From behind their bedroom doors, more than one out of every 10 teenagers has posted a nude or seminude picture of themselves or others online — a “digital tattoo” that could haunt them for the rest of their lives, according to a study released Monday.

Aside from the nudity, the survey found that at least a quarter of the young people polled had posted something they later regretted, made fun of others or created a false identity online.

While teens are spending more and more time on social-networking sites like Facebook and MySpace — with 22 percent saying they check their sites more than 10 times a day — they don’t seem to be aware of the long-term personal havoc they could create with a click of a button.

And their parents generally have little idea about what their children are up to, the poll found.

“We’ve got to stop kidding ourselves about this,”said James Steyer, chief economic officer and founder of San Francisco-based Common Sense Media, which commissioned the study. “There are enormous consequences from inappropriate behavior online.”

The survey polled 1,000 teens and 1,000 parents to gauge how much time young people are spending on social-networking sites, what they do when they’re on them — and whether their parents know.

Social-networking sites allow members to create individualized pages that often include personal information such as relationship status, age, city of residence and birth date, as well as photos, videos and personal comments.

Yet there can be enormous consequences.

That alcohol-related post-prom picture? Someday an employer or college admission officer might come across it with a quick click on Google. Hitting delete to get rid of a questionable photo won’t help. The digital imprint never goes away and could be flitting across computer screens around the world.

“Look, whether we like it or not, kids live in the 24/7 digital world,” Steyer said. “It affects virtually every aspect of their lives.”

Steyer’s organization is starting a pilot program this fall to bring media literacy into public-school classrooms. It’ll take in everything from not using the cellphone at the dinner table to the dangers of posting personal information online.

But parents need some lessons, too, he said.

var TFSMFlash_PRETAG=””;
var TFSMFlash_POSTTAG=””;
var TFSMFlash_VERSION=”9″;
var TFSMFlash_WMODE=”transparent”;
var TFSMFlash_OASCLICK=”http://ads.nwsource.com/RealMedia/ads/click_lx.ads/www.seattletimes.com/nation/L20/487071959/Middle3/Seattle/ST_House_0709_Myth-hp/ST_House_0709_Myth-mr06.html/5136706b3530613930464141412f7164?https%3A%2F%2Fservices.nwsource.com%2Fsubscribe%2Ftimes%2Findex.aspx”;
var TFSMFlash_SWFCLICKVARIABLE=”?clickTAG=”+TFSMFlash_OASCLICK ;
var TFSMFlash_SWFFILE=”http://imagec15.247realmedia.com/RealMedia/ads/Creatives/Seattle/ST_House_0709_Myth-hp/st_myth_coffee072109mr_1a.swf/1248221269″+TFSMFlash_SWFCLICKVARIABLE;
var TFSMFlash_FSCOMMAND=””;
var TFSMFlash_IMAGEALTERNATE=”http://imagec15.247realmedia.com/RealMedia/ads/Creatives/Seattle/ST_House_0709_Myth-hp/st_myth_coffee072109mr_1a.jpg/1248221269″;
var TFSMFlash_OASALTTEXT=””;
var TFSMFlash_OASTARGET=”_blank”;
var TFSMFlash_OASPROTOCOL=”http://”;
var TFSMFlash_OASDIM=”WIDTH=’300′ HEIGHT=’250′”;
var TFSMFlash_OASADID=”OAS_RMF_Middle3_FLASH”;
document.write(”);

It’s a “playground with no parental supervision,”said Dr. Dimitri Christakis, George Adkins Professor of Pediatrics at the University of Washington, who found in a study released earlier this year that 54 percent of teens demonstrate risky behavior online.

Just 4 percent of parents said their children check their sites more than 10 times a day, and only 2 percent said their teens have posted naked pictures, according to the poll released Monday.

“There’s definitely [a] not-my-kid syndrome,” Steyer said.