The importance of phenotypic defectors in stabilizing reciprocal altruism

At any one time, a population is likely to contain individuals that are eit her permanently incapable of cooperating or temporarily lack the time, ener gy, or resources to allow them to act altruistically. These individuals hav e been called "phenotypic defectors." Mie show that, rather than prevent co operation from emerging, these individuals are extremely important to the s tability of reciprocal altruism because they prevent the drift toward incre asing naivete that is generally associated with highly cooperative environm ents. By exploring a combination of simulation and analytical models, we de monstrate that both permanent and transient phenotypic defectors readily pr event the intermittent collapses of cooperation that have characterized the majority of evolutionary simulations. The incorporation of this natural cl ass of individuals not only suggests that the widespread "bang-bang" dynami cs 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 w orld.