The impact of educative and evaluative expert witness testimony on trial outcome: A meta-analytic review

The role of the expert witness is to assist the trier of fact (FRE 702), yet the degree to which an expert testifies to the specific issues of the case varies. Previous research has conceptualized case-specificity of expert testimony as a continuum, ranging from purely educative to conclusively evaluative, with four intermediate points in between (Kwartner, 2005). Twenty-nine mock jury studies in which the specificity of expert testimony was manipulated and compared to a no-expert control condition were included in this meta-analysis. The impact of expert testimony on trial outcome was measured dichotomously with juror verdict and continuously with Likert-type ratings of degree of guilt and verdict confidence. Across outcome measures, there were small, yet significant effects for expert witness testimony, with degree of guilt yielding the greatest effects. Evaluative testimony in which the expert provided an opinion about the disputed issue had a significantly greater impact on trial outcome measures than purely educative testimony, in which the expert presented an overview of research or professional practices without speaking directly to the facts of the case at hand. There was a significant amount of heterogeneity that was neither completely accounted for by including a random variance component nor the proposed moderators. However, expert discipline did have a moderating effect on trial outcome such that experts outside of the mental health profession had a greater impact on degree of guilt ratings than either psychologists or psychiatrists.; KEY WORDS: Expert witness testimony, Meta-analysis.