Figure 2.

A screenshot of the simultaneous Prisoner’s Dilemma game with costly punishment option (translated version; the version used in the experiment was in German). Participants chose to cooperate or to defect in the Prisoner’s Dilemma game by pressing the up or down arrow key that were marked yellow and blue, respectively. In the example displayed here, the participant on the left cooperates while the partner on the right defects. The participant then invests 3 cents to punish the partner by reducing her account balance by 30 cents. The facial photograph on the right side was taken from the Chicago Face Database⁵¹.