Do you know what your kids are doing on their cell phones?

More than a quarter of American teenagers have sent nude photos of themselves electronically, and those who engaged in 'sexting' were almost twice as likely as their peers to have had sex, researchers found.

About half of almost 1,000 students ages 14 to 19 from seven public high schools in Texas said they had been asked to send a naked photo electronically, according to a study published Monday in the Archives of Pediatric and Adolescent Medicine. Another third reported asking someone else to send them a nude picture.

The study found the 28 percent of teens who exchanged nude messages were more likely to have sex. Pediatricians and other adults should ask about sexual messaging to screen for other sexual behavior, said Jeff Temple, a researcher from University of Texas Medical Branch Health in Galveston and the study's lead author.

"These messages are really important to adolescents," said Megan Moreno, a University of Wisconsin adolescent medical specialist whose editorial about the connection between online and offline behavior accompanies the study. "I think we'll want to think about how do we provide education about it and how do we provide prevention."

More boys asked for a "sext," though the same percentage of boys and girls had sent one, according to the study.

Of girls who had sent a nude message, more than 77 percent reported having had sex. For those who had never sent a naked photo, 42 percent said they had had sex. For boys, those numbers were 82 percent compared with 45 percent.

Teen girls who sent the images were more likely to have multiple partners or use drugs and alcohol before sex, though the same was not true for boys.

"The main implications are that sexting is prevalent, that it's common," Temple said today in a telephone interview. "If we know that they're more likely to have sex, then maybe that gives the pediatrician or the parent information to start a conversation," he said.

Asking for and sending nude photos was most prevalent among white non-Hispanics and black teens, the study found.

White, black and Hispanic students each accounted for about 30 percent of study participants. All respondents were sophomores or juniors.

More older teens reported "sexting," and 16- to 17-year- olds were most likely to have been asked to send a naked photo.

"There is a very strong age effect. The older the person, the more likely," said Donald Strassberg, a University of Utah psychology professor who researched sexting in a study published earlier this year.

In Strassberg's study, which appeared in the Archives of Sexual Behavior, about 20 percent of teens at a private school reported sending text message photos of genitals or breasts. Another telephone survey published in the journal Pediatrics put the number at as little as 2.5 percent.

"Different samples come up with different results," Strassberg said. "Part of it depends on the specific question that was asked."

The growing number of sexting studies in the result of media attention that has sparked researcher's curiosity, Strassberg said.

"I think it just became apparent to more researchers that, hey, we don't really have a good idea of how prevalent this is," he said.