This post completely takes the wind out of the sails of a post I was planning to make on 'Self-Induced Biases' where one mistakes the environment one has chosen for themselves as being, in some sense, 'typical' and then derives lots of bad mental statistics from this. Thus, chess fanatics will tend to think that chess is much more popular than it is, since all their friends like chess, disregarding the fact that they chose those friends (at least partly) based on a commonality of interests.

A worse case is when the police start to think that everyone is a criminal because that's all they ever seem to meet.