"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. " - Fred Yankowski