IJE Advance Access published online on March 15, 2007
International Journal of Epidemiology, doi:10.1093/ije/dym012
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Income inequality and mortality: a multilevel prospective study of 521 248 individuals in 50 US states
1US Bureau of the Census, Washington, DC 20233, USA.
2Statistics Canada, Ottawa, ON K1A OT6, Canada.
3Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal QC H3A 1A2, Canada.
4Department of Epidemiology and Center for Social Epidemiology and Population Health, School of Public Health, University of Michigan, Ann Arbor MI, 48109-2029, USA.
5National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD, USA.
* Corresponding author. Eric Backlund, 6H020, Data Integration Division, US Bureau of the Census, Washington, DC 20233, USA. E-mail: eric.lars.backlund{at}census.gov
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Abstract |
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Background Some of the most consistent evidence in favour of an association between income inequality and health has been among US states. However, in multilevel studies of mortality, only two out of five studies have reported a positive relationship with income inequality after adjustment for the compositional characteristics of the state's inhabitants. In this study, we attempt to clarify these mixed results by analysing the relationship within age–sex groups and by applying a previously unused analytical method to a database that contains more deaths than any multilevel study to date.
Methods The US National Longitudinal Mortality Study (NLMS) was used to model the relationship between income inequality in US states and mortality using both a novel and previously used methodologies that fall into the general framework of multilevel regression. We adjust age–sex specific models for nine socioeconomic and demographic variables at the individual level and percentage black and region at the state level.
Results The preponderance of evidence from this study suggests that 1990 state-level income inequality is associated with a 40% differential in state level mortality rates (95% CI = 26–56%) for men 25–64 years and a 14% (95% CI = 3–27%) differential for women 25–64 years after adjustment for compositional factors. No such relationship was found for men or women over 65.
Conlcusions The relationship between income inequality and mortality is only robust to adjustment for compositional factors in men and women under 65. This explains why income inequality is not a major driver of mortality trends in the United States because most deaths occur at ages 65 and over. This analysis does suggest, however, the certain causes of death that occur primarily in the population under 65 may be associated with income inequality. Comparison of analytical techniques also suggests coefficients for income inequality in previous multilevel mortality studies may be biased, but further research is needed to provide a definitive answer.
Keywords income distribution, differential mortality, socioeconomic factors, multilevel models
The views expressed in this manuscript are attributable to the authors and do not necessarily reflect those of the US Bureau of the Census. Accepted 25 January 2007
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