“Certainty” and expert mental health opinions in legal proceedings

This pilot study addresses the legal and scientific ramifications of the “certainty” expressed by mental health professionals when functioning as expert witnesses in criminal and civil proceedings. The sporadic attention paid to “certainty” in the professional literature has typically taken the form of general policy oriented analyses as opposed to empirical, data-driven investigations. In the current study, 25 doctoral and masterʹs level mental health professionals were provided with 53 different statements. Some statements addressed “certainty” itself in the typical fashion (e.g., “Reasonable Degree of Scientific Certainty,” “Reasonable Degree of Medical Certainty,” and “Reasonable Degree of Psychological Certainty”). Other statements were confined to specifically legal standards of proof (e.g., “Beyond a Reasonable Doubt,” “Preponderance of the Evidence,” and “Clear and Convincing”). Additional statements included those that bore at least some direct forensic relevance (e.g., “Based upon All the Data at My Disposal,” “In My Medical Opinion,” and “In My Clinical Judgment”), as well as those of a non-forensic nature (e.g., “I Would Bet My Life Savings,” “On My Word of Honor,” and “I Am Personally Convinced”). Ratings were provided on one form as if the participant had uttered the statement, and on another form as if another expert witness had uttered the statement. Overall, participants did not tend to identify traditional legal terms as expressing the highest level of “certainty,” and respondents tended to ascribe more “certainty” to the same terms when uttered by themselves as opposed to when uttered by other expert witnesses. Those providing forensic testimony will do well to accommodate the courtʹs traditional requirements while developing and preparing to justify their own notions of just what “certainty” denotes in this context.