153 Effectiveness of a social network intervention 5 Moderators of treatment effects The results of our moderator analyses should be considered exploratory due to the lack of statistical power. Exploration of potential moderators showed that most of the variables (i.e., sex, age, primary personality disorder, and primary psychotic disorder) did not modify treatment effects. We did find significant effect modification of the variables primary substance use disorder, comorbidity, and sex. Stronger treatment effects were found for TAU+FNC participants without primary substance use disorders on the duration of hospitalization within 12-month follow-up (RRTAU+ = 0.016, 95% CI 0.006 to 0.014), as well as within 18-month follow-up (RRTAU+ = 0.013, 95% CI 0.005 to 0.033), compared to TAU participants. In contrast, negative effects were found for the group with primary substance use disorders (12-month RRTAU+ = 1.513, 95% CI 0.583 to 3.929; 18-month RRTAU+ = 2.019, 95% CI 0.780 to 5.227). Furthermore, the effects on duration of hospitalization within 18-month follow-up were stronger for TAU+FNC participants with comorbid disorders compared to TAU (RRTAU+ = 0.078, 95% CI 0.037 to 0.166). Again, negative effects were found in those without comorbid disorders (RRTAU+ = 1.426, 95% CI 0.320 to 6.346). Lastly, stronger treatment effects on criminal behavior were found for male participants receiving TAU+FNC compared to TAU (RRTAU+ = 0.519, 95% CI 0.203 to 1.330). The effects were negative in female participants (RRTAU+ = 13.885, 95% CI 2.090 to 92.253). Noteworthy, the effects of various potential moderating variables on the number and duration of hospitalization and incarceration could not be estimated due to an excess of 0-values of the outcome in categories of potential moderators. DISCUSSION There is an urgent and ongoing need to develop and improve effectiveness of evidence-based interventions for forensic psychiatric patients (MacInnes & Masino, 2019; MacKenzie & Farrington, 2015; McIntosh et al., 2021; Völlm et al., 2018). A supportive social network is considered an important protective factor that may improve mental health outcomes and reduce criminal recidivism in forensic psychiatric care (Bootsma et al., 2017; Klinger et al., 2020; Ullrich & Coid, 2011). Yet, the effectiveness of interventions aimed at promoting supportive social networks among forensic psychiatric patients has remained unclear. Therefore, in this RCT we examined whether an additive informal social network intervention, provided by trained volunteer coaches in the community, could improve treatment outcomes among outpatients receiving forensic psychiatric care. To this end, we aimed to (1) examine treatment effects of patients receiving an additive social network intervention versus treatment as usual alone on a broad range of outcomes,
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