Project Summary/Abstract. Substance use among justice-involved individuals constitutes a public health problem that is linked to negative consequences for offenders, their families, and their communities. As criminal justice reform becomes a reality in the United States and more justice-involved substance users join the general population, there is reason for therapeutic optimism. Our preliminary work shows that measuring heterogeneity in psychopathic personality traits can guide the use of a brief substance use intervention to people who will benefit. Our innovative assessment and matching-to-treatment approach has promise for a) revealing the true efficacy of a brief motivational intervention (BMI) for justice-involved substance users at varying levels of psychopathic traits, and b) explaining contradictions in the BMI literature. Additionally, the current proposal is a Phase III randomized clinical trial in which we will use rigorous multi-method assessment to test two putative mechanisms of change: commitment to change and self-regulation. By elucidating the mechanisms through which justice-involved substance users' change in treatment, we will make adaptations that further strengthen our intervention and increase the range of people who benefit. Hypotheses that will be examined include: 1) that a BMI, adapted for use in criminal justice supervisory settings, will reduce substance use relative to Standard Care for individuals with low/moderate affective traits of psychopathy, 2) that the reduction in substance use will be mediated by commitment to change and an increase in self-regulation, enabling adaptations that target these processes and lead to a better intervention, and 3) that the moderated reduction in substance use will mediate a reduction in violent recidivism at follow-up.