Relation between income inequality and mortality: empirical demonstration

BMJ 1999;319:953-957 ( 9 October )

Papers

Michael Wolfson, director general, ^a George Kaplan, professor and chair of epidemiology, ^b John Lynch, assistant professor, ^b Nancy Ross, analyst, ^c Eric Backlund, mathematical statistician. ^d

^a Institutions and Social Statistics Branch, Statistics Canada, Ottawa, Canada K1A 0T6, ^b Department of Epidemiology, School of Public Health, University of Michigan, Ann Arbor, MI 48109-2029, USA, ^c Social and Economic Studies Division, Statistics Canada, ^d Federal Building #3, US Bureau of the Census, Washington, DC 20233-8700, USA

Correspondence to: M Wolfson wolfson{at}statcan.ca

Objective: To assess the extent to which observed associationsat population level between income inequality and mortality arestatisticalartefacts.
Design: Indirect "what if" simulation by using observedrisks of mortality at individual level as a function of incometo construct hypothetical state level mortality specific for ageand sex as if the statistical artefact argument were 100%correct.
Setting: Data from the 1990 census for the 50 US statesplus Washington, DC, were used for population distributions byage, sex, state, and income range; data disaggregated by age,sex, and state from the Centers for Disease Control and Preventionwere used for mortality; and regressions from the national longitudinalmortality study were used for the individual level relation betweenincome and risk ofmortality.
Results: Hypothetical mortality, while correlated withinequality (as implied by the logic of the statistical artefactargument), showed a weaker association with states' levels ofincome inequality than the observedmortality.
Conclusions: The observed associations in the United Statesat the state level between income inequality and mortality cannotbe entirely or substantially explained as statistical artefactsof an underlying individual level relation between income andmortality. There remains an important association between incomeinequality and mortality at state level over and above anythingthat could be accounted for by any statistical artefact. Thisresult reinforces the need to consider a broad range of factors,including the social milieu, as fundamental determinants ofhealth.

Key messages

Evidence is accumulating that living in a society with higher inequality in income predisposes its members to higher mortality; at the same time, there is widespread evidence that, for individuals, higher income is protective
This individual level relation could "explain" the former societal level relation
The strength of observed levels of association between income inequality and mortality, however, may go well beyond what can be explained as a statistical artefact of an individual level relation between income and mortality
The empirical analysis reported here, based on 1990 data for US states, suggests that the association between income inequality and mortality is considerably stronger than can be accounted for by any statistical artefact
Research underpinning public health policy should therefore take a broad view of the importance of the social milieu as a fundamental determinant of health

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