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 50 people by using different measures of exposure and outcome; it is not akin to separate studies of a different 50 people each time. Thus, a spurious association of income inequality with health due to state level confounding will remain for each new analysis of the same natural experiment.

It seems likely and plausible that income inequality is associated with health.5 Instead of examining the possibility of state level confounding, however, we propose two research strategies. Firstly, potential state level confounders are included as covariates in analyses. This will not be without difficulty, however, owing to the high probability of (multi)collinearity of ecological variables and challenging theoretical considerations—for example, causal ordering of ecological variables, such as income inequality and welfare policies. Secondly, many different natural experiments should be analysed to look for a consistent association of income inequality with health. From this perspective, the results from Canada and the United States are just two separate natural experiments to which we want to add results from many more natural experiments.

Low mortality in Canadian cities may be driven by low mortality in immigrants

EDITOR—Ross et al compare the relation between income inequality and mortality in the United States and Canada.1 They relate the proportion of income received by the less well off 50% of households to the mortality in all working age people. For Canadian metropolitan areas, they find no significant association between this indicator of income inequality and mortality; this is visible by the lack of slope in the respective weighted regression line (figure 2). For metropolitan areas in the United States, the association between income inequality and mortality is strong.

The findings from Canada are surprising. Typically, there is a strong inverse association between individual socio-economic status and mortality, and also between overall distribution of wealth in a society and mortality. There is, however, one population group where this association tends to be absent: recent immigrants of working age frequently have an age-adjusted overall mortality that is considerably (20-30%) lower than that of the native born population.2 3 This mortality advantage may persist 10-20 years after immigration4; it is present even where immigrants are a minority group and socioeconomically disadvantaged.

In Canada, recent immigrants form a considerable proportion of the total population, and they are not uniformly distributed in the country. According to the 1996 census, immigrants represent 17.4% of the total population; 85% of all immigrants—and 93% of those who arrived between 1991 and 1996—live in a metropolitan area.5 This applies in particular to Toronto and Vancouver, which have 42% and 35% immigrants among their respective census populations (Montreal only 18%), half of whom have come to Canada since 1981. A 20-30% lower mortality among immigrants thus may have driven down the death rates in Toronto and Vancouver by as much as 10-20%.

Hence, the death rates in these two cities would be lower than what might be expected from the wealth distribution. This could be corrected for, for example, by restricting the analysis to Canadian born people. Once this is done, the overall mortality in Toronto and Vancouver would be higher. As these two cities are very populous, the slope of the regression line would become steeper, indicating some association between income inequality and mortality not only in the United States but also in Canada. In conclusion, Ross et al may wish to consider adjusting for the proportion of immigrants in future studies on social inequalities in health.

Authors' reply

EDITOR—Our study has received two types of critique—that ecological variables in the United States and immigration in Canada confound the relation between income inequality and mortality and that the range of inequality in Canada was too narrow to draw conclusions about the relation between income inequality and mortality there.

Blakely and Woodward suggest that there are probably important variables at the level of the state or metropolitan area that are correlated with income inequality and cause a spurious association of income inequality with mortality. We have, however, always conceptualised our measure of income inequality of a particular place as a marker for a wide variety of social conditions and as reflecting the outcome of layers of political, social, and economic history of that place.1 It is striking how strongly income inequality correlates with mortality at multiple geographic scales in the United States. In states income inequality is correlated (after adjusting for median income) with poverty, unemployment, incarceration, health insurance provision, and numerous educational outcomes.2

Our primary objective was to investigate the relation between income inequality and mortality in Canada compared with the United States, which has higher incomes but lower life expectancy. Our comparative analysis does not resolve causality. Instead, the results of this natural experiment provoke hypotheses about how differences in policies towards such things as health care, taxes and transfers, and urban structure in two otherwise culturally similar countries might influence population health.

Mortality among people of working age by proportion of income belonging to less well off half of households, United States (1990) and Canadian metropolitan areas (1991) over range of Canadian median share values. Mortality is standardised to Canadian population in 1991

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Razum suggests that large immigrant populations in Canadian cities lower the death rates for those places beyond what would be expected for their income distributions. Although the healthy migrant effect could contribute to lower mortality in Toronto and Vancouver, if we were to exclude immigrants, hypothetically raising mortality in Toronto and Vancouver, this would actually flatten the relation between mortality and income inequality in Canada (figure 2).

The second critique, which is not articulated above, is the claim that the Canadian range of income inequality was too narrow to allow any significant relation to emerge. We selected the subset of United States cities with income inequality measures in the same range as the Canadian cities and fitted weighted linear regression lines to the respective sets of points (figure).

A significant negative slope remained for the metropolitan areas in the United States. Thus the relation appears consistently in the United States but not in Canada. Our preliminary analysis of income inequality and mortality for Australian metropolitan areas (with comparable income distributions to Canadian metropolitan areas) has yielded similar results to the Canadian analysis, suggesting that this “Canadian paradox” may not be so paradoxical at all.