Generally, it is people with a lower ability to understand the world that overestimate their own intelligence because they are unable to imagine anyone being smarter than them, and those on the smarter end who underestimate their own intelligence because they believe everyone else must be able to see what they are able to see.

There was a paper, quite famous now, on the topic titled "Unskilled and Unaware of It: How Difficulties in Recognizing One's Own Incompetence Lead to Inflated Self-Assessments"