This review, commissioned by Nature Reviews Psychology, addresses a critical societal problem psychology is uniquely positioned to address: the challenge of distinguishing genuine- from pseudo-experts. We examine the cognitive processes that underpin genuine expertise and explore the practical methods used to evaluate it. We distinguish between ‘show-it’ and ‘know-it’ expert performances. The key difference is in their visibility, measurability, and immediacy: ‘know-it’ performances are particularly challenging and critical to interrogate. This distinction serves as a heuristic for identifying when evaluations of expertise require greater care and should incorporate a range of diagnostic factors including foundational and applied validity.

Psychological testing, based on psychometric science, often informs legal decisions that profoundly affect people’s lives, such as eligibility for disability benefits, child custody, and whether and where someone will serve a criminal sentence. We investigate psychological tests introduced as legal evidence throughout the entire history of U.S. case law, finding a sharp increase in recent years. Although the law requires judges to screen for quality before allowing an expert to testify, that rarely occurs (we estimate 1.66% of cases). This finding raises questions about the rigor of legal admissibility standards and the functioning of those rules regarding expert evidence quality.

U.S. law requires courts to recognize and screen the quality of evidence proffered by experts across fields. Yet, there is wide quality variation in expert evidence, with scant legal scrutiny. Are legal professionals able to distinguish varying qualities of expert evidence but do so sparingly for strategic or resource-related purposes, or are they unable to distinguish quality? Through preregistered experiments and surveys with judges, attorneys, and forensic psychologists, we find judges and attorneys can gauge evidence quality in laboratory conditions. However, we also report the reasons they outline for routine failure to do so in real-world cases.

Psychological expert evidence can play a substantial role in legal proceedings. However, research has identified problems with this evidence, including concerns about reasoning underlying expert opinions. Across five studies (involving 1579 participants), we constructed and evaluated the effectiveness of a reasoning checklist to improve the application of scientific reasoning in complex decision tasks. Across these studies, we find that the reasoning checklist improved reasoning and argument quality – as well as their evaluation by a legal audience. Accordingly, our findings lay a foundation for future research to continue to explore methods of improving the quality of expert opinion evidence.