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.

This study sought to investigate the relation between expert witness likeability and juror judgments of credibility and sentencing. Two actors playing expert witnesses were trained to present themselves as high and low in likeability in a standard testimony scenario involving capital trial sentencing. The effects of extraversion and gender in mock jurors in attending to expert testimony were also examined. The dependent variables were the perceptions of the witnesses’ credibility and agreement with testimony and the participants were 210 psychology undergraduates. Likeability of expert witnesses was found to be significantly related to judgments of trustworthiness of the experts, but not related to confidence or knowledge of the experts or to the mock juror sentencing decisions. Women participants rated high likeable experts as more credible than low likeable experts; men did not. For men jurors, agreement with testimony increased as extraversion increased. However, for women jurors, agreement with testimony decreased as extraversion increased. The results suggest that likeability can be an important element of source credibility, and that attorneys and trial consultants now have an empirical foundation for addressing likeability as part of witness preparation.