Conditional cooperation can hinder network reciprocity

Social networks are ubiquitous in our internet-dominated culture, and the new virtual social networks have a lot in common with traditional social networks (e.g., friends and colleagues). In all cases, it is important who interacts with whom. Lately, there has been much theoretical and empirical work on how the behavior of individuals can be influenced by their social ties and how ties are distributed within a network. Theoretical findings of the effects of such population structures on cooperative behavior in humans have been inconsistent (1⇓⇓–4), and empirical tests have lacked—from a theoretical point of view—sufficient system sizes. Gracia-Lazaro et al. (5) have now conducted an impressively large-scaled experiment of 1,229 humans simultaneously playing a prisoner’s dilemma in two different network environments. Specifically, they test whether homogenous or heterogeneous networks—both static environments—influence cooperative behavior (Fig. 1).