The Dunning-Kruger effect is a psychological effect in which people don't realize their level of knowledge on a subject. People who know little about a subject think that they know more than they actually know. People who know a lot about a subject think that they know less than they do. The more you know about a subject, the less you think you know about it. But the more you know about a subject, the better you know how well you know the subject.
This effect was shown in an experiment that was done by Justin Kruger and David Dunning at Cornell University. Their results were published in the Journal of Personality and Social Psychology in December 1999.