Living in a group is a cooperative strategy. Animals live in groups only if groups yield a net advantage compared to living solitarily. Group living inherently implies competition, however: one's worst rivals in the competition over food or mates are the conspecifics with the same needs and desires. The same is true for human social networks: common interests often go hand in hand with conflicts over the partitioning of utilities or duties. Thus both humans and non-humans are frequently caught in ‘social dilemmas’ and face similar problems: to find the optimal balance between contribution and exploitation and to keep cheaters and free-riders in check. Animals use strategies shaped by selection in the course of evolution when they deal with these challenges. Humans have the alternative option of using their cognitive capacities to plan their strategies rationally.

In the first chapter of Part I Elinor Ostrom, who has her roots in the social sciences, reviews the vast body of empirical as well as theoretical knowledge on the behaviour of humans caught in social dilemmas. She finds that humans tend to behave more cooperatively in social dilemmas than theory would predict. Charles Nunn and Rebecca Lewis look at social dilemmas with the eyes of evolutionary biologists. In this field the emphasis has traditionally been on dyadic interactions. Nunn and Lewis therefore first show how dyadic game theoretical models can be developed into the n-player models that are more meaningful to the study of social dilemmas.