Cooperation and emergence of role differentiation in the dynamics of social networks

American Journal of Sociology 110, 977 (2005) Models of cooperation grounded on social networks and on the ability of individuals to choose actions and partners aim to describe human social behavior. Extensive computer simulations of these models give important insight in the social mechanisms behind emerging collective behavior. We consider the entangled co-evolution of actions and social structure in a new version of a spatial Prisoner's Dilemma model that naturally gives way to a process of social differentiation. Diverse social roles emerge from the dynamics of the system: leaders -individuals getting a large payoff who are imitated by a considerable fraction of the population-, conformists -unsatisfied cooperative agents that keep cooperating-, and exploiters -Defectors with a payoff larger than the average one obtained by Cooperators. The dynamics generates a social network that can have the topology of a small world network. The network has a strong hierarchical structure in which the leaders play an essential role sustaining a highly cooperative stable regime. The social structure is shown to be very sensitive to perturbations acting on the leaders. Such perturbations produce social crisis described as dynamical cascades that propagate through the network.