The relationship of income inequality to mortality: does the choice of indicator matter?

Ecologic studies in the U.S. and elsewhere in the world have demonstrated that income inequality is strongly related to mortality and life expectancy: the greater the dispersion of income within a given society, the lower the life expectancy. However, these empirical studies have been criticized on the grounds that the choice of indicator may have influenced positive findings. Using a cross-sectional, ecologic design, we tested the relationships of six different income inequality indicators to total mortality rates in the 50 U.S. states. The following summary measures of income distribution were examined: the Gini coefficient; the decile ratio; the proportions of total income earned by the bottom 50%, 60%, and 70% of households; the Robin Hood Index; the Atkinson Index; and Theil's entropy measure. All were highly correlated with each other (Pearson r > or = 0.94), and all were strongly associated with mortality (Pearson r ranging from 0.50 to 0.66), even after adjustment for median income and poverty. Thus, the choice of income distribution measure does not appear to alter the conclusion that income inequality is linked to higher mortality. Furthermore, adjustment for taxes and transfers, as well as household size (using equivalence scales), made no difference to the income inequality/mortality association. From a policy perspective, the alternative income distribution measures perform differently under varying types of income transfers, so that theoretical considerations should guide the selection of an indicator to assess the impact of social and economic policies that address income inequality.