Maartje Boer

CHAPTER 1 22 To further improve our understanding of the development of SMU problems, we studied adolescents’ trajectories of SMU problems in parallel with their trajectories of SMU intensity 3 . Furthermore, based on multinomial regression, we examined which wellbeing indicators predicted these co-trajectories. Here, wellbeing referred to mental health 4 (i.e., life satisfaction, self-esteem, ADHD-symptoms) and social wellbeing 5 (i.e., friendship competencies). Finally, to further extend current knowledge on the association between SMU behaviors and wellbeing, in Chapter 8 , we used multilevel modelling on longitudinal data to study the association between SMU intensity and wellbeing in more detail. More specifically, we examined five theoretical and methodological factorsthatpossiblyaffecttheassociationbetweenadolescents’ SMU intensity and life satisfaction, namely how SMU is conceptualized (i.e., active or passive SMU), whether it is studied (non)linear, whether individual differences are considered, whether problematic SMU is taken into account, and the level of analyses (i.e., within- or between-person level). Data The chapters in this dissertationused large-scalenationally and internationally representative cross-sectional data as well as longitudinal data of adolescents aged 11 to 16 from the Health Behaviour in School-aged Children (HBSC) study (Inchley et al., 2020b; Stevens et al., 2018) and the Digital Youth (DiYo) project (Van den Eijnden et al., 2018). The HBSC-study is a cross-national study carried out every four years since 1983 in collaboration with the World Health Organization (WHO) Regional Office for Europe. It monitors adolescents’ health behaviors, wellbeing, and social context using self-report surveys administered in large-scale nationally representative samples. The HBSC- data were used in Chapters 2, 3, and 4. Table 1.1 provides an overview of the sample size and characteristics of these studies. The DiYo-project is a longitudinal study on self-report online behaviors and wellbeing among Dutch high school adolescents. Data were collected in February-April of 2015 until 2019 with yearly time intervals. Chapters 5 until 8 were based on samples from the DiYo project, although they used different 3 In Chapter 7, we refer to SMU frequency instead of SMU intensity. 4 In Chapter 7, we refer to subjective wellbeing (life satisfaction, self-esteem) and self-control (attention deficits, impulsivity). 5 In Chapter 7, we refer to social competencies instead of social wellbeing.