From a game theoretic point of view, a social norm can sometimes be considered as a Nash equilibrium in a coordination game. Here we point out a psychological reason why such a social norm might erode, even if it is beneficial and agents have rational intentions. The reason is a well-established bias in interpersonal perception, the better-than-average effect. Psychological research on this bias has mainly focused on skills and personality attributes, rather than normative behavior. In a series of online surveys, we demonstrate that the better-than-average effect applies also to judgments of the likelihood to engage in petty corruption, a very important domain of social norms. We conclude that this psychological bias may be a factor that contributes to the difficulty of establishing noncorruption.