Figure 1.

Experimental design. Participants repeatedly had to drag a ball to either the blue or orange box. Whether the decision had real financial consequences for either the participant (‘me’ block), another person (‘other person’ block), or both (‘me vs. other person’ block) changed across rounds. In the ‘free’ part, participants freely decided to drag the ball in either box, whereas in the ‘rule’ part a simple rule was given to the participant. In the ‘rule’ part, half of the participants were instructed to always place the ball in the blue box, whereas the other half was instructed to always place it in the orange box. The rule would be ‘always place the ball in above the orange box’ in the illustrated case had positive, negative or neutral consequences, or demanded to take the selfish or pro-social option.