Third explanation is plausible

EDITOR—Ross et al report that income inequality in the state or province and metropolitan area is associated with mortality in the United States but not in Canada.1 They offer two explanations for this discrepancy. Firstly, this association is observed only at levels of inequality present in the United States, not the lower levels observed in Canada. Secondly, the actual association of income inequality with mortality is modified by the social and political characteristics specific to a place.

There is, however, another possible explanation: confounding of the association of income inequality with health at the state level (or metropolitan area level) in the United States. This is not the same as confounding at the individual level by, for example, personal income, which varies between states.2 State level confounding may occur when characteristics of the states are correlated with income inequality, remain associated with mortality within strata of states by income inequality, and hence cause a spurious association of income inequality with mortality. Two possible candidates are the extent of rurality and the welfare policies of the states. Both vary between states, are plausibly related to population health, and are probably correlated with the geographically biased distribution of income inequality.

The United States provides a rich natural experiment to study the association of income inequality with health, and several studies have now replicated the association of state level income inequality with health using different data sets. 3 4 But the use of different data sets in these studies is akin to reanalysing one cross sectional study of the same …