I have long been interested in the question of how cooperation can emerge in a world of egoists without central authority. Over a period of five years, culminating in my book on the Evolution of Cooperation (1984), I published a series of studies that explored the emergence and maintenance of cooperation in the context of the two-person iterated Prisoner's Dilemma (PD). I was well aware that two-person interactions can tell only part of the story of cooperation in societies, so I thought about various ways of building and sustaining cooperation when one person's actions can affect many others. Unfortunately, the most straightforward way to extend the two-person PD game would not sustain cooperation unless something else was added to solve the collective action problem by preventing free riders.