But women are not immune from using excessive force. Ms. Fletcher identified a tendency by some rookie female officers to over-react, trying to live up to a tough image. "The cops call it the Dirty Harriet or Jane Wayne Syndrome," she said.

Many police psychologists say that youth is a major contributing factor to violence on a police force — Los Angeles police psychologist Marty Reiser calls it the "John and Jane Wayne" syndrome — and most view it as a developmental phase.