**5. Conclusions**

How does an IHEG a ffect a married individual's well-being (e.g., SRH, mental health, and objective health) and lifestyle for sustainable development (e.g., activities to improve environmental sustainability)? This study first investigated the relationship between the gap of education levels between married couples, health status, and a sustainable lifestyle using an original international survey data collected from 32 countries on six continents. A self-rated health status index, a mental health index, and an objective health status index were utilized to assess the health statuses of individuals. Objective health was a dummy variable that was equal to 1 if an individual did not experience an illness or surgery in the past half-year. Objective unhealthy includes physical illness and mental illness. Moreover, six unique indices are used to investigate sustainable lifestyles. The instrument variable method was utilized to investigate the causal relationship between the two issues above.

The main conclusions are as follows. First, in general, compared to couples with equal education levels, couples with education gaps reported worse levels of SRH, mental health, and objective health when an individual's level of education, household income, occupation, and other factors were held constant. Second, the negative e ffect of IHEGs on health di ffered in various groups. For example, the negative e ffect of IHEGs on health was greater for the high-education group than for the low-education group. Moreover, for the high-education group, the negative e ffect of IHEGs on health was greater for women, individuals in Asian countries, and couples in middle-income countries than for their counterparts (men, individuals in Europe/North America and South America/Australia, and high-income countries). However, for the low-education group, a reduction in the education gap seemed to improve the husband's health status. In this situation, however, women's mental health deteriorated. Third, for the channels of the impact of IHEGs on health, the positive e ffect of income hypothesis was not supported, whereas the results supported both the negative e ffect of the longer working hours hypothesis and the negative e ffect of a couple's skill gap hypothesis. Finally, the education gap between married couples may reduce the activities of individuals in improving environmental sustainability, such as decreasing the probability of volunteering, reducing the donation of income or goods, purchasing energy-saving products, energy-saving activities, and sorting or reducing rubbish. For a highly educated individual, the education gap between married couples reduces the likelihood of charitable activities (e.g., donation). However, low-educated individuals reduce the probability of activities regarding household consumption and a sustainable lifestyle (e.g., energy-saving activities).


### *Sustainability* **2020**, *12*, 4623

Based on the results of this study, the policy implication for improving public health (national welfare) is as follows. First, it has been shown that reducing the IHEG may also improve an individual's health status. Because a gender gap in the levels of education exists, particularly in developing countries, the gender gap in school enrollment and levels of education was high. Therefore, increasing the female school enrollment rate may contribute to reducing the IHEG, which can improve health statuses and increase the national human capital needed to increase economic growth. Second, the results showed that family-work conflicts still exist, particularly for the highly educated groups, women, individuals in Asian countries, and middle-income countries. Health improvement policies may be more important for these groups. Third, long working hours and poor support or help between couples with a higher IHEG were the main channels for the negative e ffects of IHEG on health. Therefore, regarding traditional gender-role consciousness, providing more support for family care by the government, implementing family-friendly systems, such as flexible work hours in the workplace, and improving communication between couples can lead to health status improvements. Finally, from the perspective of the SDGs, the United Nations recommended the sustainability development goals, which clearly linked 17 goals to sustaining human well-being. This study is related to responsible consumption and production (goal 12), good health and well-being (goal 3), quality education (goal 4), gender equality (goal 5), and reducing inequalities. The empirical study results sugges<sup>t</sup> that reducing the intrahousehold education gap may positively contribute to establishing a sustainable development society by improving both individual well-being and a sustainable lifestyle.

**Author Contributions:** S.M. designed and conducted the website survey; X.P. and X.M. designed the research including conceptualization and methodology. X.P. analyzed the data and wrote the original paper. X.M., X.P. and C.Z. revised the paper; writing—review and editing, S.M., X.M. and C.Z.; supervision, X.M. and S.M.; funding acquisition, X.M. and S.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by Specially Promoted Research through a Grant-in-Aid, gran<sup>t</sup> number: 26000001 from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Environment Research and Technology Development Fund (S-16-3, S-14-1, S-15-4) and Grant in Aid from the Ministry of Education, Culture, Sports, Science and Technology in Japan (MEXT): Grant in Aid (20H00648), Ministry of Environment, Japan (1-2001), and by a grand from Japan Society for the Promotion of Science (JSPS), gran<sup>t</sup> number: 20H01512. Any opinions, findings, and conclusions expressed in this material are those of the authors and do not necessarily reflect the views of the agencies.

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