A study of girls at Massachusetts health clinics found that one in 13 said they had participated in group sex -- and that the behavior was strongly associated with pornography and child abuse.
Although the study, published by the Journal of Urban Health: Bulletin of New York Academy of Medicine, is a small one, the researchers said it offers a window into a risky sex behavior that has so far been given little legitimacy.
More than half of the girls who reported experiencing group sex said they had been coerced into doing so, according to the study. Many admitted they had been "liquored up" on alcohol and drugs, often against their will.
The average age of the first group-sex experience was 15.6, according to the study, and for most, it was a one-time experience.
"I am really incredulous that this has not had more study and attention," said Emily Rothman, lead author at Boston University School of Public Health.
"The take-home message is that both consensual and non-consensual group sex is happening among youth -- and pediatricians, health organizations and rape crisis centers need to be prepared to talk about and provide the education to address it," she said.
Rothman interviewed 328 females between the ages of 14 and 20 who had used a community or school-based health center to see if they had ever had sex with multiple partners.
These girls had sought help at the clinics for a variety of reasons from strep throats to sprained ankles, not just for reproductive care.
An estimated 7.3 percent of the teens said they had experienced what researchers called "multi-person sex" -- an experience that could have ranged from a gang rape to a sex party.
"I think one of the things going on here is that boyfriends or sex partners are forcing their female partners to watch porn and also then coercing them," Rothman said. "Whether that is through peer pressure or doing things they see in the porn, we don"t know."
Those girls were also five times more likely than those who did not have group sex to have watched pornography in the last month.
In those who said they had group sex, 45 percent reported having sex without a condom in a recent encounter. They also were more likely to smoke cigarettes, have been a victim of dating violence or had a sexually transmitted disease diagnosis.
About one-third said they used alcohol or drugs prior to sex, but most of those said that it was not voluntary.
"Most said they were liquored up or drugged by their sex partner," Rothman said.
The study also found that 44 percent of the group overall had experienced dating violence. But in those who had group sex, 78 percent reported dating violence.
And even though 35 percent of the group who admitted to group sex said they had not been pressured, some may have been persuaded by videos and other media that encourages risky behavior, researchers say.
"In their minds, they were thinking it was consensual," Rothman said. "As a parent and public health advocate, it makes me heartsick."
Anecdotally, school and health officials have reported group sex parties as a new trend among young teens and all regard it as a serious risk for sexually transmitted diseases.
As far back as 1999, PBS chronicled the sex parties of middle-class teens in Georgia in the documentary, "The Lost Children of Rockdale County."
The behavior came to light when 17 teens were diagnosed with syphilis, some girls as young as 13. In the end, more than 200 others were exposed and treated.
A 2010 New York Times story highlighted "rainbow parties," where girls with different lipstick colors give oral sex to boys -- a practice some experts at the time suggested were mythical.
"What this study does is to elevate the issue up out of the realm of urban legend and hopefully make at least federal funding agencies take notice," Rothman said.
Philadelphia, with the nation"s highest HIV infection among youths, just this year made free condoms available to 11-year-olds.
They cited a 2009 CDC Youth Risk Behavior Survey that found that 15 percent of children under 13 have had their first sexual encounter.
"We hear from teachers and school counselors and sometimes the principals that kids are cutting school in the afternoon and leaving early to go have orgies -- and that's in middle school," Gary Bell, executive director of Bebashi-Transition to Hope, a local nonprofit that works with teens, told ABCNews.com at the time.
Michael Nerney, a New York consultant who specializes in risky adolescent behavior, said the numbers may be small, but nonetheless an important observation given the high rate of sexually transmitted diseases.
"I"d be astonished if it didn"t exist," he said. "And it makes sense to me that it needs more attention."
It is also not surprising that these teens tends to be younger, when their brains are not fully formed, he said.
"Their physical maturation runs four to eight years ahead of their social, emotional and relationship maturity," Nerney said.
At early adolescence risk-taking is also more exciting and "emotionally rewarding," without consideration for the consequences, he said.
This age group is also shaped by cultural influences like television shows about single teen mothers and multiple affairs.
"It's sexual roulette," he said. "They hook up with this one and then that one."
Lonely teens are also more vulnerable to the "nagging pressure" of their peers.
As for Rothman, she said she was confident of the authenticity of the teens' self-reporting.
"Some girls under-report and others over-report," she said. "One of the things that give me confidence about the accuracy of the reporting was that it was done in a health care setting. We were asking in a doctor's office and not in a middle school English class.