Is Reasonable Doubt Self-Defining?

Abstract

Many courts believe that reasonable doubt is self-defining and, therefore, do not explain the concept to their juries. The empirical evidence, however, suggests otherwise. Controlled studies demonstrate that mock jurors do not distinguish between reasonable doubt, clear and convincing evidence, or even preponderance of evidence standards when reaching their verdicts.

This Article presents our empirical study in which we sought to (1) conduct a more powerful test by remedying the methodological weaknesses of earlier studies, and (2) determine whether, instead of following their burden of proof instruction, mock jurors use a simple heuristic or rule of thumb regarding the quantum of evidence necessary to convict.

Our first finding is consistent with previous findings: we found no significant differences in conviction rates between groups that received different burden of proof instructions. Second, the data also revealed what we call “the 60/65 rule.” That is, nearly all study participants either (a) said that less than 60% of the evidence favored the State and voted not guilty, or (b) said that more than 65% of the evidence favored the State and voted guilty.

These findings demonstrate that reasonable doubt is not self-defining. Not only do mock jurors in multiple studies fail to distinguish between reasonable doubt and the two lower, civil burdens of proof, but they are also willing to convict criminal defendants on a quantum of evidence (approximately 65%) that is much lower than what judges expect and the Constitution requires.

Given these findings, we recommend that courts use a relative, comparison-based definition of reasonable doubt to properly convey to jurors the high burden the government must satisfy before depriving a person of life, liberty, or property.

Keywords: Burden of Proof, Reasonable Doubt, Jury Instructions, Empirical Research