According to procedural justice theory, when police treat people in a fair, respectful, and neutral manner, individuals are more likely to perceive the police as legitimate and comply with directives. Despite the growth in use of experimental vignettes to test PJ theory, there has been little to no attention to assessing the underlying causal assumptions, mechanisms and possible confounding pathways between police treatment and attitudinal outcomes. This study contributes to methodological knowledge by promoting recent best-practice recommendations (e.g., placebo tests, open text responses) for evaluating causal pathways between treatment and outcome in experimental vignettes. In order to assess these causal mechanisms, this study will field a between-subjects vignette with a 3x2x2 factorial design. The content of the vignette will aim to replicate similar interactions as reported in previous research (i.e., a traffic stop by police) adapted to the Dutch context. Follow-up placebo questions will ask ...