Previously I pointed out an old article from 2000 about research that suggests that incompetence can lead to self-confidence because the incompetent are to incompetent to realize they aren't competent (got that)? Well, Max Sawicky found a quote that concisely covers the bases on this matter:

"The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt." -- Bertrand Russell

Thanks Max. Update: Dre in the comments adds this:

"The best lack all conviction, while the worst Are full of passionate intensity. " -- Yeats