This paper reviews the large and growing body of literature on the apparently negative effects of income inequality on population health. Various hypotheses are identified and described that explain the empirically observed association between measures of income inequality and population health. We have concluded that data from aggregate-level studies of the effect of income inequality on health, i.e. studies at the population and community (e.g. state) levels, are largely insufficient to discriminate between competing hypotheses. Only individual-level studies have the potential to discriminate between most of the advanced hypotheses. The relevant individual-level studies to date, all on U.S. population data, provide strong support for the "absolute-income hypothesis," no support for the "relative-income hypothesis," and little or no support for the "income-inequality hypothesis." Results that provide some support for the income-inequality hypothesis suggest that income inequality at the state level affects mainly the health of the poor. There is only indirect evidence for the "deprivation hypothesis," and no evidence supports the "relative-position hypothesis." Overall, the absolute-income hypothesis, although > 20 years old, is still the most likely to explain the frequently observed strong association between population health and income inequality levels.