Empirically Validated Treatments for youth antisocial behavior target family-level risk, which according to systems theories requires direct family participation. There are multiple barriers to family interventions, and researchers emphasize exploring treatment adherence and family-level change mechanisms. Using therapy progress notes, the present study explored adherence to and effects of family member participation in Multisystemic Therapy. Contrary to hypotheses, more family participation was associated with poorer pre-treatment family functioning and less family improvement. Greater improvement in youth outcomes was associated with more general but less extended family participation. MST's Therapist Adherence Measure correlated weakly with family participation. The discussion considers influences of treatment tailoring, family complexity, family therapy training, and methodology, encouraging future mechanisms research using objective measures of family interventions
