Why do we tend to remember ugly faces more easily than attractive faces?

This may seem like a politically incorrect question, but I have always wondered why it’s easier to remember the faces of ugly people than the faces of attractive people. For example, if you’re a guy and you see a pretty girl for the first time, it’s hard to remember what she looked like even a few hours later, whereas if you saw a plain or unattractive girl, you can remember that face for days! Why is that?