Judging from what you read in books and see in the movies, rape and sexual assault behind bars may be rampant.

But a government-sponsored study has concluded that in reality, it's not a common occurrence in prison.

Cultural anthropologist Mark Fleischer says when inmates have sex, it is usually by choice. He says inmates who cry rape are usually lying and looking for a transfer, money or publicity.

The Justice Department commissioned the study, but has not yet endorsed it.

It's come under attack from other experts, including the chairman of the National Prison Rape Elimination Commission. Federal judge Reggie B. Walton calls the conclusions "totally inconsistent" with what he's learned during 30 years in the criminal justice system.