Three laboratory experiments were conducted to compare legal (unquantified) definitions of three standards of proof ("preponderance of the evidence," "clear and convincing evidence," and "'beyond a reasonable doubt") with quantified definitions, in which the standards of proof were expressed in probability terms (51%, 71%, and 91% probability of truth). In the first experiment, the quantified definitions had their intended effect--verdicts favoring the plaintiffs decreased as the standard of proof became stricter--but the legal definitions did not have their intended effect. These results were replicated in the second experiment; in addition, jury instructions that combined the two types of definitions (legal and quantified) did not have their intended effect. Results of the third experiment suggest that some legal definitions may be able to communicate their intended difficulty level when they appear in a comparative context.

Research on juror comprehension is reviewed with special attention to recent studies comparing types of definitions of three levels of the standard of proof (preponderance of the evidence, clear and convincing evidence, and beyond a reasonable doubt). Quantified definitions, (in which the standard of proof was expressed in probability terms, and combined quantified and legal definitions had their intended effect; verdicts favoring the plaintiffs decreased in number as the standard of proof became stricter. Nonquantified definitions did not achieve their intended effect on verdicts. The greater effectiveness of quantified definitions in communicating certainty levels precisely and concisely is attributed to the implicit context provided by their positions on an interval scale with known endpoints (i.e., 0-100%).

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An archival analysis of records from 206 criminal cases was used to evaluate the impact of personal and situational factors on jury verdicts. In particular, we evaluated whether repeated jury service produced bias in jurors that was sufficient to affect jury decisions. A variety of case characteristics and indices of prior jury experience was examined for their relative impact on trial outcomes. Several of the case characteristics were related to verdicts, but the personal characteristics were not. Although more than half of the juries contained experienced jurors, juror experience had little influence on verdicts in either major or subsidiary analyses. However, there was a slight tendency for small juries with large proportions of experienced jurors to convict. This result is consistent with data from Kentucky; a meta‐analysis across the two data sets indicates that it is a reliable finding. The results have implications for the determination of jury size. We suggest that future research examine the possibility that increasing jury size may reduce the influence of an individual's bias by providing a balance of other jurors with no or offsetting biases.