Association of income inequality and aggregate health indicators in Japan: analyses of prefecture-level panel data

Evidence indicates that income inequality is associated with poor health outcomes. In Japan, while the relationship between income inequality and self-rated health has been explored, the association between income inequality and aggregate health indicators remains insufficiently examined. This study seeks to add evidence on the association between income inequality and population health status in the Japanese context by investigating the relationship between the Gini coefficient and aggregate health indicators across 47 prefectures in 1999, 2004, 2009, 2014, and 2019. Associations between the Gini coefficient and health indicators, adjusted for average per capita income, population density, and year dummies, were assessed using pooled ordinary least squares (OLS), fixed-effects, and random-effects regression models. We treated year as a time variable and prefecture as the unit variable in the fixed- and random-effects regression models. In the pooled OLS analysis, we disregarded this panel structure and treated all 235 observations as independent cases. A significant association between the Gini coefficient and the infant mortality rate (b = 7.93, p = 0.008) was observed in the OLS model, which was tested as the most appropriate. The result was consistent with that derived from an alternative model using log-transformed income figures, indicating the robustness of the finding. Although the findings suggest the importance of income inequality for further improvements in Japan's infant mortality rate, more conclusive evidence requires additional studies incorporating multilevel frameworks, time-lag analyses, and a broader range of healthcare-related factors.