3. Network Reciprocity

In the previous chapter, we discussed Nowak’s five fundamental reciprocity mechanisms for adding social viscosity: kin selection, direct reciprocity, indirect reciprocity, network reciprocity, and group selection. In this chapter, we focus specifically on network reciprocity, as this mechanism has received the most attention in communities of statistical physicists and theoretical biologists who specialize in evolutionary game theory. Since 1992, when the first study of the spatial prisoner’s dilemma (SPD) was conducted by Nowak and May (1992), the number of papers dealing with network reciprocity has increased to several thousand. The main reason for this is that network reciprocity is regarded as the most important and interesting of the mechanisms from an application point of view. In fact, we can observe a lot of evidence in real life of network reciprocity working to establish mutual cooperation not only in human social systems but also in those of other animal species. The network reciprocity mechanism relies on two effects. The first is limiting the number of game opponents (that is, “depressing anonymity,” rather than having an infinite and well-mixed population), and the second is a local adaptation mechanism, in which an agent copies a strategy from a neighbor linked by a network. These two effects explain how cooperators survive in a social dilemma system, even though it requires agents to use only the simplest strategy—either cooperation (C) or defection (D), and this has attracted biologists who guess that network reciprocity may explain the evolution of cooperation even among primitive organisms without any sophisticated intelligence.