Know when to walk away: contingent movement and the evolution of cooperation.

Models of the evolution of cooperation suggest that an important characteristic of successful strategies is the ability to respond contingently to the social environment. A number of mechanisms by which this can be accomplished have been suggested, some of which require relatively complex information processing systems. This research explores relaxing the requirements on information processing while preserving the evolvability of a cooperative strategy. The agent-based computer simulations reported here show that 'Walk Away,' a behavioral rule of extremely limited complexity (move after partner defects), can outperform more complex strategies under a number of conditions. Previous simulations of exit strategies have not examined the effect of implicit and explicit movement costs, different error rates, or the simultaneous presence of TFT and PAVLOV. The simulations reported here establish that the Walk Away strategy resists invasion and can invade a population of defectors at a lower initial frequency than any other strategy. The Walk Away strategy was successful, despite its simplicity, because it exploited aspects of the physical and social environment.