Maartje Boer

CHAPTER 8 238 confounded by SMU problems. Research overcoming this limitation showed that adolescents’ time spent onSMUwas associatedwithdepressive symptoms in a bivariate model, but this association disappeared when controlling for SMU problems (Shensa et al., 2017). Overall, the few studies including both SMU intensity and SMU problems suggest that SMU problems are negatively associated with wellbeing, while SMU intensity is not. The Association Between SMU Intensity and Wellbeing Depends On the Level at Which It Is Being Analyzed Alongside the abovementioned four more theoretical factors, the association between SMU intensity and wellbeing may also depend on methodological factors. Many studies on the associationbetweenSMU intensity andwellbeing, including review studies, rely on cross-sectional data (Odgers & Jensen, 2020; Orben, 2020a). Cross-sectional data are more likely to reflect associations at the between-person level than at the within-person level . Between-person associations reveal whether adolescents who report higher SMU intensity report lower levels of wellbeing relative to adolescents who report lower SMU intensity. Longitudinal data allow for testing bothwithin- and between person associations, althoughmany longitudinal studies didnotmake this distinction (Coyne et al., 2020). Within-person associations reflect the processes occurring within the individual adolescent. These associations denote whether changes in SMU intensity relative to one’s individual average level of SMU intensity are associated with changes in wellbeing relative to one’s individual average level of wellbeing. It is not uncommon that within-person associations differ frombetween-person associations; not only in effect size, but also in direction (Dienlin & Johannes, 2020; Hamaker, 2012; Orben, 2020a). Several longitudinal studies showed small to moderate negative associations between the intensity of SMU activities and indicators of wellbeing (e.g., internalizing problems, life satisfaction) at the between-person level, while there were no or very small associations at the within-person level (Beeres et al., 2020; Coyne et al., 2020; Jensen et al., 2019; Orben et al., 2019; Stavrova & Denissen, 2020). In two other longitudinal studies, adolescents’ overall SMU intensity or text messaging was not associated with internalizing problems, depressive symptoms, and life satisfaction, neither at the between- person nor at thewithin-person level (George et al., 2020; Schemer et al., 2020).