mean
I recently read the excellent Judgment under uncertainty edited by Kahneman, Tversky and Slovic. I was particularly struck by a simple observation that's mentioned in a couple of the chapters.
Suppose you're a parent or manager or teacher. When one of your charges does something particularly good, you're likely to respond positively (praise, reward, promotion, ...). When they do something particularly bad, you're likely to respond negatively.
Now, suppose that your positive and negative responses are equally effective (or ineffective) in producing improved performance. Because of regression to the mean, they'll tend to do worse after a really good performance and better after a really bad one. Therefore, they'll tend to do worse after you respond positively and better after you respond negatively.
Therefore, we are all repeatedly exposed to misleading evidence that tends to make us think that negative responses are more effective than they are, and positive responses less effective.
(The Wikipedia article on regression to the mean quotes Kahneman making the same point. He calls the moment when he noticed it "the most satisfying Eureka experience of my career".)