Increased costs of cooperation help cooperators in the long run.

It has long been proposed that cooperation should increase in harsh environments, but this claim still lacks theoretical underpinnings. We modeled a scenario in which benefiting from altruistic behavior was essential to survival and reproduction. We used a spatial agent-based model to represent mutual cooperation enforced by environmental adversity. We studied two factors, the cost of unreciprocated cooperation and the environmental cost of living, which highlight a conflict between the short- and long-term rewards of cooperation. In the long run, cooperation is favored because only groups with a sufficient number of cooperators will survive. In the short run, however, harsh environmental costs increase the advantage of defectors in cooperator-defector interactions because the loss of resources leads to death. Our analysis sheds new light on the evolution of cooperation via interdependence and illustrates how selfish groups can incur short-term benefits at the cost of their eventual demise. We demonstrate how harsh environments select for cooperative phenotypes and suggest an explanation for the adoption of cooperative breeding strategies in human evolution. We also highlight the importance of variable population size and the role of socio-spatial organization in harsh environments.