Mean expenditure of a given punishing member as a function of the deviation between her contribution minus that of the punished member, for all pairs of subjects within a group, as reported empirically [25], [26], [59].

<p>The error bars indicate standard error around the mean. The straight line crossing zero shows the average decision rule for punishment that our agents spontaneously evolve to at long times. Its slope defines the average propensity to punish (see the main text). The anomalous punishment of cooperators, corresponding to the positive range along the horizontal axis, is not considered in our model. The inset shows the relative frequency of the pairwise deviations.