Aversion to norm-breaking: A model

In experimental games, we observe the following phenomena: (1) many subjects cooperate contrary to their material interest, (2) they cooperate in a reciprocal manner, (3) subjects often punish those others who behave unkindly, and (4) previous history usually influences subjects' choices. We propose a simple game-theoretical model to account for these and other experimental phenomena, and compare it with other models of social preferences and reciprocity.