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.

Aside from an article by Gutheil, Bursztajn, Hilliard, and Brodsky (2004), scant literature exists regarding why forensic mental health professionals refuse or withdraw from cases. The current study collected descriptive information about the reasons mental health experts decline or withdraw from forensic assessments, both early and late in the legal process. In response to an online survey, 29 practicing forensic psychologists and psychiatrists presented examples of case withdrawal from their professional experiences. Their major reasons included ethical issues or conflicts, payment difficulties, and interpersonal or procedural problems with retaining counsel or evaluees. The results indicate that there are compelling personal and professional reasons that prompt forensic mental health experts to withdraw from or turn down cases.