Updated

Fewer than three prisoners in every 1,000 report they were sexually abused or harassed, but that probably is not the whole story, a government study says.

There may be far more sexual violence in prisons than is reported, the study's authors said, because inmates fear reprisal, adhere to a code of silence, do not trust the staff or are embarrassed.

"Administrative records alone cannot provide reliable estimates of sexual violence," wrote Allen J. Beck and Paige M. Harrison of the Bureau of Justice Statistics.

"It's a real and serious problem," said Malcolm Feeley, professor of law at the University of California, Berkeley. "It may be the single largest shame of the American criminal justice system, and that's saying a lot."

The study released Sunday by the Justice Department agency is based on reports to corrections officials in 2005.

The bureau looked at more than 1,800 correctional facilities holding some 1.7 million inmates — 78 percent of the adult prison population.

The report is the second one required by the Prison Rape Elimination Act of 2003, which was an attempt to solve a problem believed to be widespread.

"What gets reported is the tip of the iceberg," said Cindy Struckman-Johnson, professor of social psychology at the University of South Dakota and a member of the National Prison Rape Elimination Commission.

Struckman-Johnson said her studies found 10 percent of male Midwestern state penitentiary inmates have been raped.

"In my research, only a third of the inmates actually reported it to anybody working in a prison," she said.

Beck and Harrison found that 38 percent of allegations involved staff sexual misconduct and 35 percent involved forced sex by an inmate on another inmate.

Also, 17 percent involved staff sexual harassment and 10 percent involved abusive sexual contact by an inmate on another.

This year's study includes more detailed information than the first study, published a year ago. The second study includes data on the circumstances surrounding each incident, the characteristics of the victims and perpetrators, the kind of force or pressure used, injuries, sanctions imposed and victim assistance.

"The greatest improvements in reporting were for staff sexual misconduct and harassment," the report said.

Only 15 percent of the sexual violence incidents were substantiated, the report said.

The bureau is working on ways to better measure sexual violence in prisons and jails, including anonymous self-administered surveys, the report said.

Prison culture makes it next to impossible for victims of such assaults to come forward, Feeley said.

The problem, he said, is exacerbated by problems of overcrowding, lax oversight and very long terms that lead to prisoner disillusionment with the system.

"There are a lot of things that could be done to reduce it," Feeley said.

The commission is warning Congress of the consequences of putting juveniles in adult prison, Struckman-Johnson said.

"When you put kids under 18 in prison with adults, the incidence of rape and abuse is five times higher," she said.

Prison rape isn't a problem limited to prisons, she said. "We get reports that people who are raped and abused in prison will rape and abuse others when they leave prison," she said.