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."