196 Chapter 7 and the belief to be important to another person), contributing to positive treatment effects (Klinger et al., 2020; Pernice, Biegel, Kim, & Conrad-Garrisi, 2017; Rosenberg & McCullough, 1981; Thoits, 2011). For instance, a study among youth with criminal behavior showed that a lost sense of mattering motivated youth to show deviant behaviors in order to force a sense of mattering and attract the attention of others (Lewis, 2017). As the contact between patients and volunteer coaches is often perceived as equal and dyads were often engaging in accessible and sociable activities in the community (chapter 4), self-esteem of patients could have been strengthened by these positive experiences. Besides, volunteer coaches were internally motivated and genuinely interested to meet patients, which might have sparked a sense of mattering. Finally, it should be noted that the measurement of hospitalization as well as criminal behavior may not fully reflect reality, which could have contributed to the positive treatment effects (chapter 5). For hospitalization, unfortunately, we only obtained information regarding the duration of hospitalizations in the local mental healthcare institute (i.e., Arkin). Although this institute is one of the largest mental healthcare institutes in the area and patients in the sample, if hospitalized, were most likely to be admitted to this institute, the positive effects do not reflect hospitalization duration in general. Nevertheless, we can conclude that the duration of hospitalization in the local mental health facility, based on official medical records, decreased more among patients who received volunteer support in addition to usual care. Furthermore, for the measurement of criminal behavior, we used self-reported data, which may be sensitive to under- and overreporting (Junger-Tas & Marshall, 1999; van der Laan & Blom, 2006). Towards a personalized approach of social network interventions In our research we zoomed in on the effectiveness of social network interventions in addition to usual care for (forensic) psychiatric patients. The main methods we used to examine effectiveness (i.e., a meta-analysis and an RCT) were both robust and broad (chapter 2, 3, and 5). We concluded that social network interventions could be used to enhance treatment outcomes in psychiatric patients (chapter 2). Furthermore, despite the challenging implementation of an informal social network intervention in forensic psychiatric patients, we still demonstrated significant benefits of the intervention in addition to usual care on group level (chapter 4 and 5). However, as mentioned, the findings are based on broad methods, not allowing us to determine which approach works best and which subgroup of patients could benefit from social network interventions. Significant treatment effects may not provide benefits for some individual patients. In chapter 2 and 5, we made a first attempt to identify subgroup differences, providing
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