Four percent of US online youths have been asked to send sexually explicit photos of themselves over the Net, and about 1.5% have done so, the Associated Press reports, citing a new study by the Crimes Against Children Research Center. That’s not just extremely unwise; those photos could be considered illegal child pornography, distribution of which is a federal crime. One of the study’s authors, Kimberly Mitchell, told the AP that kids need to know this. “Mitchell said kids also may not be aware of how quickly such photos can circulate, mistakenly thinking the image is only for the personal use of the requester,” according to the AP. Here are the conditions that she and her co-authors identified as making kids more likely to receive these requests for explicit photos of themselves: “having a close relationship with someone known only online; talking with someone online about sex or having a sexually suggestive screen name; and experiencing physical or sexual abuse offline.” The study, which is being published this week in the Journal of Adolescent Health, is an analysis of data from a 2005 phone survey of 1,500 Net users aged 10-17, the AP reports (its authors said the numbers could be higher now, with greater use of camera phones, Web cams, and other digital-photo devices). The study’s margin of error of plus or minus 2.5 percentage points.