Figure 1.

Experiment procedure and behavioral results. A, At the beginning of each trial, the participants were (anonymously and ostensibly) paired with 1 of 3 coplayers. Then the participants saw a pain-money pair and waited for the coplayer's decision. If the coplayer chose Help, then the coplayer lost the corresponding amount of bonus while the participants would be exempted from the pain stimulation on this trial. If the coplayer chose NoHelp, then the coplayer could keep the bonus while the participants had to receive the corresponding pain stimulation. The presentation of the coplayer's decision was defined as the critical events in the fMRI data analysis. At the end of the trial, the participant could allocate 20 Yuan (∼$3 U.S.) between himself/herself and the coplayer, with the knowledge that the coplayer was not aware of this procedure. B, C, Postscan gratitude rating and allocation during scanning (i.e., reciprocity) as a function of self-benefit and benefactor-cost. D, Relative weight of benefactor-cost over self-benefit in gratitude rating.