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Cooperation and Group Size in the N-Person Prisoners' Dilemma

This study explores the relationship between group size and member cooperation when individual and group interests conflict. Existing theories incorrectly imply that larger groups should experience more difficulty in providing “public goods” (goods whose benefits are not confined to the purchaser). A theoretically based typology that identifies nine systematic patterns associated with changes in group size and parameters of individual and group payoff structure is presented. The typology describes nine different ways in which group size can affect cooperation in the provision of public goods. In a computer-administered experiment using three of these nine types and group sizes of three, six, and nine, 90 college students were given sequences of opportunities to choose between “cooperation” (which benefited everyone) and “noncooperation” (which benefited the chooser but harmed everyone else). For one of these types, a counter-intuitive but theoretically predicted positive relationship between group size and cooperation was found. Two explanations for a theoretically unexpected negative relation between group size and cooperation in the other conditions are developed, one based on Steiner's concept of a conjunctive task, the other based on the decreasing information value of responses in larger groups.