Fig. 1.

Average contributions by period and group type. The graph shows average contributions in the four studies by period and group type (n = 411). Red groups are shown by solid lines and filled symbols; blue groups are shown by dashed lines and hollow symbols. For study 2, parts II–IV and study 3, part II, unconditional contributions from the Regular game are used. The four sections show parts I–IV. Part IV had a varying number of periods across studies. Exact values and SEs are provided in SI Appendix.