Forgiveness Is an Adaptation in the Iterated Prisoner's Dilemma with Memory

The prisoner’s dilemma (PD) is used to represent many real-life phenomena, whether from the civilized world of humans or from the wild world of other living things. Researchers working on the iterated prisoner’s dilemma (IPD) with limited memory inspected the outcome of different forgetting strategies in a homogeneous environment, within which all agents adopt the same forgetting strategy at the same time. In this paper, with the intention to represent real life more realistically, we improve existing forgetting strategies, offer new ones, conduct experiments in a heterogeneous environment that contains mixed agents and compare the results with previous research, as well as conduct experiments in a homogeneous environment via agent-based stochastic simulations. Our findings show that the outcome depends on the type of environment and is just the opposite for homogeneous and heterogeneous ones, opposing the existing literature on the IPD. Consequently, forgetting and forgiving defectors is the supreme memory management strategy in a competitive, heterogeneous environment. Therefore, forgiveness is an adaptation.