Maartje Boer

SUMMARY AND DISCUSSION 309 9 higher the level of SMU problems, the higher the risk of experiencing problems related to mental health, school, and sleep (Chapter 2). Another important direction for future research is to study the possible mechanisms explaining our effects. For example, the association between SMU problems and subsequent increases in ADHD-symptoms (Chapter 5) is possibly indirect: Problematic users may experience stress or anxiety when it is not possible to access their smartphone, also referred to as nomophobia (Kuss & Griffiths, 2017). This stress or anxiety may harm their ability to sustain attention on offline activities, particularly when SMU is not possible. Also, the association between low life satisfaction and SMU problems over time (Chapter 7) is in line with theoretical models proposing that psychosocial vulnerabilities cause problematic internet-related behaviors (Caplan, 2003; Davis, 2001). However, these models describe that the relation is indirect, driven by maladaptive cognitions about social media (e.g., the perception to only have a meaningful life on social media), as well as by a preference for online interaction over face-to-face-communication. To better understand our findings, more research focusing on such mediating factors is important. This dissertation highlights the outcomes of SMU problems more extensively than the predictors. Therefore, more research on the causes of SMUproblems is desired. To do so, the use of longitudinal data fromchildhood onwards until late adolescence is crucial, because SMUproblemsmay emerge at a young age (Chapter 7). Establishing the onset of SMU problems could improve estimating the causal effect of specific psychosocial vulnerabilities on developing SMU problems, because this allows to exclude the possibility that the respective psychosocial vulnerabilities are the result of earlier SMU problems. Furthermore, in future longitudinal studies on the origins of SMU problems, it is important to not only focus in psychosocial vulnerabilities, such as poor mental health or social skills, but also on genetic factors (Brand et al., 2019), as well as personality characteristics, such as extraversion (Lee et al., 2017; Sun & Zhang, 2021). Contextual factors, such as the family, peer, school, and country context, also likely play a role in adolescents’ susceptibility to SMU problems. For example, our findings showed large country differences in problematic SMU (Chapter 4), suggesting that the country context affects this risk. Overall, longitudinal research on SMU problems from childhood onwards examining the role of individual as well as contextual factors on the