Maartje Boer

CHAPTER 9 294 countries where intense SMU was more common showed a positive association between high SMU intensity and wellbeing indicators, whereas countries where this was less common reported a negative association (Chapter 4). Additionally, the association between SMU intensity and wellbeing varied per wellbeing domain. High SMU intensity was consistently associated with higher friends support. Nevertheless, in some countries, high SMU intensity was negatively associated with other wellbeing domains, such as school satisfaction (Chapter 4). Furthermore, increases in SMU intensity were associated with concurrent increases in face-to-face contact with peers (Chapter 6). Together, these findings suggest that higher SMU intensity can be both beneficial or harmful to wellbeing, depending on individual characteristics, country context, and wellbeing domain. Implications of Our Findings Conceptual Implications The finding that the SMD-scale was reliable and valid (Key finding 1; Chapters 2, 3) is an important step for the measurement of problematic SMU, because the SMD-scale may advance the conceptualization of problematic SMU. More specifically, while some other widely used scales only include the six core criteria of addiction, namely salience (i.e., preoccupation ), mood modification (i.e., escape ), relapse (i.e., persistence ), tolerance , withdrawal , and conflict (Andreassen et al., 2012, 2016; Griffiths, 2005), the SMD-scale adds three additional criteria: problems in important life domains due to SMU, displacement of important activities by SMU, and deception by lying about the time spent on SMU. This conceptual extension is considered theoretically meaningful, because the items problems and displacement indicate whether the behavior (here: SMU) has harmful implications. As such, the assessment of harmful implications is more strongly reflected in the SMD- scale than in other scales that only cover the abovementioned six criteria. This is important, because many scholars agree that harmful implications as a direct consequence of the behavior in question is an essential characteristic of behavioral addictions (Billieux, King, et al., 2017; Brand et al., 2020; Kardefelt- Winther et al., 2017). Also, by adding the three items, the conceptualization of problematic SMU is more in line with the DSM-5 definitions of other