justin cornell

BIAS OF THE MONTH: The Dunning-Kruger Effect

In 1995, a man named McArthur Wheeler robbed two banks in one day. He was confident that he would not be identified because he had covered his face in lemon juice, which, as it can be used as invisible ink and only become visible when in contact with heat, he felt sure would make his face invisible to security cameras. The police played the surveillance footage on the news that evening and, within hours, Wheeler was identified and arrested.

This fantastic tale prompted David Dunning and Justin Kruger at Cornell University to study the phenomenon of low-skilled individuals being unaware of quite how unskilled they are, suffering from an illusory superiority which leads them to believe their ability is far greater than it actually is; a bias they called the Dunning-Kruger effect. In other words, these people are too incompetent to realise how incompetent they are, and they also tend not to recognise genuine skill when it is exhibited in others.


In a series of experiments, Dunning and Kruger tested participants’ logical reasoning, grammatical skills and humour. Participants were then shown their test scores, and asked to estimate their relative rank on that skill. As hypothesised, participants who scored poorly tended to overestimate their rank, and dramatically so; those ranking in the 12thpercentile estimated they were in the 62nd. Interestingly, high-skilled individuals often underestimated their relative ability, assuming that everyone must have found the task as straightforward as they did.

This bias is greatly reduced when those suffering from the Dunning-Kruger effect are given basic training in the relevant skillset, even when this has very little impact on their ability, suggesting that a lack of exposure and feedback is to blame. But of course, afflicted individuals won’t realise there is an issue at all, let alone take action; and if this doesn’t bring anyone you know to mind, then it’s probably you.

Kruger, J., & Dunning, D. (1999). Unskilled and Unaware of It: How Difficulties in Recognizing One’s Own Incompetence Lead to Inflated Self-Assessments. Journal of Personality and Social Psychology, 77(6), 1121-34.