We propose an axiomatic approach for equilibrium selection in the discounted, infinitely repeated symmetric Prisoner's Dilemma. Our axioms characterize a unique selection criterion that is also useful as a tool for applied comparative statics exercises as it results in a critical discount factor δ* strictly larger than δ, the standard criterion that has often been used in applications. In an experimental test we find a strong predictive power of our proposed criterion. For parameter changes where the standard and our criterion predict differently, changes in observed cooperation follow predictions based on δ*.

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