Co-Evolution in Iterated Prisoner's Dilemma with Intermediate Levels of Cooperation: Application to Missile Defense

In a world torn by conflict, how can mutually beneficial cooperation can emerge spontaneously, with no higher authority than the rule of the jungle? History, economics, and especially biology provide many real-world examples of mutual cooperation without any enforcement from a higher authority. This cooperation occurs despite a bloodthirsty option offering more payoff to those might exploit their cooperative partners. While it’s clearly a good thing to keep such a partnership going, there are many question about how such cooperative partnerships are formed and maintained. In order to study how this kind of cooperation comes about, many researchers use the abstract game of Iterated Prisoner’s Dilemma (IPD). Normally, the 2-choice game is studied, where players can choose only between full cooperation or full defection. Some studies have looked at intermediate choices between these two extremes. They found that intermediate choices made full cooperation less likely to dominate, but did not explain why this is so. This paper examines why intermediate choices make full cooperation less likely. This question is relevant to the proposed National Missile Defense, which (its opponents claim) would allow intermediate choices between full peace and all-out war, and thus make a partial nuclear war more likely.