The Dunning–Kruger effect is a cognitive bias in which "people reach erroneous conclusions and make unfortunate choices but their incompetence robs them of the metacognitive ability to realize it". The unskilled therefore suffer from illusory superiority, rating their own ability as above average, much higher than in actuality; by contrast, the highly skilled underrate their abilities, suffering from illusory inferiority. This leads to a perverse result where less competent people will rate their own ability higher than more competent people. It also explains why actual competence may weaken self-confidence because competent individuals falsely assume that others have an equivalent understanding. "Thus, the miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others."
“ The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt. ”
— Bertrand Russell
The phenomenon was demonstrated in a series of experiments performed by Justin Kruger and David Dunning, then both of Cornell University. Their results were published in the Journal of Personality and Social Psychology in December 1999. However, the phenomenon had been assumed by many philosophers for nearly a century prior to Kruger and Dunning's study (see Russell quote above).
Kruger and Dunning noted a number of previous studies which tend to suggest that in skills as diverse as reading comprehension, operating a motor vehicle, and playing chess or tennis, "ignorance more frequently begets confidence than does knowledge" (as Charles Darwin put it). They hypothesized that with a typical skill which humans may possess in greater or lesser degree,
1. Incompetent individuals tend to overestimate their own level of skill.
2. Incompetent individuals fail to recognize genuine skill in others.
3. Incompetent individuals fail to recognize the extremity of their inadequacy.
4. If they can be trained to substantially improve their own skill level, these individuals can recognize and acknowledge their own previous lack of skill.