Maartje Boer

INTRODUCTION 11 1 Figure 1.2 Percentage of Adolescents Reporting Problematic Social Media Use Note . Data source: HBSC 2017/2018, n = 227,441 adolescents from 45 countries in Europe and Canada (Inchley et al., 2020b) particularlypopular after the rise in smartphone adoptionaround2012 (Twenge, Martin, et al., 2018), which implies that SMU is a relatively new phenomenon. It generally takes decades of research before a certain behavioral pattern is recognized as an addiction in a classification system. Therefore, in line with the commonly used definition among scholars, we define SMU that is characterized by addiction-like behaviors as problematic SMU (Lee et al., 2017). In many studies, problematic SMU is defined by the presence of a number of addiction criteria, for example, when at least four out of six criteria are reported (Andreassen et al., 2012; Cheng et al., 2021). Such a definition is useful to identify and study the most extreme behaviors that are possibly the most clinically relevant. Some studies in this dissertation defined problematic SMU on a continuous scale, indicated by the number of present criteria, which in some chapters is referred to as the level of SMU problems . Throughout the dissertation, we use the terms problematic SMU and SMU problems interchangeably. Review studies showed that problematic SMU is positively correlated with the intensity of SMU with a small to moderate effect size (Frost & Rickwood, 2017; Parry et al., 2020). This correlation and its magnitude seem plausible, because some problematic users may engage in high SMU intensity as a result of their inability to control their SMU and to fulfil their