DEFINING THE STANDARD OF PROOF IN JURY INSTRUCTIONS

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%). © 1990, Association for Psychological Science. All rights reserved.