Adaptation-Level Phenomena and the Prevalence of Cooperation

Abstract

We conducted a computer simulation study to examine the relation between an individual-level decision process and the prevalence of self-sacrificial cooperation in large groups of interacting individuals. In each simulated interaction, two randomly paired individuals played a single round of the prisoner's dilemma. Individual decisions about whether or not to cooperate were made on the basis of a simple rule known as win-stay, lose-change. For each individual, wins and losses were defined in relation to a simulated adaptation level. The rate of adaptation was varied systematically, and we made a distinction between adaptation to gains and adaptation to losses. We found that the overall prevalence of cooperation was determined mainly by the rate of downward adaptation, whereas the rate of upward adaptation had very little effect. Moreover, we found a strongly negative relation between the prevalence of cooperation and the rate of downward adaptation. This latter finding is particularly significant in view of evidence that people generally adapt to losses slowly, for it suggests that slow downward adaptation at the individual level may fuel the