Spatial Prisoner's Dilemma

In Nowak & May (1992), placing the Prisoner's Dilemma on a spatial lattice allows cooperators to survive by forming clusters. Each cell copies the strategy of its most successful neighbour. With b > 1 (temptation to defect) but not too large, spatial structure sustains cooperation that would collapse in a well-mixed population. Kaleidoscopic patterns emerge from simple local rules.