Many researchers and advocates believe that income inequality affects individual health, but empirical evidence has been inconclusive. A large body of research has found that income inequality is negatively correlated with average life expectancy, partly because a transfer of income from the poor to the rich is likely to harm the health of the poor more than it improves the health of the rich. A smaller body of work has investigated socioeconomic disparities in life expectancy, which widened in many countries after 1980, at the same time that income inequality was increasing. These two lines of work should be seen as complementary, because high and rising income inequality is unlikely to affect the health of all socioeconomic groups equally.

Understanding the effects of income inequality on health requires attention to the mechanisms that affect the health of different income groups, changing average health, disparities in health, or (more likely) both. Rising income inequality can affect individuals in two ways. Direct effects change individuals’ own income. Indirect effects change other people’s income, which can then change a society’s politics, customs, and ideals, altering even the behavior of those whose own income remains unchanged. Indirect effects can thus change both average health and the slope of the relationship between individual income and health.