Fig. 4.

Mean free-riding ratios in the trial following a trial with the indexed number of free riders. Both groups displayed comparable free-riding rates within their groups in (a-b) condition I and II. (c) In condition III, healthy subjects showed significantly higher free-riding rates when all participants defected than when 0, 1, or 2 players free-rode in the preceding trial. Black asterisk: within-group difference; gray asterisk: between-group difference. SEs of each condition are represented as error bars. *P < .05; **P < .01; ***P < .001