Teens who watch a lot of sexually suggestive television programming -- which is to say much of it -- are almost twice as likely to have sex earlier than teens who don't.

That's the conclusion of a national Rand Corp. survey of 1,792 adolescents released yesterday.

The connection didn't come as much of a surprise to Brenda Green, a local expert in adolescent health, who recalled the plot line of Tuesday night's "Two and a Half Men," a CBS sitcom starring Charlie Sheen.

"The entire half-hour was devoted to Sheen trying to get his brother and nephew out of his beach house so he could engage in something called 'revenge sex,' which is defined as having sex with someone to get even with the person who broke up with you," said Green, vice president for education for Planned Parenthood of Western Pennsylvania.

"There's a casual attitude about sex that's just pervasive on television that has an effect on teens, and it's something we've been saying for years."

While the link between television and risky teen behaviors -- including violence, drinking and sex -- has been shown in other studies, the California-based organization's study is the first to find a direct cause-and-effect link specifically to sexual activity, said Rebecca Collins, a psychologist and the study's lead researcher.

The Rand study, published in this month's "Pediatrics" online journal, resulted from phone interviews of adolescents ages 12 to 17, first in 2001 and again a year later in 2002. It found that children who watched the most sex-saturated television -- defined as popular comedies and dramas on the major networks and on cable television, such as "Friends, "Sex and the City" and "That '70s Show" -- were twice as likely to engage in sexual activity within the next year than those who reported a low level of television watching. (Researchers did not include MTV because its format is not easily comparable to sitcoms or other shows.)

So, the question is, why?

"TV may create the illusion that sex is more central to daily life than it truly is and may promote sexual initiation as a result," said Collins. "They make kids think that everyone is thinking about sex and trying to get sex all the time, when in fact that's not true."

"The more people watch anything, the more it makes that behavior look normative," added Melanie Gold, an associate professor in pediatrics at the University of Pittsburgh School of Medicine and director of family planning services.

"When there was an 'ER' episode on emergency contraception, a number of kids who came to the family planning clinic said they'd heard about it on 'ER.' Others talked about a program where it was mentioned that St. John's wort could decrease the effectiveness of birth control pills."

Then there was an episode from "Friends" that made her wince. "They kept saying ... condoms were 97 percent effective. In fact, there's a 15 percent failure rate, but that information was not conveyed correctly."

The average teen watches three hours of TV a day, and during the 2001-02 season, 64 percent of the programming (excluding newscasts, sports events and children's shows) contained sexual content, most of it talk about sex rather than a depiction of sexual behavior itself.

"A lot of it is just joking or casual references in scenes -- a lot of stuff that gets under the radar of most parents," said Collins. But whether it was just talk about sex or the act itself, such television programs appear to hasten sexual initiation at a young age, she added.

Would a reduction of sexual content in such programming have the effect of delaying sexual activity among teens? Collins, in her report, suggests that would be the case, but it's not clear if television programmers are buying it: Spokespersons for CBS, NBC and Fox Television declined requests for comment.

In the meantime, what can parents do?

Don't turn off the television immediately, suggests Green.

"Censorship is not the way to go," she said. "They'll just go over to their neighbors' to watch this stuff. Instead I encourage parents to sit down and watch these programs with their children and use it as a teachable moment. Tell your kids what you like and don't like, how you respond to the violence, the language, how women are portrayed."

And ask open-ended questions, she adds.

"Ask them, 'Do you know people who behave like that? Who looks like that? Do you think that relationship was realistic?' And tell them why you don't think certain characters are good role models."

It will be a tough battle, she acknowledges. "In a lot of these shows, there's 30 or 60 minutes of behavior, but not a whole lot of consequences."

Still, there are some bright spots. On "The O.C.," a show that's wildly popular with teens, one of the teen characters got pregnant "and the whole show dealt with the consequences of that and how her life would change dramatically."

But for the most part, Green longs for the days of "The Cosby Show," which she used to tape and show in health education classes. "It was hands down the best. It dealt with everything -- from getting your period to dating and onward. But those days are long gone."