"Kruger and Dunning set out to test... hypotheses on human subjects consisting of Cornell undergraduates who were registered in various psychology courses. In a series of studies, they examined self-assessment of logical reasoning skills, grammatical skills, and humor. After being shown their test scores, the subjects were again asked to estimate their own rank, whereupon the competent group accurately estimated their rank, while the incompetent group still overestimated their own rank. As Dunning and Kruger noted,
Across four studies, the authors found that participants scoring in the bottom quartile on tests of humor, grammar, and logic grossly overestimated their test performance and ability. Although test scores put them in the 12th percentile, they estimated themselves to be in the 62nd.
Meanwhile, people with true knowledge tended to underestimate their competence. A follow-up study suggests that grossly incompetent students improve both their skill level and their ability to estimate their class rank only after extensive tutoring in the skills they had previously lacked.
They won Ig Nobel Prizes in Psychology in 2000 with their report Unskilled and Unaware of It: How Difficulties in Recognizing One's Own Incompetence Lead to Inflated Self-Assessments."
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Also see Downing Effect
