 This tree is an example of a game in extensive form. At each stage \(i\), the agent who moves can either choose \(C^i\), which corresponds to helping or cooperating, or \(D^i\), which corresponds to not helping or defecting. The relative preferences of the two agents over the various outcomes are reflected by the ordered pairs of payoffs each receives at any particular outcome. If, for instance, Fiona chooses \(C^i\) and Alan chooses \(D^i\), then Fiona’s payoff is 0, her worst payoff, and Alan’s is 4, his best payoff. In a game such as the Figure 1.1.a game, agents are (Bayesian) rational if each chooses an act that maximizes her expected payoff, given what she knows.