the short of it

A study carried out by Cornell University psychology professor David Dunning concluded that incompetent people do not know that they are incompetent, and actually are more confident that they know what they are doing than competent people. The researchers said that the reason incompetent people do not recognize their own incompetence is because the cognitive skills required to be competent are the same required to recognize competence. Are you following this? If you think you are it could be because you're too incompetent to realize that you don't understand what you just read.

New York Times

And the long of it can be found here.

Justin Kruger & David Dunning. Unskilled and Unaware of It: How Difficulties in Recognizing One's Own Incompetence Lead to Inflated Self-assessments. Journal of Personality and Social Psychology, December 1999 Vol. 77, No. 6, 1121-1134.