Credit: Tarnita et al.
Evolutionary set theory is a powerful modeling framework that can be used to study how cooperation is affected by population structure. In this framework, people belong to groups and can simultaneously have multiple group affiliations. They interact only with others who are in the same groups. These interactions assume the form of the “donation game” (a modified version of the prisoner’s dilemma). In this example, there are 10 people and 5 groups. Figure originally appeared in Tarnita, et al. 2009 PNAS.