You may not know about the Dunning-Kruger effect, but it explains how hard it can be to get underperforming employees to improve. The Dunning-Kruger effect is a cognitive bias that causes people to overestimate their own skill level and fail to recognize other’s genuine skills. The effect occurs whether you’re looking at physical skills (like playing tennis or golf) or mental skills (like reading comprehension or chess). In one study by David Dunning and Justin Kruger of humor, grammar, and logic, participants in the 12th percentile estimated that they were in the 62nd percentile.

Bryan Garner, in the March 2013 ABA Journal, used the effect to explain why lawyers who can’t write well think that they can. Paul Krugman in the New York Times used it to explain “experts” whose economic predictions prove wrong time after time.

The application to human resources is clear. Poor performers don’t know they’re poor performers. And they aren’t going to figure it out from stern looks or exasperated sighs. They need clear, consistent feedback on what’s expected of them.

As I’ve said before, if you’re evaluating employees, you’re not doing them any favors by glossing over their shortcomings. Supervisors are paid to help their workers perform at their highest level and they can’t do that without letting people know where they can improve. Because of the Dunning-Kruger effect, you can’t rely on poor performers to figure it out on their own.