There are tens of thousands of male sex offenders in U.S. prisons, and only 1,200 female offenders. But sex offenses by women are more common than most people would think, according to one study.

Psychologist Katherine Peterson of the Kentucky Department of Corrections (search) began studying female sex offenders a few years ago and found that the problem is much more prevalent than people realize because the majority of men who are sexually assaulted by women never report it.

Of course, there are famous cases — like that of Seattle teacher Mary Kay Letourneau (search), who is serving seven years in prison after having sexual relations with a 13-year-old student and bearing two of his children.

But Peterson said there are many more women like Letourneau who have never been brought to justice. In addition, she said, those who are caught generally receive lighter sentences than men.

Prosecutors disagree, saying that male and female sex offenders are treated equally under the law.

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