Evolution of altruism in optional and compulsory games.

In "optional" variants of the iterated prisoner's dilemma, players may choose whether or not to participate. Members of evolving populations playing optional variants of the iterated prisoner's dilemma by following inherited strategies tend to cooperate more than do members of populations playing the standard, "compulsory" version. This result is due to dynamical properties of the evolving systems: the populations playing the compulsory game can become stuck in states of low cooperation that last many generations, while the optional games provide routes out of such states to states of high cooperation. Computational simulations of the evolution of populations playing these games support these analytic results and illustrate the interactions between the genetic representation of strategies and the composition of populations in which those strategies are deployed.