Innumeracy and unpacking: bridging the nomothetic/idiographic divide in violence risk assessment.

Structured methods to assess violence risk have proliferated in recent years, but such methods are not uncontroversial. A "core controversy" concerns the extent to which an actuarial risk estimate derived at the group level should-morally, logically, or mathematically-apply to any particular individual within the group. This study examines the related psychological question: When do people apply group-level risk estimates to the individual case? We manipulated whether an actuarial risk estimate is "unpacked;" that is, whether the risk factors on which the estimate is based are articulated. Our findings indicate that the degree of unpacking (e.g., listing six vs. three risk factors) increased the likelihood that jury-eligible citizens will apply an actuarial risk estimate in their decision to civilly commit a particular respondent. Unpacking also increased the perceived relevance of the group-level risk estimate to the individual case. We then split the sample based on self-reported numeracy, defined as "ability with or knowledge of numbers." Numeracy moderates the unpacking effect in that unpacking only made a difference for the innumerate participants. The psychological approach we take to the question of group-to-individual inference is not limited to violence-risk assessment, and may apply to many other areas of law in which group-to-individual inferences are frequently, if controversially, made.