Title: A systems approach to the evolution of cooperation
Description: The field of systems biology emphasizes the integration of theoretical and experimental approaches. In this dissertation, I apply that methodology to the evolution of cooperation. Cooperation, where one pays a cost to confer a benefit on another, is a fundamental building block of all life on earth and particularly of human societies. Yet cooperation poses an evolutionary puzzle: how can natural selection support such seemingly altruistic behavior? Here I present a collection of evolutionary game theoretic models and behavioral experiments that explore questions related to the origins and maintenance of cooperation in humans. Chapters 1, 2, and 3 address the role of costly punishment in promoting cooperation. Chapter 1 describes an experimental study of a repeated three option Prisoners Dilemma in which pairs of subjects can cooperate, defect, or punish every round. The option for punishment does not improve the average payoff relative to a control where subjects can only cooperate or defect. Moreover, there was a strong negative correlation between an individuals payoff and her propensity to punish defection. Chapter 2 describes an evolutionary computer simulation of the game from Chapter 1. The model quantitatively reproduces the human behavior observed in the experiment, and finds that natural selection disfavors the use of costly punishment for all model parameters that were explored. Chapter 3 describes an experimental study of the repeated four player public goods game, comparing reward and punishment. Both reward and punishment work equally well to stabilize cooperation when added to the public goods game, and reward leads to much higher average payoffs.

Together these chapters challenge to contention that costly punishment played a central role in the evolution of human cooperation. Chapter 4 describes an evolutionary model of the centipede game, a fixed length cooperation game. While human behavior in centipede game experiments sharply contradicts the predictions of classical game theory, the evolutionary model presented here quantitatively reproduces the observed human behavior. Chapter 5 describes a behavioral experiment demonstrating that in-group bias can be remodeled over time in the face of a changing social environment. This raises important challenges for existing theoretical models, which have only considered static groupings.