An Experimental Study of Jury Decisions

Richard D. McKelvey and Thomas Palfrey

No 1034, Working Papers from California Institute of Technology, Division of the Humanities and Social Sciences

Abstract: We present experimental results on individual decisions in juries. We consider the effect 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 find 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 theoretic 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 find 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.

Pages: 53 pages
Date: 2000-03
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