One cognitive bias is the tendency of a person to regard themself as better than average—even when there is no evidence for that belief. Surveys illustrate this bias: most Americans rank themselves as above average in everything ranging from leadership ability to accuracy in self-assessment.

Obviously, most people cannot be better than average—that is just how averages work. As to why people think the way they do, the disparity between what is claimed and what is the case can be explained in two ways. One is another cognitive bias, which is the tendency people have to believe their performance is better than it is. Teachers see this often—students generally believe that they did better on a test or paper than they really did. I have lost count of the number of students who have told me their papers “felt like an A.” I never doubted this; having felt the same thing about various C and D papers I wrote as a student. Given that people regard their own performance as better than it is, it certainly makes sense that they would see their abilities as better than average.

Another reason is yet another bias: people tend to give more weight to the negative over the positive. As such, when assessing other people, we tend to consider negative things about them as more significant than the positive. So, for example, when Sally is assessing the honesty of Bill, she will give more weight to incidents in which Bill was dishonest relative to those in which he was honest. So, Sally will probably see herself as being more honest than Bill. After enough comparisons, she will probably see herself as above average.

This self-delusion probably has some positive effects. It no doubt allows people to maintain a sense of worth. But there are downsides—after all, a person who does not do a good job assessing himself and others will be operating based on inaccurate information and this rarely leads to good decision making.

The better-than-average delusion holds up well even in the face of clear evidence to the contrary. For example, the British Journal of Social Psychology did a survey of British prisoners asking them to compare themselves to other prisoners and the general population in terms of such traits as honesty, compassion, and trustworthiness. Not surprisingly, the prisoners ranked themselves as above average. They did, however, only rank themselves as average when it came to the trait of law-abidingness. This suggests that reality has some slight impact on people, but not as much as one might hope.