Figure 2

The figure shows defection rates across experimental conditions. The rates hardly differ without a penalty imposed on punished defectors. With a penalty, the rates drop significantly to levels that are inversely proportional to the punishment levels in the respective conditions. Except for the difference between the asymmetric MHD and the symmetric VOD, all differences between defection rates in the conditions with penalty are statistically significant. This shows that groups with an unequal distribution of punishment costs are more effective in deterring defections than groups of all equals. See section S2 in the SI for further details on the data analysis.