Fig. 5: Trial setup.

Each phase began with a presentation of a figure indicating whose turn it was. This was followed by a fixation cross and then the two pictures indicating the choice options were shown. In the observation phase, first the pictures for the choice options were shown, followed by a fixation cross of jittered duration before the demonstrator’s choice was shown, indicated by the fixation cross tilting 20° towards the left or right (to indicate left or right choice). Jittered durations were randomly sampled from a Poisson distribution, with removal of durations outside of the wanted range. Jittered durations ranging between 1 and 7 s were drawn from a distribution with a mean of 2, and durations ranging between 2 and 5 s were drawn from a distribution with a mean of 2.5. In the action phase, the pictures were shown for a maximum of 2 s, and the presentation terminated when the participant made their choice. The participant’s choice was indicated as for the demonstrator, with the cross tilting 20° towards the left or right, but without being preceded by a neutral fixation cross. In both phases, the fixation cross was then shown in neutral position before the outcome of the choice was indicated on the screen with a T-shaped symbol for the shock and the same symbol turned upside down when there was no shock. During the action phase, an electric shock was delivered to the participant at the onset of the presentation of the symbol for shock. No shock was administered during the observation phase or during training.