Abstract
Background: This study explored adolescent's mental health trajectories over the course of a school-based mindfulness-based intervention trial (MYRIAD). It examined whether intervention condition (mindfulness vs. teaching-as-usual), individual-level and contextual-level factors were associated with different trajectories. Methods: This pre-registered study used data from 11- to 14-year-olds who participated in the MYRIAD trial. We used growth mixture models to examine distinct trajectories in risk for depression, social-emotional-behavioural functioning, and wellbeing (co-primary outcomes), and anxiety (secondary outcome), across pre-intervention, post-intervention and 12-month follow up (ns = 7198-7727). We then used multinomial and binomial logistic regression models to examine factors associated with individual trajectory membership. Results: Distinct trajectories emerged for each outcome: A five-trajectory model best explained the changes in risk for depression, whilst four-trajectory models best explained changes in social-emotional-behavioural functioning, wellbeing, and anxiety. While 69%-80% of adolescents followed stable low-problem trajectories for each outcome, 11%-23% experienced stable high-problem trajectories, 2%-16% experienced increasing-problem trajectories and 1%-5% experienced decreasing-problem trajectories. Receiving the mindfulness intervention was not associated with any mental health trajectory in models adjusted for confounders. Several individual-level factors, including executive functioning difficulties and risk of mental health problems at baseline, and school-level factors, such as school climate, predicted adolescents' classification into different trajectories, but they did not vary according to intervention group. Conclusions: Individual differences in mental health trajectories emerged over the course of a 1-year mindfulness-based intervention, with most adolescents experiencing low-stable problem trajectories for each outcome. However, the intervention itself had no impact on individual trajectory membership, mirroring null results found in the main trial. Our findings suggest that universal interventions may not be sensitive enough to address the diverse needs of all students, however, tailoring interventions to address a range of different individual and contextual factors might maximise their impact.