Lifestyle - A snapshot of Teen sex;US

Kids complain that there's nothing to do in the Midwestern town scientists are calling "Jefferson City." For fun, teenagers drive to the outskirts of this largely white, working-class community and get wasted. Another favorite activity? Sex. A little more than half the 1,000 students in the only high school are sexually active; the average age of initiation: 15 1/2.

Shocked? Actually, it makes Jefferson's kids typical American teens. But in one way the town is highly unusual: it was the site of a unique study in which researchers tried to document every romantic and sexual liaison among high school students over an 18-month period. The purpose of the research--part of the huge National Longitudinal Study of Adolescent Health--was to learn how sexually transmitted diseases (STDs) travel through teen populations. But what is most remarkable about the study, published recently in the American Journal of Sociology, is the accompanying chart-- the first to map the sexual geography of a U.S. high school.

The map took researchers by surprise. Overall, 573 out of 832 surveyed students reported at least one relationship during the previous 18 months. The majority probably involved an "exchange of fluids," say the authors. There were 63 couples who had no outside partners, but an astonishing 288 students were linked together in an elaborate network of liaisons. Many students had just one or two romances, but they were at risk of contracting STDs from everyone in the chain. This, wrote the authors, is "the worst-case scenario for potential disease diffusion."

Adult sexual networks look very different and usually involve clusters of wanton individuals known to public-health experts as "core transmitters." (Think prostitutes, NBA stars.) Another surprise was the absence of tightly closed loops in which a foursome trades partners--what co-author Peter Bearman calls the Bob & Carol & Ted & Alice phenomenon, after the 1969 film. Teenagers seem to shy from such post-breakup swaps. Bearman, who heads the sociology department at Columbia University, suggests that dating the former boyfriend of your ex-boyfriend's new girlfriend may involve a loss of status or cross a line of loyalty. "It's an incest taboo of sorts," suggests co-author James Moody, an Ohio State sociologist. The behavior is a big factor in creating the long chains that spread germs.

Though girls tended to date older boys, the study found few behavioral differences between the sexes. There are promiscuous boys who prey on less experienced girls, says Bearman, "and girls who are predators of boys." Most relationships were "romantic"; only about 5% were sex-only "hookups."

Just 1% of the relationships were homosexual; nationally, says Moody, the figure is about 2.5% for teens, whose sexual identities are still emerging. The data were collected in the kids' homes back in 1995 using a secure, computerized survey. Says Bearman: "There was no incentive to lie."

The study shows how easily STDs could spread in a high school, but paradoxically, it also indicates how easily the chains of contagion could be snapped. "If you could get person A or B to change his or her behavior--through abstinence, using a condom, or getting treated for an STD--then you could prevent transmission from B to C and down the network," says Kathleen Ethier, of the CDC's Division of STD Prevention. It's much harder to intervene in the adult core-transmitter model. So, scary as that map may look to parents, Ethier says, understanding how it works "is very encouraging for us." •