Instructions on reasonable doubt: Defining the standard of proof and the juror’s task

Reasonable doubt (RD) is the standard of proof used in criminal trials. It is, however, a fuzzy concept in most jurors’ minds, and several jurisdictions have developed judicial instructions defining RD to help jurors understand and correctly apply the standard. We investigated the effectiveness of two instructions that have been used and challenged in the US (briefly RD is “proof that makes you willing to act”, and RD is “doubt that makes you hesitate to act’”. In order to describe and explain the effectiveness of these two instructions (i.e., abbreviated as “proof-willing” and “doubt-hesitate”), we used an experiment to isolate the effects of the precise language used in them (i.e., “willing to act”, “hesitate to act”, “proof”, and “doubt”). Two hundred mock jurors provided their quantitative interpretations of RD following one of four instructions (i.e., “doubt hesitate”, “proof-willing”, “doubt-willing”, and “proof-hesitate”). Participants had significantly lower interpretations of RD under the “doubt-hesitate” than “proof-willing” instruction. There was also greater inter-individual (i.e., across juror) variability and greater intra- individual (i.e., within-juror) variability in interpretations of RD under the “doubt-hesitate” instruction. Finally, there was also a significant interaction effect of the “proof v. doubt” language and the “willing v. hesitate to act” language on interpretations of RD. The “willing v. hesitate to act” language reduced the standard when it was used in combination with the word “doubt”. The present findings have implications for the study of how people understand and interpret standards of proof as well as the development of RD instructions.