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Morality Games 291 Fig. 1 The Prisoner’s Dilemma. Player 1’s available C D strategies (C and D, which stand for cooperate and defect, respectively) are represented as rows. Player 2’s available strategies (also C b-c -C C and D) are represented as columns. Player 1’s payoffs are represented at the intersection of each row and column. For example, if player 1 plays D and player 2 plays C, player 1’s payoff is b b. The Nash equilibrium of the game is (D, D). It is indicated with a circle Game theory has traditionally been applied in situations where players are ratio- nal decision makers who deliberately maximize their payoffs, such as pricing decisions of firms (Tirole, 1988) or bidding in auctions (Milgrom & Weber, 1982). In these contexts, behavior is expected to be consistent with a Nash equilibrium, otherwise one of the agents—who are actively deliberating about what to do— would realize she could benefit from deviating from the prescribed strategy. However, game theory also applies to evolutionary and learning processes, where agents do not deliberately choose their behavior in the game, but play according to strategies with which they are born, imitate, or otherwise learn. Agents play a game and then “reproduce” based on their payoffs, where reproduction represents off- spring, imitation, or learning. The new generation then play the game, and so on. In such settings, if a mutant does better (mutation can be genetic or can happen when agents experiment), then she is more likely to reproduce or her behavior imitated or reinforced, causing the behavior to spread. This intuition is formalized using mod- els of evolutionary dynamics (e.g., Nowak, 2006). The key result for evolutionary dynamic models is that, except under extreme conditions, behavior converges to Nash equilibria. This result rests on one simple, noncontroversial assumption shared by all evolutionary dynamics: Behaviors that are relatively successful will increase in frequency. Based on this logic, game theory models have been fruitfully applied in biological contexts to explain phenomena such as animal sex ratios (Fisher, 1958), territoriality (Smith & Price, 1973), coop- eration (Trivers, 1971), sexual displays (Zahavi, 1975), and parent—offspring con- flict (Trivers, 1974). More recently, evolutionary dynamic models have been applied in human contexts where conscious deliberation is believed to not play an important role, such as in the adoption of religious rituals (Sosis & Alcorta, 2003), in the expression and experience of emotion (Frank, 1988; Winter, 2014), and in the use of indirect speech (Pinker, Nowak, & Lee, 2008). HOUSE_OVERSIGHT_015503