*4.2. Limitations*

For this review, we narrowed our article pool by searching for only "smartphone"/"mobile phone" and "addiction"/"problematic use" terms. We did not actively search for "android", "iPhone", "cell phone", "cellular device", "compulsive use", "phubbing", "snubbing", and "nomophobia", which could have potentially made us miss articles relevant to this review. However, we were able to locate a few relevant articles after reviewing the reference sections of selected articles. Much more research is needed that investigates differences as a function of demographic variables, such as gender, age, or ethnicity. Another limitation is that our interpretations of categorizing certain variables into their respective groupings may not match others' opinions. For example, while we may propose that a certain subjective report is suggestive of a causal relationship with SA/PSU, another researcher may disagree and say that it is insufficient evidence. Another example would be missing a guiding theory used in articles, making Table 3 incomplete.

### **5. Conclusions**

Most studies to date seem to assume that SA is a valid behavioral addiction, many forming their assumptions based on Griffiths' component model of addiction, and framing their study based on that assumption. The interchangeable use of the terms SA and PSU, inconsistent methodological approaches used to study SA (e.g., varied use of SA/PSU scales among research), lack of standardized diagnostic criteria, and unclear distinctions of SA from other related addictions make it difficult to make a conclusive statement on the status of SA, which could be considered "an ill-defined and heterogeneous construct" [67]. With no unifying theory on SA, all theories mentioned in the current research highlight the complexity of SA: One theory cannot simply explain SA, but rather several theories and models possibly need to be integrated to better explain its distinct addictive traits in this new technologically advanced era. Much more research is needed to confirm the uniqueness of SA, which encompasses the addictive activities engaged in on the smartphone, which includes apps that are not available on other devices. It is most plausible, based on the current studies, to infer that SA falls on a continuum of additive behaviors, from mild PSU to more extreme addictive behavior, where the consequences need to be addressed, prevented, and potentially treated before the adverse health effects debilitate the smartphone user.

**Author Contributions:** Conceptualization, methodology, analysis, writing—original draft preparation, S.Y.; writing—Review and editing, supervision, S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** We would like to thank Dmitri Rozgonjuk and Regina van den Eijnden for their support on this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
