Maartje Boer

CHAPTER 9 290 identified: two subgroups with relatively high levels of SMU problems over time, of which one reported high and one reported average levels of SMU intensity, and two subgroups with low levels of SMU problems over time, of which one reported low and one reported high levels of SMU intensity. In the two subgroups with high levels of SMU problems, the level of SMU problems remained high over time. The subgroup with low levels of SMU problems and high SMU intensity was the largest subgroup. Compared to this subgroup, adolescents in the two subgroups with high levels of SMU problems reported lower life satisfaction, but differed on other characteristics: The subgroup with high levels of SMU problems and high SMU intensity also showed more attention deficits and impulsivity, whereas the subgroup with high levels of SMU problems and average SMU intensity also showed poorer friendship competencies. Together, these findings highlight the emergence of different co-trajectories of SMU problems and SMU intensity throughout adolescence, depending on adolescents’ psychosocial profile. Chapters 3 until 7 highlighted the differences between SMU intensity and SMU problems in their associations with several indicators of wellbeing. However, to improve our understanding of the association between SMU and wellbeing, further scrutiny was desired. Therefore, Chapter 8 investigated whether theassociationbetweenSMU intensity and life satisfactiondepended on (1) the type of SMU activity the adolescent engages in, (2) the (non)linear assumptions of the association, (3) individual characteristics, (4) whether SMU problems were considered, and (5) the level of analysis. Multilevel analyses on four waves of longitudinal data among 1,419 adolescents indicated that, at the within-person level, on average, changes in the intensity of any type of SMU activity were not associated with changes in life satisfaction, regardless of whether we controlled for changes in SMU problems. However, for some adolescents, increases in SMU intensity were associated with decreases in life satisfaction, whereas for others, increases in SMU intensity were associated with increases in life satisfaction. This individual variation could not be explained by adolescents’ tendency to engage in upward social comparisons. At the between-person level, adolescents with higher averages in SMU intensity reported lower average levels of life satisfaction than adolescents with lower averages in SMU intensity, although this association was small in effect size, and disappeared when controlling for adolescents’ average level