An old study, but would explain Bush.
Why the ignorant are blissful
Inept individuals ooze confidence, study finds
by Erica Goode, New York Times
snip
People who do things badly, Dunning has found in studies conducted with a graduate student, Justin Kruger, are usually supremely confident of their abilities -- more confident, in fact, than people who do things well.
snip
One reason that the ignorant also tend to be the blissfully self-assured, the researchers believe, is that the skills required for competence often are the same skills necessary to recognize competence.
The incompetent, therefore, suffer doubly, they suggested in a paper appearing in the December issue of the Journal of Personality and Social Psychology.
"Not only do they reach erroneous conclusions and make unfortunate choices, but their incompetence robs them of the ability to realize it," wrote Kruger, now an assistant professor at the University of Illinois, and Dunning.
http://www.zenspider.com/RWD/Thoughts/Inept.html