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 Monday.

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, NJ, 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.

The Recording Industry Association of America, which represents the US recording industry, declined to comment on the findings.

Benjamin Chavis, chief executive officer of the Hip-Hip Summit Action Network, a coalition of hip-hop musicians and recording industry executives, said explicit music lyrics are a cultural expression that reflect "social and economic realities."

"We caution rushing to judgment that music more than any other factor is a causative factor" for teens initiating sex, Chavis said.

Martino said the researchers tried to account for other factors that could affect teens' sexual behavior, including parental permissiveness, and still found explicit lyrics had a strong influence.

However, Yvonne K. Fulbright, a New York-based sex researcher and author, said factors including peer pressure, self-esteem and home environment are probably more influential than the research suggests.

"It's a little dangerous to just pinpoint one thing. You have to look at everything that's going on in a young person's life," she said. "When somebody has a healthy sense of themselves, they don't take these lyrics too seriously."

David Walsh, a psychologist who heads the National Institute on Media and the Family, said the results make sense, and echo research on the influence of videos and other visual media.

The brain's impulse-control center undergoes "major construction" during the teen years at the same time that an interest in sex starts to blossom, he said.

Add sexually arousing lyrics and "it's not that surprising that a kid with a heavier diet of that ... would be at greater risk for sexual behavior," Walsh said.

Martino said parents, educators and teens themselves need to think more critically about messages in music lyrics.

Fulbright agreed.

"A healthy home atmosphere is one that allows a child to investigate what pop culture has to offer and at the same time say 'I know this is a fun song but you know that it's not right to treat women this way or this isn't a good person to have as a role model,'" she said.