The importance of phenotypic defectors in stabilizing reciprocal altruism

At any one time, a population is likely to contain individuals that are either permanently incapable of cooperating or temporarily lack the time, energy, or resources to allow them to act altruistically. These individuals have been called ‘‘phenotypic defectors.’’ We show that, rather than prevent cooperation from emerging, these individuals are extremely important to the stability of reciprocal altruism because they prevent the drift toward increasing naivete that is generally associated with highly cooperative environments. By exploring a combination of simulation and analytical models, we demonstrate that both permanent and transient phenotypic defectors readily prevent the intermittent collapses of cooperation that have characterized the majority of evolutionary simulations. The incorporation of this natural class of individuals not only suggests that the widespread ‘‘bangbang’’ dynamics are a modeling artifact, but also highlights the need to reconsider the types of cooperative strategy that we should expect to see in the natural world. Key words: cooperation, phenotypic defectors, Prisoner’s Dilemma, reciprocal altruism. [Behav Ecol 12:313–317 (2001)]