A study of sexual and romantic relations at a high school found students connected by long chains, rather than in a tight network with a core group of a promiscuous few.

Sharing of partners was rare, but many students were indirectly linked through one partner to another and another.

The unexpected result could help shape strategies for combating sexually transmitted diseases among young people.

"We went into this study believing we would find a core model, with a small group of people who are sexually active," said James Moody, a professor of sociology at Ohio State University. "We were surprised to find a very different kind of network."

In the most striking chain, 52 percent of the romantically involved students were connected in a manner of student A having relations with B, and B having relations with C, and so on down the line over the 18 months of the study.

Students couldn't possibly know of all the connections, the scientists conclude.

"Many of the students only had one partner," Moody said. "They certainly weren't being promiscuous. But they couldn't see all the way down the chain."

The study was detailed in a recent issue of the American Journal of Sociology. Peter Bearman of Columbia University and Katherine Stovel of the University of Washington participated in the research.

The work was based on the National Longitudinal Study of Adolescent Health, a 1995 survey of students at an unidentified Midwestern high school. The students were mostly white, in the only public school in a mid-sized city more than an hour away from a metropolis. Of about 1,000 students at the school, 832 were interviewed and asked to identify their sexual and romantic partners over the previous 18 months. Just more than half reported having sexual intercourse, a rate comparable to the national average, the researchers say.

Of all the pairings, 63 involved two students who had not partnered with anyone else.

The research reveals a semantically complex rule that seems to guide adolescent sexual conduct. Here goes: A girl is loath to date her old boyfriend's new girlfriend's old boyfriend.

Adults don't generally adhere to any similar rule, so core populations of sexually active adults tend to be prime spreaders of disease. But with adolescents, the study suggests, "there aren't any hubs to target, so you have to focus on broad-based interventions," Moody said.