After the instructions to the game were explained, all pictures were presented one at a time to the entire group. While the pictures were being presented, each participant played in the role of the Investor with the pictured participant and was endowed with

$10 for the round. After making an investment on the round, they were then asked how much of this amount (multiplied by 4) they believed their partner would return to them. At the end of the session, participants were paid $5 for their participation. A subset of participants (n = 17) were recruited from Session 1 to participate in the second session, in which they played the TG in the role of the Trustee while being scanned ISRIB cost using functional magnetic resonance imaging (fMRI). Each participant had an individually tailored paradigm, in which they decided how much money they wanted to return to the other participants in the experiment, based on these partners’ actual proposals to them from Session 1. Each participant played a total of 28 rounds, distributed over four runs. Each run lasted exactly 7 min including an extra 14 s fixation cross display at the beginning of the run to allow for T1 equilibrium, and another 21 s fixation cross at the end of the

run (210 volumes per run). The timeline of events in a typical round can be seen in Figure 1B. The stimuli were presented using E-Prime software via VisuaStim goggles (Resonance Technologies Inc, IL, USA), and participants DNA ligase indicated their answers by using a two-button fiber optic response Androgen Receptor Antagonist purchase box. Responses changed in 10% increments on each button press. These increments were randomly selected to either increase from $0 or decrease from the maximum amount of money for that round (which varied depending on how much had been sent by the partner), ensuring that the number of button presses was orthogonal to the amount of money selected, removing effects of any motor confounds. After participants selected their chosen amount of money, they used the second button to confirm this response. After participants completed scanning, they rated their

counterfactual guilt by indicating on a 7-point Likert scale the amount of guilt they believed they would have experienced had they returned a different amount of money, and were then paid a $20 participation fee. Finally, at the conclusion of the entire experiment, all participants were paid 50% of their earnings for one randomly selected trial. If participants participated in both sessions, they were paid for two separate trials. Participants in the first session that correctly predicted their partner’s behavior for the trial selected received an additional $2 bonus (Charness and Dufwenberg, 2006 and Dufwenberg and Gneezy, 2000). Only identification numbers were provided at the time of payment, thus ensuring that Trustees’ responses were completely anonymous. No deception was employed in this study.