Maartje Boer

SUMMARY AND DISCUSSION 289 9 panel modelling showed that within adolescents, increases in SMU problems predicted subsequent increases in ADHD-symptoms, in particular attention deficits and impulsivity (but not hyperactivity), with moderate to large effect sizes. Changes in SMU intensity did not predict subsequent changes in any ADHD-symptoms. Reversely, changes in ADHD-symptoms neither changed adolescents’ level of SMU intensity nor SMU problems over time. Together, these findings suggest a unidirectional relation, whereby SMU problems, and not the intensity of SMU, affect adolescents’ ADHD-symptoms. Building on the previous chapter, Chapter 6 investigated the direction of the association between SMU intensity, SMU problems, and depressive symptoms as well as life satisfaction, using three waves of data from 2,109 Dutch adolescents. Also, it investigated whether these associations were mediated by increased upward social comparisons, cybervictimization, worsened subjective school achievements, and fewer face-to-face contact with friends. Findings from random intercept cross-lagged panel models showed that, within adolescents, increases in SMU problems predicted subsequent decreases in life satisfaction and increases in depressive symptoms, with small effect sizes. Also, SMU problems increased upward social comparison and cybervictimization over time with small to moderate effect sizes. These changes, however, did not predict subsequent changes in depressive symptoms and life satisfaction, which suggests that upward social comparison and cybervictimization did not mediate the observed effect of SMU problems on these aspects of mental health. Reversely, changes in depressive symptoms and life satisfaction did not predict subsequent changes in SMU problems. Adolescents’ SMU intensity was not associated with depressive symptoms and life satisfaction in any direction. Overall, in line with findings fromChapter 5, a unidirectional association was found between SMU problems and mental health. Little is known about how SMU problems develop over time during adolescence. Using four waves of data from 1,419 adolescents, Chapter 7 investigated how SMU problems developed over time and which wellbeing indicators predicteddevelopments in SMUproblems. Indoing so, adolescents’ trajectories of SMU problems were analyzed in parallel with their trajectories of SMU intensity 8 using latent class growth modelling. Four subgroups were 8 In Chapter 7, we referred to SMU frequency instead of SMU intensity.