Cooperative behavior cascades in human social networks

Fowler and Christakis, in an open access article in PNAS, demonstrate that one cooperative act has a multiplying effect. In an iterative game, when a subject gives money to help someone else, the recipients of that cash then became more likely to give their own money away in the next round. This leads to a cascade of generosity, in which the itch to cooperate spreads first to three people and then to the nine people that those three people interact with, and then to the remaining individuals in subsequent waves of the experiment. (Uncooperative behavior can spread and persist as well.)  Here is one of their figures. 

Figure - A hypothetical cascade. This diagram illustrates the difference between the spread of the interpersonal effects across individuals and the persistence of effects across time. We abstract from the numerous interactions that take place between individuals in these experiments to focus on a specific, illustrative set of pathways. Cooperative behavior spreads three degrees of separation: if Eleni increases her contribution to the public good, it benefits Lucas (one degree), who gives more when paired with Erika (two degrees) in period 2, who gives more when paired with Jay (three degrees) in period 3, who gives more when paired with Brecken in period 4. The effects also persist over time, so that Lucas gives more when paired with Erika (period 2) and also when paired with Lysander (period 3), Bemy (period 4), Sebastian (period 5), and Nicholas (period 6). There is also persistence at two degrees of separation, because Erika givesmore not only when paired with Jay (period 3) but also when paired with Harla (period 4) and James (period 5). All the paths in this illustrative cascade are supported by significant results in the experiments, and it is important to note that if Eleni decreases her initial contribution, her uncooperative behavior can spread and persist as well.