The less after-school supervision teens have, the more likely they are to have sex, new research says.

And that sexual activity is likely to happen at the home of one of the teens, claims a new study in the December issue of Pediatrics.

Dr. Deborah Cohen, a researcher at the RAND Corp. in Santa Monica, Calif., and her colleagues evaluated more than 2,000 students in grades 9 through 12 in six public high schools. They found that 56 percent were home without an adult present at least four hours each day after school.

Then they compared supervision with sexual activity and found a strong relationship between the number of unsupervised hours and sexual activity. Those who were unsupervised for at least 30 hours a week, or roughly six hours each weekday, were more likely to be sexually active compared with those left alone for no more than five hours a week. Those who were unsupervised for more than five hours weekly also reported more sexually transmitted diseases.

They also found that among those who had intercourse, 91 percent said the last time occurred at their home, their partner's home, or a friend's home -- usually after school.

The students who were evaluated were 98 percent black, many from low-income families and more than half from single-parent homes. While the study results may not be applicable to all populations, Cohen says, it "should raise red flags."

She adds there were no differences between single parent and two-parent families, further lending credence that it was indeed the lack of supervision, not the family structure, that was associated with the sexual activity.

She also points to a report done earlier this year by Child Trends, a Washington, D.C.-based nonprofit group, in which those researchers found that more than half of boys and girls aged 16 to 18 had sex for the first time in their own home or that of their partner's -- presumably when parents were not around, although the researchers didn't ask that question.

In the current study, the most surprising finding to Cohen was the amount of unsupervised time. "These are high school kids," she says. "People expect them to be more responsible."

While teens are physically more able to take care of themselves than younger children, she says teens would still "be better off if adults were around more."

If that's impossible because of work schedules, parents might investigate after-school activities that are supervised, she suggests.

The findings of the latest study don't surprise Julia Davis, a senior program officer at the Kaiser Family Foundation. "You can only assume the less time parents spend with the kids, the less opportunity [they have] to talk about sex."

Even though teens are uncomfortable talking to their parents about sex, says Davis, citing her own research on teen sexuality, they still look to their parents for guidance.

Spending more time with teens will present more opportunities to open that dialogue about sex, Davis says.

"Make it clear what your opinion is of being sexually active, what they need to know about protection, risks and consequences," she adds. "The key issue is to communicate to your teen there can be a dialogue. It isn't just one talk, but ongoing dialogue."

The dialogue, she adds, must change as your teen's needs change. A 15-year-old, for instance, probably needs very different information than an 18-year-old about to go off to college.