The U.S. Supreme Court has repeatedly assumed the functional equivalence of different sized juries (at least in the range of 6- to 12-person groups). Several formal models of jury decision making predict that larger juries should hang more often, particularly for very close cases. Failures to confirm this prediction in several previous studies were attributed to inadequate sample sizes or to insufficiently close cases. An experimental simulation study that minimized these problems was undertaken to test the models' prediction. Social decision scheme and social transition scheme analyses permitted comparisons of the decision-making processes of the different-sized mock juries. The effect of the method used to poll group members' verdict preferences was also examined. As group size increased, the observed probability of a hung jury increased significantly. No process differences between 6- and 12-person groups were detected, but 3-person groups did exhibit several process differences from the larger groups. When cases were very close, the likelihood of a hung jury for typically sized juries was found to be lower when the group was polled by secret ballot than when a show-of-hands polling method was used.
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