 Szabó and Hauert have investigated spatial versions of the optional PD in some detail. Among their findings is that, for a particular (intermediate) range of payoffs, a population of agents playing “pure” strategies on a square lattice will evolove towards a unique equilibrium in which all three strategies are present. This contrasts with the continuous cycles for the non-spatialized versions of the evolutionary optional PD's discussed above. Like the earlier observations, it may help to explain how a group might achieve a state other than universal defection, but not how it might achieve a state of universal cooperation.