Prior studies have shown that selfish behavior is reduced when co–players have the opportunity to approve/disapprove a player’s choice, even if that has no consequences on the player’s material payoff. Using a prisoner’s dilemma, we experimentally study the causes of this phenomenon, which seems crucial to understand compliance with social norms. Our data is consistent with a model based on the assumption that people feel badly if they expect to be disapproved by others. Furthermore, we find suggestive evidence in line with the following assumptions: (i) People become more aware about the others opinion if feedback is available, and (ii) even if the feedback is ex post and has no effect on their ex ante expectations about disapproval, people prefer not to receive negative feedback.