People in more equal societies live longer, a smaller proportion of children die in infancy and self-rated health is better.
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.
Wilkinson RG. The Impact of Inequality: how to make sick societies healthier. New Press, New York, and Routledge, London. 2005
Subramanian SV, Kawachi I. Whose health is affected by income inequality? A multilevel interaction analysis of contemporaneous and lagged effects of state income inequality on individual self-rated health in the United States. Health and Place 2006; 12: 141-56.
Wilkinson RG, Pickett KE. Income inequality and social gradients in mortality American Journal of Public Health 2008; 98(4): 699-704.
Subramanian SV, Kawachi I. Income inequality and health: what have we learned so far?Epidemiologic Reviews 2004; 26: 78-91.
Wilkinson RG, Pickett KE. The Spirit Level. Penguin. 2009. Buy the book from Amazon.