I took a class in business school that fundamentally changed the way I think about a lot of things. The core premise of the class (Managerial Decision Making class taught by Thaler) is that we are subject to a lot of biases by virtue of being humans.
One of these biases is overconfidence. He surveyed the class asking how many thought they would get an above average grade in the class. It was about 75%. He also asked how many thought they would be in the top decile. About 20%. Obviously neither of these situations is possible. This bias holds true outside an MBA classroom (clearly a biased sample). Consider asking people if they are an above average driver.