The evolution of degrees of cooperation.

The Prisoner's Dilemma has been widely studied as a model for the evolution of cooperation, and most of this work has dealt with agents who either cooperate or not. In this paper we look at the consequences of allowing agents to have intermediate levels of cooperation, and to update these levels over time. The familiar strategy of "tit for tat" emerges as a robust mode of behaviour, yet there are important differences between this case and that of "all or nothing" cooperation.