The income inequality-health hypothesis seeks to explain differences in average levels of health among countries, or among regions in one country, by the extent of disparity among income levels. Income inequality is linked to other components of social policy and its influence on health can be hard to isolate. In order to examine the value of income inequality as a determinant of population health the authors reviewed 98 peer-reviewed studies that address the issue. Evidence suggests that, among wealthy countries, income inequality is not associated with differences in population health. Some evidence supports the influence of income inequality on particular health outcomes, such as homicide. Within the United States there is evidence of an association at the state level. While some effects may occur within the United Kingdom, the authors did not find regional effects among other wealthy nations. These results do not contradict the large body of evidence indicating that higher income individuals are healthier than those of lower income. Income inequality is an attribute of a social system, while income is associated with an individual. The authors suggest that reducing income inequality by increasing the income of disadvantaged individuals will lead to their improved health, reduce health inequalities and raise the level of population health.