Prying Open the Black Box of Causality: A Causal Mediation Analysis Test of Procedural Justice Policing

Abstract

This paper reviews causal mediation analysis as a method for estimating and assessing direct and indirect effects in experimental criminology and testing procedural justice theory by examining the extent to which procedural justice mediates the impact of contact with the police on various outcomes. Causal mediation analysis permits one to better interpret data from a field experiment that has suffered from a particular type of implementation failure. Data from a block- randomised controlled trial of procedural justice policing (the Scottish Community Engagement Trial) was analysed. All constructs were measured using surveys distributed during roadside police checks. The treatment implementation was assessed by analysing the treatment effect consistency and heterogeneity. Causal mediation analysis and sensitivity analysis were used to assess the mediating role of procedural justice. The results suggest that the treatment effect was consistent and fairly homogeneous, indicating that the systematic variation in the study is attributable to the design. Moreover, procedural justice acts as a mediator channelling the treatment’s effect towards normative alignment (NIE=-0.207), duty to obey (NIE=-0.153), sense of power (NIE=-0.078), and social identity (NIE=-0.052), all of which are moderately robust to unmeasured confounding. The NIEs for risk of sanction and personal morality were highly sensitive, while for coerced obligation and sense of power they were non-significant. This paper shows that causal mediation analysis is a versatile tool that can salvage experiments with systematic yet ambiguous treatment effects by allowing researchers to “pry open” the black box of causality. Most of the theoretical propositions of procedural justice policing were supported. Future studies are needed with more discernible causal mediation effects.