The Impact of Expert Witness Knowledge on Perceived Credibility: Implications for Not Guilty by Reason of Insanity (NGRI) Endorsement

Abstract

Previous research demonstrates that expert witness education and experience have an influence on mock juror perceptions of credibility. However, whether this relationship extends to cases involving the insanity defense remains unclear, leaving an important gap in the literature given the high stakes of such trials. The current study used an experimental design to examine the impact of expert witness knowledge (high vs. low) on perceived credibility and subsequent NGRI endorsement. Participants (N = 425) read a case summary and the credentials and testimony of the expert witness, completed questionnaires, and reported the likelihood that they would endorse NGRI for the defendant. Results indicated that, regardless of expert witness testimony, prior attitudes about the insanity defense (IDA-R) predicted NGRI endorsement. Specifically, positive attitudes towards the insanity defense resulted in an increased likelihood of NGRI endorsement. These findings underscore that juror attitudes toward the insanity defense, rather than expert witness characteristics, may be the decisive factor shaping NGRI endorsement. This highlights the need for courts to consider such attitudes during jury selection in NGRI cases, paralleling the practice of death qualification in capital trials.