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Half the romantically active students at a large U.S. high school were linked by up to 37 degrees of separation, according to a study that mapped the teenagers' sexual patterns.

"Instead of the stereotype of a jock or somebody having lots and lots of partners, what you have is everybody who is active has one, maybe two [former]partners. A few people have three. But that's just enough to connect everyone," James Moody, co-author of the study and a sociology professor at Ohio State University, said in an interview.

The researchers, whose work was published in the American Journal of Sociology, found that 288 students -- or 52 per cent of the teens who were romantically active in the previous 18-month period -- were linked by up to 37 degrees of separation. In other words, when researchers plotted the sexual activity as a graph, there were 37 individuals between the two most-distant people in the large network.

While some of the teens reported being involved in relationships that did not involve sexual intercourse, they engaged in other behaviours, such as oral sex, that could have exposed them to sexually transmitted diseases.

Prof. Moody likened the teens' romantic relations to rural telephone lines, which run from a long main trunk line to individual houses. He had expected they would be similar to many adult sexual networks which, aside from monogamous couples, are like an airline system where many points are connected to a small number of hubs.

The study, which the researchers say is the first to map the sexual undercurrents of an entire high school, used data from a 1995 survey of 832 students in grades 10 to 12 at an unidentified, almost all-white, working-class Midwestern school. Students privately answered questions on computers at their homes and the data was later stripped of identifying information.

Slightly more than half the teens reported having sexual intercourse, a rate comparable to the U.S. national average. The researchers found 63 pairs of students whose only partnership was with each other.

The study also discovered a form of "incest taboo," meaning the teens were loathe to sleep with those closely linked to them in the network. Girls, for example, would not have sex with their ex-boyfriend's current girlfriend's old boyfriend, a practice known as taking "seconds."

"Instead you have this fanning out to new people," Prof. Moody said.

The students' taboo pattern holds a valuable lesson for sexual-health educators: In such a population, sexually transmitted diseases have a greater chance of spreading because the students' former partners are widely dispersed. "More people are reached faster," he explained.

But Prof. Moody said broad-based education campaigns have a greater chance of breaking the chain of STD transmission in such an environment. (The study did not ask about sexually transmitted diseases, because self-reporting of STDs, many of which have no symptoms, is not considered accurate.)

"If you're effective, even with a small proportion of the population, in getting them to use condoms or change their behaviour or whatever, you're going to be effective in stopping the disease," he said.

Because most of the teens who took part in the survey were not promiscuous, Prof. Moody said he suspects they were unaware of the extended sexual histories that linked half of them, which illustrates the principle that one is not only having sex with one's current partner, but also all of his or her previous lovers.

"They can say, 'Well, I know sort of theoretically that I'm sleeping with all my past partners' partners,' but it's not until you really see that there are 300 people here that are in that chain that it becomes real," he said.

But Alex McKay, research co-ordinator of the Sex Information and Education Council of Canada, noted that STDs do not have a 100-per-cent infection rate.

Mr. McKay said researchers have long known that broad-based teenage sexual education, as well as intensive targeting of high-risk groups, is the best way to reduce STDs. "That's not particularly new."

He also noted that the study focused on the sexual behaviour of students at an isolated school, which was the only public high school in a medium-sized city, and that the practices of urban teens would likely be very different.

The study was based on data from the U.S. National Longitudinal Study of Adolescent Health.