Why is baldness so often ociated with evil in Hollywood?

My friend made a joke one time when I was accidentally short a dollar while paying at a restaurant. He said, "It's always the bald guy." I realized that in Hollywood movies, at least, he's right. When you see bald guys in movies, they are almost always villains. Not always, you get your Bruce Willis, Jason Statham, and Samuel L. Jackson types in good guy roles now and then, but most of the time the bald guys you see in movies are depicted as evil. Why is that?