BY THE NUMBERS BY THE NUMBERS Students are increasingly likely to be sexually active as they move through adolescence. Percentage who have had intercourse: Age Group Boys Girls Before age 13 9 4 9th grade 39 29 10th grade 42 44 11th grade 51 52 12th grade 64 62 Source: Centers for Disease Control and Prevention (2006) There's a "recipe" that raises the odds of a teen starting sex early, and the more risky ingredients in a child's life — for example, not feeling close to parents, low self-esteem and lots of TV — the more likely he is to be sexually active by age 15, suggests a study released over the weekend. "It isn't any one thing. It's cumulative, and the more risks there are, the greater the chances that they'll begin sex early," says Janet Shibley Hyde, a psychologist at the University of Wisconsin-Madison. She and co-author Myeshia Price reported on their two-year study of 273 children at the Society for the Scientific Study of Sexuality meeting in Indianapolis. The students were in a larger database study, which gave many facts about their families. Using anonymous surveys, the researchers asked about sexual activity at age 13 and again two years later. By 15, one out of five boys had participated in oral sex and about one in 10 said they'd had intercourse; the numbers were somewhat lower for girls. (Because the teens were mostly middle class and white, they had lower rates of sexual experience than the U.S. average.) Each risky factor raised the odds of sexual activity by 44%. Boys with more advanced puberty development started sex early. Teens with low self-esteem may start sex to boost their self-images or gain popularity, Price speculates. Defiant kids with attention-deficit hyperactivity disorder, those whose parents had little education or those who regularly watched certain types of TV also tried sex sooner. The researchers asked about viewing six kinds of programming and channels: MTV, BET, music videos, wrestling, daytime soap operas and sports shows. The more TV watched, the greater the likelihood of beginning sex between 13 and 15, Price says. Children from 13 to 15 see about three hours of television a day, says Vicky Rideout, vice president of the Kaiser Family Foundation. "And there's a lot of sex on TV," she adds. But it's unfair to blame just TV, Rideout says. Heavy viewers may be left unsupervised a lot by parents. Kids who start sex early have more partners than those who wait, and they're much more likely to get pregnant or catch a sexually transmitted disease, says Bill Albert of the National Campaign to Prevent Teen and Unplanned Pregnancy. Young teens also are more apt to feel coerced into sex, adds Michael Resnick, director of the Healthy Youth Development Prevention Research Center at the University of Minnesota. "Maybe he says, 'I'll break up with you if you don't,' or 'I'll say terrible things about you.' " But feeling close to parents may offset pressure for sex. "You have to remain close to your kids," Hyde says. "You can't just say, 'They're teenagers, they're obnoxious. I'm checking out, and I'll see them again at 20.' Warmth from parents and clear, firm guidelines can make a big difference to kids this age." Conversation guidelines: USA TODAY welcomes your thoughts, stories and information related to this article. Please stay on topic and be respectful of others. Keep the conversation appropriate for interested readers across the map.