In Defense of Unanimous Jury Verdicts: Communication, Mistrials, and Sincerity

It is a widely held belief among legal theorists that the requirement of unanimous jury verdicts in criminal trials reduces the likelihood of convicting an innocent defendant. This belief is, to a large extent, dependent upon the assumption that all jurors will vote sincerely based on their own impression of the trial evidence. Recent literature, however, has drawn this assumption into question, and simple models of jury procedure have been constructed in which, except under very strict conditions, it is never a Nash equilibrium for all jurors to vote sincerely. Moreover, Nash equilibrium behavior in these models leads to higher probabilities of both convicting an innocent defendant and acquitting a guilty defendant under unanimity rule than under a wide variety of alternative voting rules, including simple majority rule. The present paper extends these models by adding minimal enhancements that we argue bring the existing models closer to actual jury procedures. In particular, we separately analyze the implications of (1) incorporating the possibility of mistrial and (2) allowing limited communication among jurors. Under each of these enhancements, we identify general conditions under which sincere voting is, in fact, a Nash equilibrium. We further demonstrate that under such sincere voting equilibria, unanimous jury verdicts perform better than any alternative voting rule in terms of minimizing probability of trial error and maximizing expected utility.