It seemed like more troubling evidence that kids these days engage in behavior they wouldn't want to write home about. Researchers recently found that one in five teenagers have shared nude or semi-nude photos of themselves by cellphone or online. That statistic has become a fixture in articles about "sexting" and its social and legal implications.

But that number may be inflated, because the same teenagers who have engaged in such behavior could be the ones most likely to say they have done so in an online poll. To find out how many teenagers are sharing personal information over new media, researchers last year asked teenagers personal questions using one of those new media, skewing the sample.

"These kinds of samples select Internet cowboys and cowgirls," says David Finkelhor, director of the Crimes against Children Research Center at the University of New Hampshire, who has used the telephone for his studies of teens and online behavior. "These are more likely to be the kind of people who engage in this kind of activity." He guesses that online poll-takers might be two to four times more likely to send nude photos of themselves than the average teen.

The sponsors of the poll, the National Campaign to Prevent Teen and Unplanned Pregnancy and CosmoGirl.com, counter that teens are more comfortable telling a Web page than an adult about their risqué behavior. "We wanted to reach teens where they are -- which is online," a CosmoGirl.com spokeswoman says.

The debate has played mostly in the background of a flurry of sexting stories in the news media, which have focused on whether such images are child pornography. But the dispute highlights a growing challenge for pollsters: Fewer people are responding to them over the phone, yet they haven't yet agreed on how to move their business online.