A survey of young teens in Los Angeles found that 40 percent who'd sent explicit messages or photos said they'd been sexually active compared to just 5 percent of other kids with cellphones that could display text messages.
"The surprise is that for younger kids -- 11- to 13-year-olds -- sexting is not an alternative to real-life sexual activity. It's actually a part of it," said study author Eric Rice, an assistant professor at the University of Southern California, Los Angeles.
"Also, kids who reported 100 or more text messages per day were much more likely to report sexting, so being an excessive texter may be an indication of risky behaviors," Rice added.
"Sexting" refers to sexually explicit messages sent via computer or cellphone -- in this case with or without images. Some sexts may just be explicitly flirtatious, while others include nudity.
Experts disagree about the level of risk posed by "sexting," and there's no definitive proof that the electronic messages are a gateway to early sexual activity.
"Sexting isn't harmful unto itself, but it can have harmful implications," Rice said. "It can ruin reputations and cause legal problems and may encourage kids to be more sexually active."
For the study, researchers surveyed more than 1,300 students aged 10 to 15 in the Los Angeles school district in 2012. Among those who answered questions about their ethnicity and sexuality, 60 percent were Latino and 96 percent were heterosexual.
The researchers adjusted their statistics so they'd better reflect the ethnic makeup of the school district.
More than two-thirds of the students owned their own cellphone. Three-fourths had access to a cell phone that could display text messages, according to the study, published online June 30 and in the July print issue of the journal Pediatrics.
Only a minority of kids with access to texting admitted to sexting. Twenty percent said they'd received a sexually explicit message or photo, and 5 percent said they'd sent one.
But young teens who received sexts were six times more likely to report having had sex, defined as either oral, vaginal or anal intercourse, according to the study.
Those who sent sexts were about four times more likely to report sexual activity.
The study has caveats, however. For one thing, self-reported information may be biased. "Sexual behavior is notoriously difficult to measure because you're relying on people reporting about themselves," said Amy Hasinoff, an assistant professor at the University of Colorado, Denver, who studies sexting.
Hasinoff cautions parents not to worry excessively about kids who spend a lot of time texting. "I don't know that texting a lot is a particular problem," she said.
"We need to think of cellphones as a way that kids communicate. You'd never say kids are talking to their classmate too much at recess, or that they used 1,000 words at recess, and that's excessive," she added.
What should parents do?
"Start talking to your kids about sexting early. If you suspect that your child is sexting, be aware that he or she is probably also sexually active," study author Rice said.
"Remember that teen pregnancy and sexually transmitted diseases can be prevented, but teens need to be educated in thoughtful ways by parents, teachers and pediatricians," Rice added.
The study says parents may wish to "openly monitor" their kids' cellphones. But Hasinoff said "that sends the exact wrong message."
Parents should talk about the importance of privacy and serve as role models, she said. "We want to be teaching kids to respect the privacy of other kids and develop the sense that privacy really matters," she explained.
Kids should also learn about the need to gain consent for any kind of sexual behavior, including sending someone sexually explicit photos, she added.
The American Academy of Pediatrics has more about how to discuss sexting with your teen.