# Debunking the Dunning-Kruger effect – the least skilled people know how much they don’t know, but everyone thinks they are better than average

John Cleese, the British comedian, once summed up the idea of the Dunning–Kruger effect as, “If you are really, really stupid, then it’s impossible for you to know you are really, really stupid.” A quick search of the news brings up dozens of headlines connecting the Dunning–Kruger effect to everything from work to empathy and even to why Donald Trump was elected president.

As a math professor who teaches students to use data to make informed decisions, I am familiar with common mistakes people make when dealing with numbers. The Dunning-Kruger effect is the idea that the least skilled people overestimate their abilities more than anyone else. This sounds convincing on the surface and makes for excellent comedy. But in a recent paper, my colleagues and I suggest that the mathematical approach used to show this effect may be incorrect.

## What Dunning and Kruger showed

In the 1990s, David Dunning and Justin Kruger were professors of psychology at Cornell University and wanted to test whether incompetent people were unaware of their incompetence.

To test this, they gave 45 undergraduate students a 20-question logic test and then asked them to rate their own performance in two different ways.

First, Dunning and Kruger asked the students to estimate how many questions they got correct – a fairly straightforward assessment. Then, Dunning and Kruger asked the students to estimate how they did compared with the other students who took the test. This type of self-assessment requires students to make guesses about how others performed and is subject to a common cognitive mistake – most people consider themselves better than average.

Research shows that 93% of Americans think they are better drivers than average, 90% of teachers think they are more skilled than their peers, and this overestimation is pervasive across many skills – including logic tests. But it is mathematically impossible for most people to be better than average at a certain task.

After giving students the logic test, Dunning and Kruger divided them into four groups based on their scores. The lowest-scoring quarter of the students got, on average, 10 of the 20 questions correct. In comparison, the top-scoring quarter of students got an average of 17 questions correct. Both groups estimated they got about 14 correct. This is not terrible self-assessment by either group. The least skilled overestimated their scores by around 20 percentage points, while the top performers underestimated their scores by roughly 15 points.

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The results appear more striking when looking at how students rated themselves against their peers, and here is where the better-than-average effect is on full display. The lowest-scoring students estimated that they did better than 62% of the test-takers, while the highest-scoring students thought they scored better than 68%.

By definition, being in the bottom 25% means that, at best, you will score better than 25% of people and, on average,…