Judicial error by groups and individuals

tIn criminal cases judges evaluate and combine probabilistic evidence to reach verdicts.Unavoidably, errors are made, resulting in unwarranted conviction or acquittal of defen-dants. This paper addresses the questions (1) whether hearing cases by teams of threepersons leads to less error than hearing cases alone; (2) whether deliberation leads to betterdecisions than mechanical aggregation of individual opinions; and (3) whether participat-ing in deliberations improves future individual decisions. We find that having more thanone judge consider cases reduces error effectively. This does not mean that it is necessaryto deliberate about all cases. In simple cases many errors can be avoided by mechani-cal aggregation of independent opinions, and deliberation has no added value. In difficultcases discussion leads to less error. The advantage of deliberation goes beyond the case athand: although we provide no feedback about the quality of verdicts, it improves individualdecisions in subsequent cases