A minimum number of rounds per generation is needed for cooperation to prevail. This number can be very small: each player needs to be chosen for only about two interactions per lifetime. (In this case, there is a probability of only 1/4 that a defector can be punished, if he is chosen first as a donor and then as a recipient.)

Builds on: "Frequent interactions per generation favor the evolution of cooperation."

Nowak, M. A., & Sigmund, K, Evolution of Indirect Recip…, loc. 68