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Original research
State-level association between income inequality and mortality in the USA, 1989–2019: ecological study
  1. James R Dunn1,
  2. Gum-Ryeong Park1,2,
  3. Robbie Brydon1,
  4. Michael Veall1,
  5. Lyndsey A Rolheiser3,
  6. Michael Wolfson4,
  7. Arjumand Siddiqi2,5,
  8. Nancy A Ross6
  1. 1McMaster University Faculty of Social Sciences, Hamilton, Ontario, Canada
  2. 2University of Toronto Dalla Lana School of Public Health, Toronto, Ontario, Canada
  3. 3York University Schulich School of Business, Toronto, Ontario, Canada
  4. 4University of Ottawa Faculty of Medicine, Ottawa, Ontario, Canada
  5. 5The Hospital for Sick Children, Toronto, Ontario, Canada
  6. 6Queen's University, Kingston, Ontario, Canada
  1. Correspondence to Dr James R Dunn, Health, Aging & Society, McMaster University, Hamilton, Canada; jim.dunn{at}mcmaster.ca

Abstract

Background Prior studies have shown a positive relationship between income inequality and population-level mortality. This study investigates whether the relationship between US state-level income inequality and all-cause mortality persisted from 1989 to 2019 and whether changes in income inequality were correlated with changes in mortality rates.

Methods We perform repeated cross-sectional regressions of mortality on state-level inequality measures (Gini coefficients) at 10-year intervals. We also estimate the correlation between within-state changes in income inequality and changes in mortality rates using two time-series models, one with state- and year-fixed effects and one with a lagged dependent variable. Our primary regressions control for median income and are weighted by population.

Main outcome measures The two primary outcomes are male and female age-adjusted mortality rates for the working-age (25–64) population in each state. The secondary outcome is all-age mortality.

Results There is a strong positive correlation between Gini and mortality in 1989. A 0.01 increase in Gini is associated with more deaths: 9.6/100 000 (95% CI 5.7, 13.5, p<0.01) for working-age females and 29.1 (21.2, 36.9, p<0.01) for working-age males. This correlation disappears or reverses by 2019 when a 0.01 increase in Gini is associated with fewer deaths: −6.7 (−12.2, –1.2, p<0.05) for working-age females and −6.2 (−15.5, 3.1, p>0.1) for working-age males. The correlation between the change in Gini and change in mortality is also negative for all outcomes using either time-series method. These results are generally robust for a range of income inequality measures.

Conclusion The absence or reversal of correlation after 1989 and the presence of an inverse correlation between change in inequality and change in all-cause mortality represents a significant reversal from the findings of a number of other studies. It also raises questions about the conditions under which income inequality may be an important policy target for improving population health.

  • epidemiology
  • health policy
  • public health
  • economics

Data availability statement

Data are available upon reasonable request. All data used in this study are publicly available and also available from the corresponding author.

https://doi.org/10.1136/jech-2024-222262

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    Data availability statement

    Data are available upon reasonable request. All data used in this study are publicly available and also available from the corresponding author.

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    Footnotes

    • Contributors JRD conceived the idea for the work and is the guarantor. The research design and analysis plan were jointly developed by all authors. G-RP and RB acquired data and conducted analyses. All authors interpreted results. The manuscript was drafted by JRD, G-RP and RB, then revised by all authors.

    • Funding This work was supported by grant #162117 from the Canadian Institutes of Health Research (CIHR). The sponsor had no involvement in or control over the study and does not endorse it in any way. All authors have completed the ICMJE uniform disclosure form at http://www.icmje.org/disclosure-of-interest/ and declare: all authors had financial support from CIHR for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

    • Competing interests None declared.

    • Provenance and peer review Not commissioned; externally peer reviewed.

    • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.