Maartje Boer

INTRODUCTION 21 1 Item Response Theory, Latent Class Analysis, and multivariate regression analysis. To test whether the scale has good psychometric properties across multiple national settings, in Chapter 3 , we compared the psychometric properties of the SMD-scale across 44 countries within the European region and Canada. In particular, we compared the structural validity, reliability, and criterion validity of the scale across countries. In addition, we examined whether the factor structure of the scale was measurement invariant across countries, gender, and age. In light of the abundance of single-country studies on the association betweenSMUbehaviors andwellbeing, in Chapter 4 , we conductedmultilevel models oncross-national data to investigatewhether intense andproblematic SMU and their associations with wellbeing varied by the way in which social media were adopted within the countries’ adolescent population, indicated by the country-level prevalence of intensive and problematic SMU. Here, wellbeing was indicated by mental health 2 (i.e., life satisfaction, psychological complaints), social wellbeing (i.e., family support, friends support), and school wellbeing (i.e., school satisfaction, schoolwork pressure). The remaining chapters zoom in on the association between SMU behaviors and wellbeing by studying adolescents’ SMU intensity, SMU problems, and wellbeing over time within adolescents. More specifically, in the next two chapters, we aimed to address the question whether lower wellbeing, in particular mental health, precedes or follows from SMU intensity and SMU problems, using longitudinal data and Random Intercept Cross-Lagged Panel Modelling. In Chapter 5 , we studied the direction of the association between SMU intensity, SMU problems, and ADHD-symptoms. Extending the work in Chapter 5, in Chapter 6 , we investigated the direction of the association between SMU intensity, SMU problems, and life satisfaction as well as depressive symptoms. In addition, we examined whether these associations were mediated by different mechanisms, including upward social comparisons, cybervictimization, decreased face-to-face contacts, and worsened subjective school achievements. In addition, considering the lack of studies investigating problematic SMU from a developmental perspective, to gain knowledge on how SMU problems develop throughout adolescence, in Chapter 7 , we explored trajectories of SMU problems over time using Latent Class Growth Analysis. 2 In Chapter 4, we refer to mental wellbeing instead of mental health.