Horowitz, I. A., & Kirkpatrick, L. C. (1996). A concept in search of a definition: The effects of reasonable doubt instructions on certainty of guilt standards and jury verdicts. Law and Human Behavior, 20, 655-670.
Eighty (6-person) juries heard 1 of the 5 reasonable doubt instructions in a trial that either had strong evidence for guilt or favored acquittal. None of the instructions produced acceptable criteria of self-reported reasonable doubt, although instructions that informed juries to be firmly convinced (FC) elicited the highest standards of proof. The FC instructions yielded verdicts that tended to correspond with the evidence in both versions of the trial. FC juries focused proportionately more on the evidence and less on non-probative issues than juries given other instructions. Juries had lower self- reported standards of proof when the trial evidence was weak for conviction as opposed to when it was strong. The discussion addresses the surprisingly low self- reported standards of proof.
Strong main effect of evidence strength on jury verdicts, plus interaction with “reasonable doubt” definition. Juries given “firmly convinced” reasonable doubt definition reported higher required certainty, convicted less often with weak evidence, spent more time discussing instructions and evidence, and took fewer polls than other definitional conditions. All instructions regarding reasonable doubt produced relatively low subjective certainty requirements.