they'll be less likely to have multiple partners or get pregnant early."

"But until now, no one had tested this assumption."

Bornovalova and her colleagues—Brooke M. Huibregtse, Matt McGue, and William Iacono of University of Minnesota and Brian Hicks of the University of Michigan—tested it. Their finding, published in an upcoming issue of Psychological Science, a journal of the Association for Psychological Science, should spark serious rethinking wherever sex educators are seeking the facts as their guide.

Yes, there's a correlation between early sexual initiation (this study defined this as 16 or younger) and later sexual risk-taking. But, as a causal factor for sexual risk-taking—multiple partners, drug and alcohol use during sexual encounters, or unprotected intercourse—"it doesn't really matter whether you delay sex or not."

The researchers looked at more than 1,000 pairs of identical and fraternal twins enrolled in the longitudinal Minnesota Twin Family Study (MTFS). These twins, aged 11, upon the time of enrollment, were questioned on biological, social, and psychological factors, from parental drug use to age of puberty to friendliness. Then, at age 24, they were asked about the risks they were taking in their sex lives. In some pairs, one twin had early sex and the other didn't – and the two twins were compared on their sexual risk-taking in adulthood.

Numerous runs of the data led to the same conclusion: "You take two twins who share 100 percent of their genes. One has sex at 15 and one at 20. You compare them on risk-taking at 24—and they don't differ."

So why does someone end up sexually promiscuous? The researchers think it's a combination of genetic factors—such as the strong inherited tendency to be impulsive or anti-social – and environmental ones, such as poverty or troubled family life.

Most important, though – biology and life experience both give rise to early sexual initiation and risk-taking later on. The former does not cause the latter. The psychologists aren't advocating sex at a very early age; it very well might have other harmful effects on a teenager, such as depression or poor school performance. "But if our goal is to reduce sexual risk-taking, we need to be focusing on something else," says Bornovalova. More study is needed to zero in on what that something else is. But for now, one thing should be clear to the people writing sex ed curricula: "Whatever is causing sexual risk-taking, it is not early sexual initiation."

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Association for Psychological Science: http://www.psychologicalscience.org