Maartje Boer

SUMMARY AND DISCUSSION 291 9 of SMU problems. We did not find support for the proposition that different types of SMU activities have differential associations with wellbeing. Also, no evidence was found for curvilinear associations between the intensity of SMU activities and life satisfaction. In sum, results suggest that taking into account individual heterogeneity, considering SMU problems, and disentangling within- from between-person effects are important for understanding the relation between SMU intensity and wellbeing. Key Findings (1) The Social Media Disorder-Scale Is a Reliable and Valid Instrument to Measure Problematic SMU There are concerns about adolescents who display problematic SMU (Griffiths & Kuss, 2017; Kuss & Billieux, 2017; Marino et al., 2018b). Therefore, it is essential that an instrument is available for research on problematic SMU that adequately reflects the theoretical conceptualization of problematic SMU and that this instrument is valid and reliable. Findings from Chapters 2 and 3 showed that thenine-itemSMD-scalehas goodpsychometricproperties, both in the Netherlands as well as throughout the European region and Canada. Our findings suggest that the scale should be used as a unidimensional scale, including nine items which are all important indicators of problematic SMU. Also, researchers can use the scale to identify subgroups of users, including normative, at-risk, and problematic users. Furthermore, the scale can be used to reliably compare the prevalence of problematic SMU across countries and adolescent groups differing in gender, age, and socioeconomic status. These findings are an important step for providing researchers with a psychometrically sound instrument to measure problematic SMU, which is pivotal considering the possible detrimental effects of problematic SMU, as shown by our longitudinal findings (Chapters 5, 6, 8). (2) Rather Than the Intensity of SMU, Problematic SMU is Negatively Related to Wellbeing Our cross-sectional as well as longitudinal studies showed that SMU problems werenegatively relatedtomultipledomainsof adolescentwellbeing, including their mental health, school wellbeing, and social wellbeing, while higher SMU