CHICAGO — Teens whose iPods are full of music with raunchy, sexual lyrics start having sex sooner than those who prefer other songs, a study found.

Whether it's hip-hop, rap, pop or rock, much of popular music aimed at teens contains sexual overtones. Its influence on their behavior appears to depend on how the sex is portrayed, researchers found.

Songs depicting men as "sex-driven studs," women as sex objects and with explicit references to sex acts are more likely to trigger early sexual behavior than those where sexual references are more veiled and relationships appear more committed, the study found.

Teens who said they listened to lots of music with degrading sexual messages were almost twice as likely to start having intercourse or other sexual activities within the following two years as were teens who listened to little or no sexually degrading music.

Among heavy listeners, 51 percent started having sex within two years, versus 29 percent of those who said they listened to little or no sexually degrading music.

Exposure to lots of sexually degrading music "gives them a specific message about sex," said lead author Steven Martino, a researcher for Rand Corp. in Pittsburgh. Boys learn they should be relentless in pursuit of women, and girls learn to view themselves as sex objects, he said.

"We think that really lowers kids' inhibitions and makes them less thoughtful" about sexual decisions and may influence them to make decisions they regret, he said.

The study, based on telephone interviews with 1,461 participants aged 12 to 17, appears in the August issue of Pediatrics, being released today.

Most participants were virgins when they were first questioned in 2001. Follow-up interviews were done in 2002 and 2004 to see if music choice had influenced subsequent behavior.

Natasha Ramsey, a 17-year-old from New Brunswick, N.J., said she and other teens sometimes listen to sexually explicit songs because they like the beat.

"I won't really realize that the person is talking about having sex or raping a girl," she said. Even so, the message "is being beaten into the teens' heads," she said. "We don't even really realize how much."

"A lot of teens think that's the way they're supposed to be, they think that's the cool thing to do. Because it's so common, it's accepted," said Ramsey, a teen editor for Sexetc.org, a teen sexual health Web site produced at Rutgers University.

"Teens will try to deny it, they'll say 'No, it's not the music,' but it IS the music. That has one of the biggest impacts on our lives," Ramsey said.