The study published Monday in the journal Pediatrics analyzed 17,220 public health surveys and found that more than a third of teens who had same-sex encounters identified themselves as straight. Advocates said the results point to the need for public health messages to target behavior, not identity.
It's troubling, though not surprising, that the youths who reported encounters with both sexes also had higher rates of risky behavior and violence, said Dr. Susan Blank, assistant commissioner of the New York City Department of Health and Mental Hygiene.
"It has been shown in the literature that students who have both male and female partners have a lot of adverse health problems," she said.
The report is based on surveys administered in New York City high schools in 2005 and 2007. The Youth Risk Behavior Survey is distributed every two years by the health department to a representative sampling of city high schools.
Of the students who completed the survey, 7,261 said they had had sexual intercourse. The survey asked questions including the number of sex partners in the teen's lifetime, as well as in the previous three months, and whether there was a history of forced sex or dating violence.
Among the sexually active males, 93.1 percent said they had had sexual contact only with female partners, 3.2 percent said they'd had contact only with other males, and 3.7 percent said they'd had both male and female partners.
Among the sexually active females, 88.1 percent said they'd had sex only with males, 3.2 percent said they'd had only female partners and 8.7 percent said they'd had partners of both sexes.
Of all the sexually active teens, 9.3 percent reported at least one same-sex partner.
Study author Preeti Pathela, a research scientist at the health department, said that there are no comparable national numbers but that other states have reported lower numbers of teens who have had partners of the same or both sexes.
Students also were asked whether they considered themselves straight, gay or lesbian, bisexual, or "not sure." Of the teens with at least one same-sex partner, 38.9 percent answered "heterosexual or straight."
Other studies have shown a similar divergence between behavior and sexual identity, but Blank said this study provides a reminder that "our public health prevention messages really need to look at behavior, not identity."
The study found that teens who reported partners of both sexes also reported higher-than-average rates of risky sexual practices, such as not using a condom during intercourse, as well as higher rates of partner violence and forced sex.
Of the girls with both male and female partners, 35.8 percent said they had experienced dating violence in the previous year. Of the boys with both male and female partners, 34.8 percent reported it in the previous year.
Far lower percentages of teens with only male or only female partners answered yes to the question about dating violence.
Of the males who reported both male and female partners, just 44.1 percent said they'd used a condom during their last sexual encounter.
The percentages for condom use during their last sex were 79.8 for males with only female partners and 62.3 for males with only same-sex partners.
Laura Lindberg, senior research associate at the Guttmacher Institute, which focuses on sexual and reproductive health issues, said the teens with male and female partners "are obviously facing challenges."
"These are kids in New York City where there's more awareness and perhaps acceptance of non-heterosexual behavior, and you're still finding such high reports of risk behavior and violence," Lindberg said.
Thomas Krever, executive director of the Hetrick-Martin Institute, a youth advocacy organization that runs an alternative high school for gay teens in New York City, said the survey results did not surprise him.
Many teens with partners of both sexes lack supportive adults and peers in their lives and may experience depression because social stigma, Krever said.
"Young people who are exhibiting characteristics of depression and lower self-worth can indeed place themselves in more risky situations including risky sexual practices," he said.
Blank, who heads the health department's efforts to fight sexually transmitted diseases, said teens need to get the message that help is available.
She said health department clinics will test anyone over 12 for sexually transmitted diseases free of charge.