  Experimental evidence suggests that human decisions involve a mixture of
self-interest and internalized social norms which cannot be accounted for by
the Nash equilibrium behavior of Homo Oeconomicus. This led to the notion of
strong reciprocity (or altruistic punishment) to capture the human trait
leading an individual to punish norm violators at a cost to himself. For a
population with small autonomous groups with collective monitoring, the
interplay of intra- and intergroup dynamics shows this to be an adaptive trait,
although not fully invasive of a selfish population. However, the absence of
collective monitoring in a larger society changes the evolution dynamics.
Clustering seems to be the network parameter that controls maintenance and
evolution of the reciprocator trait.
