Likelihood ratios in psychological expert opinion, and their reception by professional judges

Abstract

In various countries, forensic scientists have begun to express their expert opinion in terms of the likelihood of observing the evidence under the primary and under an alternative hypothesis (i.e. the likelihood-ratio approach). This development is often confined to technical domains such as fingerprint analyses. In forensic psychological expertise, likelihood ratios are largely absent. In this contribution, we explain how forensic psychologists can employ likelihood ratios, and we describe two illustrating cases. We also present two studies in which we examined how (Dutch) professional judges appreciate psychological expertise framed in likelihood ratios. Findings suggest that judges ( N = 39) appreciate a fictitious expert witness report framed in likelihood-ratios similarly to an opinion framed one-dimensionally. Judges’ ( N = 79) understanding of a psychological expert opinion framed in likelihood ratios was satisfactory as measured by self-report and an actual test We conclude that, as is custom in forensic technical domains, psychological expert opinion can be expressed in likelihoods. Two of the hypothesised flipsides, namely, lawyers’ dislike of likelihoods, and their lack of proper understanding, may be surmountable.

Dark personality traits and deception, and the short dark tetrad (SD4) as integrity screening instrument

“…However, there is an alternative approach that may make the SD4 more useful in this regard, namely the likelihood-ratio approach. This approach is common in forensic sciences 15 and was introduced in forensic psychology recently 16 . Applied to our analysis, findings suggest that liars are approximately six times more likely to score in the range of 27 to 31 (5.4%) than are truth tellers (0.9%).…”

If generalization is the grail, practical relevance is the nirvana: Considerations from the contribution of psychological science of memory to law.

“…This document is copyrighted by the American Psychological Association or one of its allied publishers.This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.1 Apart from using the smallest effect size of interest, psychological scientists have suggested that the use of likelihood ratios in expert testimony can help in conveying the practical relevance of research(Rassin et al, 2022).…”

“…1Apart from using the smallest effect size of interest, psychological scientists have suggested that the use of likelihood ratios in expert testimony can help in conveying the practical relevance of research (Rassin et al, 2022). …”