Chicago ? Children who watched a lot of TV with sexual content were about twice as likely to start having intercourse during the subsequent year as those with little exposure to televised sex, researchers found.

High exposure to TV sex among those age 12 to 17 also was linked with a lower but still substantially increased risk of starting nonintercourse behavior, including passionate kissing and oral sex, the researchers found. Even shows that only refer to sex but don’t depict it had the effect, they found.

“Exposure to TV that included only talk about sex was associated with the same risks as exposure to TV that depicted sexual behavior,” said Rand Corp. behavioral scientist Rebecca Collins and colleagues.

From innuendoes to depictions of intercourse, sex is pervasive on TV, present in about two-thirds of all shows other than news and sports, and teens watch an average of three hours of television daily, previous research has shown.

TV thus “may create the illusion that sex is more central to daily life than it truly is and may promote sexual initiation as a result,” the researchers said.

“When they’re watching it for three hours a day, it really does become their social world. Those characters are people they identify with and pay attention to,” said Collins, the lead researcher.

TV sex rarely deals with negative aspects most teens aren’t prepared to deal with, including unwanted pregnancy, AIDS and other sexually transmitted diseases, she said.

That “sends kids the message that everybody’s having sex and nobody’s thinking about responsibility and nothing bad ever happens,” Collins said. “You don’t see the fade to black, the couple has sex, and the next morning says, ‘You gave me an STD.”‘

The study appears in September’s Pediatrics, released today.

The results are based on nationwide telephone surveys of 1,792 adolescents queried in 2001 and again in 2002. Parental consent for participation was obtained before the interviews.

The researchers devised a list of 23 popular shows that on average featured abundant sexual content. Programs the researchers considered high in sexual content included “That ’70s Show,” “Friends” and “Sex and the City” — all popular with teens.

Participants then were asked how often they watched those 23 shows. They also were asked whether they engaged in various sexual activities; results were compared from the two surveys.

The number of teens who reported having had intercourse climbed from about 18 percent to 36 percent. The number who’d had sexual experiences other than intercourse climbed from 62 percent to 75 percent, Collins said.

Factors that increased the likelihood of having intercourse included being older, having older friends and getting poor grades. But even considering those factors, television still remained a strong influence, the researchers said.