The first “map” of teen sexual behavior gives new meaning to the old warning that you don’t just have sex with a person, but with everyone that person ever had sex with, researchers said on Monday.

They found a chain of 288 one-to-one sexual relationships at a high school in the U.S. Midwest, meaning the teenager at the end of the chain may have had direct sexual contact with only one person, but indirect contact with 286 others.

The sociologists who conducted the study said they were surprised by the findings, which also showed that despite reputations and popularity, most teens in their study did not engage in promiscuous behavior with many others.

“From a student’s perspective, a large chain like this would boggle the mind,” said sociologist James Moody, who led the study.

“They might know that their partner had a previous partner. But they don’t think about the fact that this partner had a previous partner, who had a partner, and so on.”

This means that teens need a different approach to sexual health education and especially prevention of sexually transmitted diseases, the team at Ohio State University said.

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