Substantial evidence has accumulated in recent empirical works on the limited ability of the Nash equilibrium to rationalize observed behavior in many classes of games played by experimental subjects. This realization has led to several attempts aimed at finding tractable equilibrium concepts...

The evolution of large-scale cooperation among genetic strangers is a fundamental unanswered question in the social sciences. Behavioral economics has persuasively shown that so called "strong reciprocity" plays a key role in accounting for the endogenous enforcement of cooperation. Insofar as...