## 5 Discussion
### Limitations and Ethical Considerations  
One study cannot include everything that exists. This study also has some limitations of which the reader should be aware. First, this study focused on social, not individual, aspects of e-learning taking place in 3D virtual environments. That is, the focus has been on studies which have used multi-user not single-user mode. Social interaction is a vital affordance of SVREs. Second, this study reviewed published empirical e-learning studies which have been conducted between 2004 and 2019. That is, the period from the launch of Second Life to the time before the coronavirus pandemic erupted. There are studies that were conducted before 2004 and during the COVID-19 pandemic, but this study focused on the maturing era of SVREs, not the early years or the coronavirus pandemic era. Third, this study included only published studies which were written in English. There are many studies conducted in other languages, but they were excluded since we assumed that the reader might not have the necessary language skills to read those studies too. Fourth, this study focused on the higher education context. For this reason, primary and secondary education contexts were not included in this study. Hence, this study focused on adult learners rather than children as learners. Fifth, this study paid attention to the formal learning rather than informal learning context. Sixth, this study focused on humans' deep and meaningful learning rather than machines' deep learning, which is actively studied by artificial intelligent researchers. Seventh, concerning humans' deep and meaningful learning, this study paid attention to the published studies which handled at least two of the following three aspects of deep and meaningful learning: cognitive, social, or affective. Therefore, those studies that focused on only one of the aspects were excluded from this study. Every study has some ethical issues which both the authors and the readers should be aware of. First, there might be some ethical issues arising from perceived or real conflicts of interest such as financial interests, which may influence the findings of systematic literature reviews. This study was not a sponsored systematic literature review. In so being, no external funding source influenced the findings. Second, there might be some ethical issues concerning whose voice is heard (more). That is, less influential and marginalized groups might have difficulties to be heard. This study, for example, did not focus only on the Second Life studies, which are plenty, but also included other voices. Third, there might be some ethical issues concerning how appropriately the relevant published studies were searched and selected--more explicitly, how well the researchers managed to consider the impact of potential search biases such as availability bias, database bias, citation bias, country bias, familiarity bias, language bias, and multiple publication biases. In addition, how well the researchers managed to consider the impact of potential publication biases such as funding bias, methodological bias, and outcome bias is also worth mentioning as studies with certain methodological orientations might be more likely to be funded. Studies with a large sample size might be more likely to be published. Studies with significant positive effects might be more likely to be cited. In this study, we included only high-quality published research that has undergone the peer-review process. In so doing, we excluded, for example, potential government reports, published and unpublished, as well as book chapters. We designed a search strategy which gave us access to the most relevant primary research from a variety of sources without excluding diverse contextual viewpoints. We held appropriate transparency to the reader during the whole review process. In this research work, we reviewed published empirical e-learning studies taking place in 3D virtual environments that feature social elements.