To compare the effects of group size on cooperation, we introduce a class of simple multiperson games. These games can be regarded as n-person generalizations of the two-person prisoner's dilemma with expected value payoffs. In order to ensure that identical expected-value monetary alternatives are available to players in different-sized groups, we introduced a certain formal constraint on payoffs. Results of an experimental study comparing three-and seven-person groups show that the smaller-sized group is markedly more cooperative than the larger group. The primary emphasis of this paper is methodological. It demonstrates a technique to measure cooperation as a function of group size unconfounded by role-playing and utility considerations.

Four-person groups played repeated trials of an n-Person Prisoner's Dilemma game under two information conditions. In one condition, each player's choice was made known to the others following each trial. In the other information condition each subject was informed only of the number of others who cooperated or competed on each trial. The first condition was a “public choice” situation while the second was an “anonymous choice” situation. The data supported the hypothesis that there would be greater cooperation in the “public choice” condition than in the “anonymous choice” condition since the former created a situation in which the individual would be made accountable to the group for the consequences of his choices. The results are discussed in terms of the various manipulations of group process which can produce deindividuation, that is, a lessening of the individual's propensity to behave in a socially approved manner.

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