The knowledge of experts presumably affects their credibility and the degree to which the trier of fact will agree with them. However, specific effects of demonstrated knowledge are largely unknown. This experiment manipulated a forensic expert’s level of knowledge in a mock trial paradigm. We tested the relation between low versus high expert knowledge on mock juror perceptions of expert credibility, on agreement with the expert, and on sentencing. We also tested expert gender as a potential moderator. Knowledge effects were statistically significant; however, these differences carried little practical utility in predicting mock jurors’ ultimate decisions. Contrary to hypotheses that high knowledge would yield increased credibility and agreement, knowledge manipulations only influenced perceived expert likeability. The low knowledge expert was perceived as more likeable than his or her high knowledge counterpart, a paradoxical finding. No significant differences across expert gender were found. Implications for conceptualizing expert witness knowledge, credibility, and their potential effects on juror decision-making are discussed.

Two experiments examined how mock jurors’ beliefs about three factors known to influence eyewitness memory accuracy relate to decision-making (age of eyewitness and presence of weapon in Study 1, length of eyewitness identification decision time in Study 2). Psychology undergraduates rendered verdicts and evaluated trial participants after reading a robbery-murder trial summary that varied eyewitness age (6, 11, 42, or 74 years) and weapon presence (visible or not) in Study 1 and eyewitness decision length (2-3 or 30 seconds) in Study 2 (n=200 each). The interactions between participant belief about these variables and the manipulated variables themselves were the heart of this study. Participants’ beliefs about eyewitness age and weapon presence interacted with these manipulations, but only for some judgments – verdict for eyewitness age and eyewitness credibility for weapon focus. The exploratory meditational analyses found only one relation: juror belief about eyewitness age mediated the relation between eyewitness age and credibility ratings. These results highlight a need for juror education and specialized voir dire in cases where legitimate concerns exist regarding the reliability of eyewitness memory (e.g., child eyewitness, weapon presence during event, long eyewitness identification time). If erroneous juror beliefs can be corrected their impact may be reduced.