> I'm not convinced that there are really two distinct sets of people in the world: stupid and not stupid. In my experience, most people are pretty good at some things and stupid with other things.

I've read a quite thorough explanation similar to this in one of Scott Adams' books : most people are somewhat intelligent, but behave stupidly at times - In fact, we may assume most people behave stupidly most of the time. Maybe he called that the "Dogbert principle" or something similar.