## 4 Conclusions
### Limitations and Future Directions  
This study provides new findings regarding the relationships among people's U-turn motivations and their subjective evaluations of living environments. These findings can help local governments to create policies for promoting U-turns and improving population disparities in Japan. However, there are some limitations that call for further research. First, more advanced analytical methods should be utilized. The information root node variation (IRNV) method [58] is one such method. This would overcome the weakness of the decision tree method in which the adopted branch rules are constrained by the root variable. The IRNV method would provide more robust explanations for the reasons for U-turn migration. Second, additional steps in individuals' decision-making processes should be taken into account. To directly examine the influence of the evaluations of living environments, we focused on the motivation of people toward U-turn migrations. However, we did not examine the influence of possibility, such as economic conditions or social positions. These factors are considered when people decide to migrate. We did not compare the importance of these factors in the decision-making process; such a comparison will provide invaluable findings regarding the common factors among decision stages or stage-specific factors. It is likely that stage-specific factors are not essential in decision making, but they may be important considerations for enhancing the satisfaction of lives and encouraging a longer settlement. Outcomes from such research would enable local governments to plan and develop policies more effectively. Third, future research should not only consider the subjective evaluations of the living environments, but also their objective measures. Although we determined how much development each region needs by using respondents' subjective evaluations, we did not yet examine the relationships between their subjective evaluations and objective conditions. Clarification of this relationship will provide standards for development targets with objective indices. Finally, considerations of other destinations of migration are also necessary. In this study, we selected a specific destination--the hometown. However, the destinations of people's spontaneous migration should be more varied in future research. Destinations other than hometowns may potentially improve population disparities. Although we prioritize U-turn migration in this study, for some regions, new residents who have never lived in the region may contribute to population sustainability. Among the living environments that affect U-turn motivations, developing the transportation or daily shopping environments in rural and smaller cities would also enhance the attractiveness of the regions for new residents. If another survey is conducted, we can verify the impact of the development of these living environments not only on return migrants, but also on new residents. The development of one living environment that enhances both types of people's motivation to migrate would be a more effective countermeasure for improving population disparities in Japan. Future studies should focus on understanding the larger context of decisions to migrate and the motivations behind them. Conceptualization, S.T. and H.N. ; methodology, S.T., H.N. and S.H. ; validation, S.T., S.H. and H.N. ; formal analysis, S.T. ; investigation, S.T., H.N., S.H. and T.Y. ; data curation, S.T. and T.Y. ; writing--original draft preparation, S.T. ; writing--review and editing, S.T. ; visualization, S.T. ; supervision, S.T. ; project administration, S.T. ; funding acquisition, H.N. This research was funded by the Ministry of Land, Infrastructure, Transport and Tourism in 2015 (for [PERSON], [PERSON]) and the APC was funded by JSPS KAKENHI GRANT NUMBER 17K14785. This work is supported by the Ministry of Land, Infrastructure and Transport and JSPS KAKENHI GRANT NUMBER 17K14785. The authors have no conflict of interest to declare.