Extra Credit-Serial Killers

For my final paper I am talking about serial killers in film and something I've noticed is that almost all of them are always male. Why is it that men are the sick sadistic ones. Maybe women have more common sense than to do something so risky or their mental states just won't allow them. I'm not sure about a lot of these questions but I still think it's very interesting. Say for instance you walk down an alley late at night and another man is there you'd be freaked out probably, but if it was a woman you'd think nothing of it probably except that she may be a hooker or something. So why is it that there aren't many female serial killers? Just a thought any comments are welcome.