In 1996, when discussing studies of income inequality and health, the editor of the BMJ wrote, “The big idea is 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.”1

Since then, the “big idea” has been widely tested. More than 200 peer reviewed studies of the association have been published. Income inequality has been variously associated with lower life expectancy, higher rates of infant and child mortality, shorter height, poor self reported health, low birth weight, AIDS, depression, mental illness, and obesity. In the linked meta-analysis (doi:10.1136/bmj.b4471), Kondo and colleagues further assess the association between income inequality and health.2

Perhaps because of the deep political implications of a causal relation between better health of the population and narrower differences between incomes, interpretations of the evidence have come to different conclusions.3 4 5 6 The …