2 69 REVIEW OF APPLIED & CASUAL GAMES FOR MENTAL HEALTH either applied or casual games showed promising effects. It remains unclear why certain clinical populations benefitted more from applied games and others more from casual games. It might be that the casual games were more engaging and intrinsically motivating than the applied games, providing (better) disruption of negative automatic thoughts and enhancing positive emotions in youth with anxiety and depressive symptoms, and increased motivation for therapy in youth with ADHD. Due to the fact that we were not able to distinguish between gamified and serious game interventions, however, further explanations remain speculative. Given the promising results of games on a wide range of mental health outcomes, more research is needed that looks into the working mechanisms of these games. A second recommendation to improve methodological rigor and to advance research in this field, is to establish standards for evaluation and type of control groups (cf. Boot et al., 2013; Connolly et al., 2012; Zayeni et al., 2020). As this review showed, a wide variety of control and comparison groups has been investigated to establish the effectiveness of digital games. These groups consist of active and passive conditions, multi-modal interventions, and other game conditions including those in which multiple games were provided that participants could choose from. As outlined earlier, effectiveness may vary depending on the comparison group, and although the specific effect of the game remains unknown in multi-modal interventions, they are valuable control conditions as they are more ecologically valid (Spielmans et al., 2010; Weisz et al., 2013). Additionally, conditions providing participants with free choice from a set of games may instil intrinsic motivation and autonomy, increasing engagement and the likelihood for positive intervention outcomes (Granic et al., 2014; Scholten & Granic, 2019). Proper standards for effectiveness evaluation should be established, taking into account additional benefits of certain control groups. In addition, future research may want to take a closer look at the relative effectiveness of different types of control conditions. Finally, our review uncovered a great deal of variability in outcomes and variables typically measured in studies using games for mental health interventions. The lack of harmonised and/or comparable outcome measures in some areas of mental health limited the conclusions that could be drawn from this. Future research should aim to use standard sets of outcome measures, particularly for nonspecific factors. Furthermore, relatively few studies measured positive outcomes, such as positive affect and well-being. Both the promising effects on positive affect as well as the mixed evidence on well-being that the current review found suggest that there might be interesting and
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