Yet others say that the results, while significant, risk tarring some men unfairly. The findings, based on offenders serving prison time who volunteered for the study, do not necessarily apply to the large and diverse group of adults who have at some point downloaded child pornography, and whose behavior is far too variable to be captured by a single survey.

Adding to the controversy, the prison bureau in April ordered the paper withdrawn from a peer-reviewed academic journal where it had been accepted for publication, apparently concerned that the results might be misinterpreted. A spokeswoman for the bureau said the agency was reviewing a study of child pornography offenders but declined to comment further.

Ernie Allen, who leads the National Center for Missing and Exploited Children, which is mandated to coordinate the nation’s efforts to combat child pornography, said he was surprised that the full study had not been released. “This is the kind of research the public needs to know about,” Mr. Allen said. Others agreed that the report should be published but were more cautious about the findings. “The results could have tremendous implications for community safety and for individual liberties,” said Dr. Fred Berlin, founder of the Johns Hopkins Sexual Disorders Clinic. “If people we thought were not dangerous are more so, then we need to know that and we should treat them that way. But if we’re wrong, then their liberties aren’t going to be fairly addressed.”

Everyone agrees that researchers need to learn more about online consumers of illegal child images. The volume of material seized from computers appears to be doubling each year — the National Center collected more than eight million images of explicit child pornography in the last five years — and Attorney General Alberto R. Gonzales made child protection a national priority in 2006.

Those who are arrested on charges of possession or distribution of child pornography generally receive lighter sentences and shorter parole periods than sexual abusers. They do not fit any criminal stereotype; recent arrests have included politicians, police officers, teachers and businessmen.