This paper reviews the large and growing body of literature on the apparent
ly negative effects of income inequality on population health. Various hypo
theses are identified and described that explain the empirically observed a
ssociation between measures of income inequality and population health. We
have concluded that data from aggregate-level studies of the effect of inco
me 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 discriminat
e between most of the advanced hypotheses. The relevant individual-level st
udies to date, all on U.S, population data, provide strong support for the
"absolute-income hypothesis," no support for the "relative-income hypothesi
s," and little or no support for the "income-inequality hypothesis." Result
s that provide some support for the income-inequality hypothesis suggest th
at income inequality at the state level affects mainly the health of the po
or. There is only indirect evidence for the "deprivation hypothesis," and n
o evidence supports the "relative-position hypothesis." Overall, the absolu
te-income hypothesis, although >20 years old, is still the most likely to e
xplain the frequently observed strong association between population health
and income inequality levels.