**5. Conclusions**

In this longitudinal study, diagnostic stability was observed for PSU, with the majority of participants displaying persistent PSU six months after the beginning of the study. Prognostic factors of PSU were individual traits such as harm avoidance and impulsivity; psychological vulnerabilities such as high SAS-SV, low quality of life, low perceived happiness, and goal instability; and social risk factors such as shorter weekday conversations with mothers. We suggest that the bio-psychosocial framework could be useful in the evaluation of PSU and in predicting its course. The exploration of causality between PSU and psychiatric problems showed that PSU increased the risk of developing psychiatric problems later, which supports the interpretation of PSU as a primary disorder. While the recovered group demonstrated fewer psychological risk factors at baseline than persistent problematic users, they also benefited from further gains in psychological well-being and were less afflicted by physical consequences. This Matthew effect in the recovery of PSU calls for additional attention to be paid to disadvantaged children or adolescents with PSU, to better facilitate their recovery and reduce the harmful long-term consequences of PSU as a secondary prevention strategy.

**Author Contributions:** S.-Y.L. analyzed and interpreted the data and drafted the manuscript; J.-S.C., S.-y.B. and Y.-S.K. participated in the recruiting and data collection; H.K.L., and Y.-S.K. developed concepts and supervised; M.-H.P., and K.-I.J. reviewed and edited the manuscript. All authors have read and agree to the published version of the manuscript.

**Funding:** This research was funded by the Korean Mental Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea (HM14C2603).

**Acknowledgments:** This study was supported by a grant of the Korean Mental Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea (HM14C2603).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

### **References**


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International Journal of *Environmental Research and Public Health*
