  The exclusion of freeriders from common privileges or public acceptance is
widely found in the real world. Current models on the evolution of cooperation
with incentives mostly assume peer sanctioning, whereby a punisher imposes
penalties on freeriders at a cost to itself. It is well known that such costly
punishment has two substantial difficulties. First, a rare punishing cooperator
barely subverts the asocial society of freeriders, and second, natural
selection often eliminates punishing cooperators in the presence of
non-punishing cooperators (namely, "second-order" freeriders). We present a
game-theoretical model of social exclusion in which a punishing cooperator can
exclude freeriders from benefit sharing. We show that such social exclusion can
overcome the above-mentioned difficulties even if it is costly and stochastic.
The results do not require a genetic relationship, repeated interaction,
reputation, or group selection. Instead, only a limited number of freeriders
are required to prevent the second-order freeriders from eroding the social
immune system.
