This article reviews 98 aggregate and multilevel studies examining the association between income inequality and health. Overall, there is little support for the idea that income inequality is a major determinant of population health differences within or between rich countries. However, income inequality may directly influence some health outcomes, such as homicide in some contexts. The strongest evidence for direct health effects is among U.S. states, though this is somewhat mixed. Reducing income inequality by raising the incomes of the most disadvantaged can improve their health, reduce health inequalities, and generally improve population health. Several theories have been proposed to understand differences and trends in population health. McKeown's thesis suggests that better nutrition is the most important factor in explaining mortality declines in 19th-century Britain. Easterlin argues that improvements in population health are a byproduct of economic growth. Preston suggests that efficient public health technology is most important for rising life expectancies in poor countries. Caldwell emphasizes the importance of female autonomy, education, and maternal and child health. Health behaviors, medical treatment, and genetic explanations have also been offered to explain health differences. Over the last decade, the income inequality hypothesis has been considered among the "meta-theoretical" frameworks for the determinants of population health. This research theme has coincided with concerns over income inequality within and between countries. The interest in the health effects of income inequality has also coincided with the long-standing epidemiological interest in whether physical and social environmental characteristics can affect the health of individuals. There has been ample discussion about the extent and consequences of income inequality within and among countries. It is widely acknowledged that income inequality rose markedly in wealthy nations starting in the 1970s. Figure 1 shows U.S. trends in income inequality and poverty from 1913 to 1996 and age-adjusted total mortality from 1900 to 1998. These trends have several noteworthy features, including the largest change in income inequality during the interwar period and the Depression, followed by large declines after World War II. The income inequality–health hypothesis suggests that what matters in determining mortality and health in a society is less the overall wealth of that society and more how evenly wealth is distributed. The more equally wealth is distributed, the better the health of that society. This idea is important because of its relevance to achieving greater social justice and better population health through redistributive fiscal and tax policies. The income inequality–health hypothesis has been described in various ways, emphasizing different aspects at different times. Table 1 summarizes how these have been described in the literature. The hypothesis has been interpreted in several ways, including the absolute income hypothesis, relative income hypothesis, deprivation hypothesis, relative position hypothesis, and income inequality hypothesis. The income inequality hypothesis suggests that there is a direct effect of income inequality on health, that the amount of income inequality in a community matters for health in addition to absolute income. The article reviews the empirical evidence for theThis article reviews 98 aggregate and multilevel studies examining the association between income inequality and health. Overall, there is little support for the idea that income inequality is a major determinant of population health differences within or between rich countries. However, income inequality may directly influence some health outcomes, such as homicide in some contexts. The strongest evidence for direct health effects is among U.S. states, though this is somewhat mixed. Reducing income inequality by raising the incomes of the most disadvantaged can improve their health, reduce health inequalities, and generally improve population health. Several theories have been proposed to understand differences and trends in population health. McKeown's thesis suggests that better nutrition is the most important factor in explaining mortality declines in 19th-century Britain. Easterlin argues that improvements in population health are a byproduct of economic growth. Preston suggests that efficient public health technology is most important for rising life expectancies in poor countries. Caldwell emphasizes the importance of female autonomy, education, and maternal and child health. Health behaviors, medical treatment, and genetic explanations have also been offered to explain health differences. Over the last decade, the income inequality hypothesis has been considered among the "meta-theoretical" frameworks for the determinants of population health. This research theme has coincided with concerns over income inequality within and between countries. The interest in the health effects of income inequality has also coincided with the long-standing epidemiological interest in whether physical and social environmental characteristics can affect the health of individuals. There has been ample discussion about the extent and consequences of income inequality within and among countries. It is widely acknowledged that income inequality rose markedly in wealthy nations starting in the 1970s. Figure 1 shows U.S. trends in income inequality and poverty from 1913 to 1996 and age-adjusted total mortality from 1900 to 1998. These trends have several noteworthy features, including the largest change in income inequality during the interwar period and the Depression, followed by large declines after World War II. The income inequality–health hypothesis suggests that what matters in determining mortality and health in a society is less the overall wealth of that society and more how evenly wealth is distributed. The more equally wealth is distributed, the better the health of that society. This idea is important because of its relevance to achieving greater social justice and better population health through redistributive fiscal and tax policies. The income inequality–health hypothesis has been described in various ways, emphasizing different aspects at different times. Table 1 summarizes how these have been described in the literature. The hypothesis has been interpreted in several ways, including the absolute income hypothesis, relative income hypothesis, deprivation hypothesis, relative position hypothesis, and income inequality hypothesis. The income inequality hypothesis suggests that there is a direct effect of income inequality on health, that the amount of income inequality in a community matters for health in addition to absolute income. The article reviews the empirical evidence for the