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,...

In experimental games, many subjects cooperate contrary to their material interest and they do that in a reciprocal manner. In addition, many subjects punish those others who behave unkindly, and previous history usually influences subjects’ choices. We propose a simple game-theoretical model...