An Experimental Study of Jury Decision Rules 1

periments. We also thank Tim Feddersen, Susanne Lohmann, Krishna Ladha, the audiences at several academic conferences and seminars, and three referees for their comments. We present experimental results on individual decisions in juries. We consider the ef-fect of three treatment variables: the size of the jury (three or six), the number of votes needed for conviction (majority or unanimity), and jury deliberation. We nd evidence of strategic voting under the unanimity rule, where the form of strategic behavior involves a bias to vote guilty to compensate for the unanimity requirement. A large fraction of jurors vote to convict even when their private information indicates the defendant is more likely to be innocent than guilty. This is roughly consistent with the game theo-retic predictions of Feddersen and Pesendorfer (FP) [1998]. While individual behavior is explained well by the game theoretic model, at the level of the jury decision, there are numerous discrepancies. In particular, contrary to the FP prediction, we nd that in our experiments juries convict fewer innocent defendants under unanimity rule than under majority rule. We are able to simultaneously account for the individual and group data by using Quantal Response Equilibrium to model the error.