Abstract
Selective preventive interventions aim to reduce mental illness in high-risk populations, yet the reasons why some children benefit while others do not remain unclear. This study explores participants' perceptions of mechanisms contributing to change in a family-based preventive intervention for children of parents with severe mental illness. Using an exploratory sequential mixed methods approach, we conducted an abductive qualitative analysis of focus groups (eight parents, eight children) to identify narratives of mechanisms contributing to change. The qualitative findings informed a subgrouping variable for a quantitative post hoc exploratory subgroup analysis of secondary data from the VIA Family trial (N = 110). The qualitative findings indicate that child mental health problems, parents' personal unmet needs from childhood, children's relatedness to peers and family, and contextual family-focused activities contribute to change within selective prevention. Quantitative results indicated that parents motivated by a need for support at baseline exhibited meaningful improvements in the home environment upon enrollment in the experimental preventive intervention compared with families motivated to support science (mean change: 5.07, 95% CI 2.11 to 8.03). However, no significant subgroup differences were observed in changes in children's global functioning between the allocation groups. Parents' perceived need for support facilitated engagement and home improvements, while children's relatedness to peers and family contributed to their intervention experiences. These findings emphasize the importance of motivation and social connections in intervention outcomes, contributing to the growing field of precision prevention. Future research should explore these mechanisms as potential mediators or mechanisms of action for selective prevention. ClinicalTrial.gov Identifier: NCT03497663.