The evolution of extortion in repeated games

In evolutionary biology and the social sciences, repeated games serve as a fundamental tool to describe individual behavior in long-term relationships. When playing a repeated game, subjects can condition their next moves on the outcome of previous interactions, which may give rise to reciprocal cooperation and mutualistic symbioses. However, for the most commonly used metaphor for repeated interactions, the infinitely iterated prisoner`s dilemma, it was recently shown that a sophisticated player could also apply a strategy that permits manipulation and extortion of opponents. Such extortion strategies have three important properties: (1) they ensure that the payoffs of the players are linearly related, (2) they incentivize the co-player to cooperate, and (3) they ensure that any surplus of cooperation is unevenly distributed among the players, such that the co-player`s payoff is lower by a fixed percentage. Evolutionary theory suggests that behaviors that lead to a relative advantage are likely to spread; it is therefore natural to ask whether extortionate behaviors can emerge and succeed in evolving populations. Recent work has shown that their prospects are limited: in reasonably large populations, extortion strategies are not stable and they promote (rather than inhibit) the evolution of cooperation. However, if the population size is small, or if the players originate from distinct populations that evolve at different rates, then extortion is a major force in the evolutionary dynamics. The present project intends to assess the role of extortion strategies in general repeated games. In particular, we will use evolutionary game theory to clarify under which conditions extortion evolves, and what are the consequences for the evolution of cooperation. Moreover, we aim to show that the realm of extortion is not restricted to pairwise interactions; instead, such strategies do also exist for large-scale collective actions, for an arbitrary number of players. In this way, we hope to shed new light on the emergence of conditionally cooperative behaviors in the repeated public goods game and the volunteer`s dilemma. This new angle on repeated games could then be used to re-evaluate existing data on human behavior in social experiments.

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