Why Some Representations Are More Cooperative Than Others For Prisoner´s Dilemma

In the work of D. Ashlock et al. (2006) it was shown that the representation used has a large impact on the cooperativeness of evolved prisoner´s dilemma strategies. Why is this? This paper examines the look-up table representation, the finite state machine representation, and the neural net representation to gain insight into this somewhat surprising result. A tool called a prisoner´s dilemma fingerprint is used to compare the strategies produced by the different representations, and a Voronoi tiling (based on which of 12 reference strategies is the closest neighbor) of the strategy space is done. The initial random populations are shown to have significantly different distributions, and the evolved populations are shown to favor different parts of the strategy space