Intelligence leads to hubris
The research that sheds some light on the way-too-confident middle manager you're constantly battling at work found that intelligence does play a role in confidence… but it's way more complex than "dumb = overconfident." It has more to do with how a person views intelligence, rather than her actual intelligence level.
For starters, there are two basic beliefs about intelligence: entity theory and incremental theory. Incremental theory says that as people learn, they become more intelligent, which means there’s a near-limitless potential for growth if you’re not afraid to work at it.
Entity theory, however, is the idea that everyone is born with a certain amount of intelligence, and that’s it. Or, rather, everyone is born with a set potential for intelligence, because obviously people progress from infant to adult (life isn’t an endless screening of Look Who’s Talking). Sorry if you got the short straw. It’s the people from this second category, the entity-believers, who are prone to overconfidence.