The geometrical patterns of cooperation evolution in the spatial prisoner's dilemma: An intra-group model

The prisoner's dilemma (PD) deals with the behavior conflict between two agents, who can either cooperate (cooperators) or defect. If both agents cooperate (defect), they have a unitary (null) payoff. Otherwise the payoff is T for the defector and null for the cooperator. The temptation T to defect is the only free parameter in the model. Here the agents are represented by the cells of a L×L lattice. The agent behaviors are initially randomly distributed according to an initial proportion of cooperators ρc(0). Each agent has no memory of previous behaviors and plays the PD with his/her eight nearest neighbors. At each generation, the considered agent copies the behavior of those who have secured the highest payoff. Once the PD conflict has been established (1<T<2), this system shows that cooperation among agents may emerge even for reasonably high T values giving rise to the well-known strategy: join to conquer, fight to share. Contrary to previous studies, in which the lattice cells are viewed as groups and are allowed to self-interact (inter-group situation), here the cells are viewed as individuals and are not allowed to self-interact (intra-group situation). Although the short time and asymptotic behavior of ρc are similar in both cases, the intermediate behavior is different. Oscillations in the intra-group ρc(t) forbids data collapse. The cooperators clusters geometrical configurations are distinct between inter and intra-group models, which explains the ρc(t) differences.