Innumeracy and Unpacking: Bridging the Nomothetic/Idiographic Divide in Violence Risk Assessment

Abstract

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 6 versus 3 risk factors) affected 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.

Keywords: actuarial risk assessment, decision making, numeracy