A simple rule for the evolution of cooperation on graphs and social networks

Your cheating art The evolution and maintenance of cooperative behaviour take some explaining. Cooperative groups can be undermined by ‘cheaters’ who selfishly exploit common resources, and a large body of theory predicts that cheats will usually displace cooperators. But a possible explanation of why cheats don't always prosper emerges from competition experiments between strains of yeast that act as cooperators and cheaters, competing for glucose and utilizing it either efficiently or ‘selfishly’. The results show that both strategies can coexist, because both are associated with costs and benefits. There is a cost to cheating; in this instance the production of fewer offspring than the opposition. A graphic — really — demonstration that natural selection can favour cooperation comes in a study by Ohtsuki et al. of the evolutionary dynamics of structured ‘virtual’ populations formed of points on a graph. Cooperation is favoured if the benefit of the altruistic act divided by the cost exceeds the average number of neighbours. So cooperation can evolve as a consequence of this ‘social viscosity’ even in the absence of reputation effects or strategic complexity.