Fig. 1: People tend to conform with an arbitrary rule against their self-interest, even a stylized, asocial and unenforced rule stated by the experimenter.

a, Experimental task in which people are instructed to follow a rule in a minimalist traffic light setting where the rule read ‘The rule is to wait at the stop light until it turns green’⁵⁵. b, Abstract task setting without a traffic light context, where the rule read ‘The rule is to wait in the grey area until the cross disappears’. In a and b, violating the rule maximizes own earnings without affecting anyone else. Participants must move a circle across the screen by clicking a ‘Move’ button. The endowment indicated on top of the decision screen starts at 20 MU and ticks down by 1 MU per second. c–e, Frequencies of moving patterns and percentages of rule-conformity (green) and rule violations (purple), in settings with control questions (c,e) and without control questions (d) that test the understanding of payoff consequences. The Methods and Supplementary Sections 1.1 and 1.2 report further background details on procedures and the traffic light task and abstract task, respectively. The video TrafficLightTask_nopeers.mov in the Supplementary Information shows how the task looked like to participants.