Double disadvantage: income inequality, spatial polarization and mortality rates in Taiwan.

Background: Previous studies have suggested that social and economic spatial polarization is associated with various health outcomes. However, few studies have examined the joint effect of income inequality and spatial polarization on health. Methods: Data on mortality in 2008-12 were from the Ministry of Health and Welfare. We constructed economic spatial polarization using the Index of Concentration at the Extremes (ICE) by tax data from the Ministry of Finance. The Gini coefficient was from the Family Income and Expenditure Survey. Using multilevel datasets of 352 townships nested within 20 cities in Taiwan, we examined the association between township-level ICE and mortality, and further examined whether city-level income inequality moderate this association. Results: In 2008-12, the average age-standardized mortality in Taiwan was 470.5 per 100 000 populations. As compared to the highest income-based ICE quintile, the lowest ICE quintile was associated with an excess 171.7 deaths per 100 000 people (95% CI = 116.1, 227.3) after controlling for income inequality and population size. One unit rise in the Gini coefficient further increased 29.9 deaths (95% CI = 12.4, 47.5) for the lowest ICE quintile, as compared to the highest. Conclusion: The joint effect of income inequality and small-scale economic polarization may shed light on how inequalities increase mortality.