Limitations and Future Directions for Research There were several limitations to this study. The small sample, missing values, trends during the A phase (PT only) and the lack of any interrater reliability and social validity measures limit the explanatory power and the generalizability of the results. To further validate these effects, larger, more differentiated studies should be implemented. Future studies might be further concerned with the potential moderators of the effectiveness of playing the GBG/KKS during PT (e.g., student characteristics, different types of rewards), transfer effects (student behavior outside the intervention) and teacher experiences (e.g., self-efficacy, the perception of stress, acceptance).