One day in 1995, a large, heavy middle-aged man robbed two Pittsburgh banks in broad daylight. He didn’t wear a mask or any sort of disguise. And he smiled at surveillance cameras before walking out of each bank. Later that night, police arrested a surprised McArthur Wheeler. When they showed him the surveillance tapes, Wheeler stared in disbelief. “But I wore the juice,” he mumbled. Apparently, Wheeler thought that rubbing lemon juice on his skin would render him invisible to videotape cameras. After all, lemon juice is used as invisible ink so, as long as he didn’t come near a heat source, he should have been completely invisible.

Police concluded that Wheeler was not crazy or on drugs—just incredibly mistaken, The saga caught the eye of the psychologist David Dunning at Cornell University, who enlisted his graduate student, Justin Kruger, to see what was going on. They reasoned that, while almost everyone holds favorable views of their abilities in various social and intellectual domains, some people mistakenly assess their abilities as being much higher than they actually are. This “illusion of confidence” is now called the “Dunning-Kruger effect,” and describes the cognitive bias to inflate self-assessment.

To investigate this phenomenon in the lab, Dunning and Kruger designed some clever experiments. In one study, they asked undergraduate students a series of questions about grammar, logic, and jokes, and then asked each student to estimate his or her score overall, as well as their relative rank compared to the other students. Interestingly, students who scored the lowest in these cognitive tasks always overestimated how well they did—by a lot. Students who scored in the bottom quartile estimated that they had performed better than two-thirds of the other students!

This “illusion of confidence” extends beyond the classroom and permeates everyday life. In a follow-up study, Dunning and Kruger left the lab and went to a gun range, where they quizzed gun hobbyists about gun safety. Similar to their previous findings, those who answered the fewest questions correctly wildly overestimated their knowledge about firearms. Outside of factual knowledge, though, the Dunning-Kruger effect can also be observed in people’s self-assessment of a myriad of other personal abilities. If you watch any talent show on television today, you will see the shock on the faces of contestants who don’t make it past auditions and are rejected by the judges. While it is almost comical to us, these people are genuinely unaware of how much they have been misled by their illusory superiority.