"This is the strongest evidence yet that the sexual content of television programs encourages adolescents to initiate sexual intercourse and other sexual activities," said Rebecca Collins, a psychologist at the RAND Corp. who headed the study.

"The impact of television viewing is so large that even a moderate shift in the sexual content of adolescent TV watching could have a substantial effect on their sexual behavior," she added.

The study found that youths who watched large amounts of programming with sexual content were also more likely to initiate sexual activities short of intercourse, such as oral sex.

It found that shows where sex was talked about but not depicted had just as much impact as the more explicit shows. "Both affect adolescents' perceptions of what is normal sexual behavior and propels their own sexual behavior," Collins said.

She said the 12-year-olds who watched a lot of sexual content behaved like the 14- or 15-years-olds who watched the least amount. "The advancement in sexual behavior we saw among kids who watched a lot of sexual television was striking."

Her comments were released in a statement in conjunction with publication of the study in the September issue of "Pediatrics," the journal of the American Academy of Pediatrics.

The survey did not break down the amount of sexual exposure in terms of hours per week or percentages of material viewed, Collins said in an interview.

It did find that the 10 percent of those who watched the most television with sexual content were twice as likely to have initiated sexual intercourse when checked a year later than adolescents who were among the 10 percent who watched the least amount of sexual content.

"The best way for parents who are trying to figure out what is a lot versus little is to realize that the average (U.S.) child watches about three hours of television a day, and that the heaviest rates of sexual content are in prime time which is probably what those hours are made of," she said.

The report said earlier studies found that about two-thirds of TV entertainment programs contain sexual content, ranging from jokes and innuendo to intercourse and other behaviors.

The study was funded by the National Institute of Child Health and Human Development.