BMJ 1999;319:953-957 ( 9 October )

Papers

Relation between income inequality and mortality: empirical demonstration

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 associations at population level between income inequality and mortality are statistical artefacts.
Design: Indirect "what if" simulation by using observed risks of mortality at individual level as a function of income to construct hypothetical state level mortality specific for age and sex as if the statistical artefact argument were 100% correct.
Setting: Data from the 1990 census for the 50 US states plus Washington, DC, were used for population distributions by age, sex, state, and income range; data disaggregated by age, sex, and state from the Centers for Disease Control and Prevention were used for mortality; and regressions from the national longitudinal mortality study were used for the individual level relation between income and risk of mortality.
Results: Hypothetical mortality, while correlated with inequality (as implied by the logic of the statistical artefact argument), showed a weaker association with states' levels of income inequality than the observed mortality.
Conclusions: The observed associations in the United States at the state level between income inequality and mortality cannot be entirely or substantially explained as statistical artefacts of an underlying individual level relation between income and mortality. There remains an important association between income inequality and mortality at state level over and above anything that could be accounted for by any statistical artefact. This result reinforces the need to consider a broad range of factors, including the social milieu, as fundamental determinants of health.

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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© BMJ 1999

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