Causal assessment of income inequality on self-rated health and all-cause mortality: a systematic review and meta-analysis

Shimonovich, M. , Campbell, M. , Thomson, R. M. , Broadbent, P., Wells, V. , Kopasker, D. , McCartney, G. , Thomson, H. , Pearce, A. and Katikireddi, S. V. (2024) Causal assessment of income inequality on self-rated health and all-cause mortality: a systematic review and meta-analysis. Milbank Quarterly, 102(1), pp. 141-182. (doi: 10.1111/1468-0009.12689) (PMID:38294094)

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Context: Whether income inequality has a direct effect on health or is only associated because of the effect of individual income has long been debated. We aimed to understand the association between income inequality and self-rated health (SRH) and all-cause mortality (mortality) and assess if these relationships are likely to be causal. Methods: We searched Medline, ISI Web of Science, Embase, and EconLit (PROSPERO: CRD42021252791) for studies considering income inequality and SRH or mortality using multilevel data and adjusting for individual-level socioeconomic position. We calculated pooled odds ratios (ORs) for poor SRH and relative risk ratios (RRs) for mortality from random-effects meta-analyses. We critically appraised included studies using the Risk of Bias in Nonrandomized Studies – of Interventions tool. We assessed certainty of evidence using the Grading of Recommendations Assessment, Development and Evaluation framework and causality using Bradford Hill (BH) viewpoints. Findings: The primary meta-analyses included 2,916,576 participants in 38 cross-sectional studies assessing SRH and 10,727,470 participants in 14 cohort studies of mortality. Per 0.05-unit increase in the Gini coefficient, a measure of income inequality, the ORs and RRs (95% confidence intervals) for SRH and mortality were 1.06 (1.03-1.08) and 1.02 (1.00-1.04), respectively. A total of 63.2% of SRH and 50.0% of mortality studies were at serious risk of bias (RoB), resulting in very low and low certainty ratings, respectively. For SRH and mortality, we did not identify relevant evidence to assess the specificity or, for SRH only, the experiment BH viewpoints; evidence for strength of association and dose–response gradient was inconclusive because of the high RoB; we found evidence in support of temporality and plausibility. Conclusions: Increased income inequality is only marginally associated with SRH and mortality, but the current evidence base is too methodologically limited to support a causal relationship. To address the gaps we identified, future research should focus on income inequality measured at the national level and addressing confounding with natural experiment approaches.

Item Type:Articles
Additional Information:Funding/Support:This work is supported by Wellcome Trust (218105/Z/19/Z and 205412/Z/16/Z), NHS Research Scotland (SCAF/15/02), Medical Research Council(MC_UU_00022/2), Chief Scientist Office (SPHSU17) and the European Research Council(949582).
Keywords:Causality, income inequality, systematic review.
Glasgow Author(s) Enlighten ID:Katikireddi, Professor Vittal and Thomson, Dr Hilary and Kopasker, Dr Daniel and Thomson, Dr Rachel and Wells, Ms Valerie and Shimonovich, Ms Michal and McCartney, Professor Gerard and Broadbent, Dr Philip and Pearce, Dr Anna and Campbell, Ms Mhairi
Authors: Shimonovich, M., Campbell, M., Thomson, R. M., Broadbent, P., Wells, V., Kopasker, D., McCartney, G., Thomson, H., Pearce, A., and Katikireddi, S. V.
College/School:College of Medical Veterinary and Life Sciences > School of Health & Wellbeing > MRC/CSO SPHSU
College of Medical Veterinary and Life Sciences > School of Medicine, Dentistry & Nursing
College of Social Sciences > School of Social and Political Sciences > Sociology Anthropology and Applied Social Sciences
Journal Name:Milbank Quarterly
ISSN (Online):1468-0009
Published Online:31 January 2024
Copyright Holders:Copyright © 2024 The Authors
First Published:First published in Milbank Quarterly 102(1): 141-182
Publisher Policy:Reproduced under a Creative Commons License

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
3048230021Inequalities in healthAlastair LeylandMedical Research Council (MRC)MC_UU_00022/2HW - MRC/CSO Social and Public Health Sciences Unit
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306430Predicting the impacts of universal basic income on mental health inequalities in the UK population: a microsimulation modelRachel ThomsonWellcome Trust (WELLCOTR)218105/Z/19/ZSHW - MRC/CSO Social & Public Health Sciences Unit
174091Improving life chances & reducing child health inequalities: harnessing the untapped potential of existing dataAnna PearceWellcome Trust (WELLCOTR)205412/Z/16/ZSHW - MRC/CSO Social & Public Health Sciences Unit
308851HEEDSrinivasa KatikireddiEuropean Research Council (ERC)949582SHW - MRC/CSO Social & Public Health Sciences Unit