Convicting the Innocent: The Inferiority of Unanimous Jury Verdicts

It is often suggested that requiring juries to reach a unanimous verdict reduces the probability of convicting an innocent defendant while increasing the probability of acquitting a guilty defendant. We construct a model that demonstrates how strategic voting by jurors undermines this basic intuition. We show that unanimity rule may lead to high probabilities of both kinds of errors and that the probability of convicting an innocent may actually increase with the size of the jury. Finally, we demonstrate that a wide variety of voting rules, including simple majority rule, lead to much lower probabilities of both kinds of errors.