Law and psychology must think critically about effect sizes

This comment examines a threat to the development of law and psychology as a “public science” (i.e., one that goes beyond theory to address important issues in society), a failure to think critically about effect sizes. Effect sizes estimate the strength or magnitude of the relationship between variables and therefore can help decision makers understand whether scientific results are relevant to some legal or policy outcome. Accordingly, I suggest that those conducting and reporting law and psychology research should: (1) justify why observed effect sizes are meaningful and report them candidly and transparently, (2) scrutinize effect sizes to determine if they are plausible, and (3) plan studies such that they fit with the researchers’ inferential goals. I explore these points by way of case studies on influential law and psychology studies, such as implicit bias in the courtroom. I end with suggestions for implementing my recommendations, includinga metaresearch agenda for law and psychology.