There are now over 170 studies of income inequality in relation to various aspects of health. Life expectancy, infant mortality, low birth weight and self-rated health have repeatedly been shown to be worse in more unequal societies. These studies have been reviewed in the journal Social Science and Medicine.

Researchers sometimes disagree about the pathways leading from inequality to worse population health. The most consistent interpretation of all the evidence is that the main route hinges on the way inequality makes life more stressful. Chronic stress is known to affect the cardiovascular and immune systems and to lead to more rapid aging. Inequality makes social relations more stressful (see section on Trust and Community Life), by increasing status differences and status competition. These effects are important: Americans living in more equal states live around 4 years longer than those living in more unequal states.