More than half of boys and girls aged 16 to 18 said the first place they had sex was in their own home or the home of their partner -- 10 times as many as who said they did it in a car or truck. And 70 percent said the act occurred in the evening or at night.
"Parents really aren't aware of the fact that their children are having sexual intercourse," says Angela Papillo, a researcher at Child Trends, the Washington, D.C. nonprofit group that published the report. "They need to be more involved in their kids' life and be a little more aware of what is actually taking place in their household."
The report used data from 2000. It found that 22 percent of teens said their first time happened at home, while 34 percent said it occurred at their partner's house. Boys were more likely than girls to have their first act of intercourse at their house or that of a friend.
Papillo says the survey didn't ask teens if their parents or their partner's parents were home at the time. And it's likely that in many cases the answer was no. "There are situations where the parents may be working, out for the evening, or there was a party at home where sex was taking place."
Still, Bill Albert, a spokesman for the National Campaign to Prevent Teen Pregnancy, calls the survey "a wake-up call" for parents.
"The idea that has developed since 'Happy Days' was on the air -- that sex happens in the back seat of a car at Lookout Point -- is simply an antiquated notion. Teens only have to go down to the rec room in the basement or the bedroom upstairs," Albert says.
Similarly, Albert says, parents need to realize that the few hours between school and dinner aren't necessarily when their kids are getting themselves into trouble.
While parents might clueless about their kids, another new study shows that they have a little less to worry about these days than they did a decade ago. American teens are less active sexually than they used to be. And those that do have sex are more careful than in the past, putting themselves at lower risk of HIV and other sexually transmitted diseases.
The report, from the U.S. Centers for Disease Control and Prevention (CDC), found the number of high-school students nationwide who said they'd ever had sex fell 16 percent between 1991 and 2001. For boys, the figure dropped from 57 percent to 48.5 percent; for girls, it was 43 percent versus 51 percent in 1991.
At the same time, the number who reported four or more sex partners dropped 24 percent --- to 17 percent for boys and 11 percent for girls -- while the share who said they used a condom when last having sex rose through the decade before leveling off at the end of the period at 65 percent for boys and 51 percent for girls.
By 2001, a third of boys and girls said they were sexually active in the three months leading up to the survey, down from 37 percent for girls and 38 percent for boys a decade earlier.
Laura Kann, chief of research for the CDC's division of adolescent and school health, says those trends are consistent with a decline over the last 10 years in teen rates of gonorrhea and pregnancy. Births to girls aged 15 to 19, for example, have plunged 26 percent since 1991, from 62 per 1,000 to about 46 per 1,000.
However, the study did turn up one alarming finding: The number of teens who said they did drugs or drank alcohol before their most recent sexual encounter spiked 18 percent.
"This is one of the first times that we've been able to monitor this particular behavior," says Kann, who adds the report doesn't address why the various trends have occurred.
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