 The “geographical” aspect of SPD's need not be taken too literally. For social applications, and probably even for many biological ones, there seems to be no motivation for any particular geometrical arrangement. (Why not a “honeycomb,” for example, where each agent has six neighbors, rather than a grid where each agent has four or eight?) The interest in SPDs presumably lies in the insight that my “neighborhood” of interaction and my “neighborhood” of comparison, is much smaller than the population as a whole even if it turns out not to be limited by details of physical geography. Nevertheless SPD models of the evolution of cooperation in particular geometrical arrangements have given us some suggestive and pretty pictures to contemplate. Several examples are accessible through the links at the end of this entry.