Abstract
BackgroundIncome inequality has been linked to health and mortality. While there has been extensive research exploring the relationship, the evidence for whether the relationship is causal remains disputed. We describe the methods for a systematic review that will transparently assess whether a causal relationship exists between income inequality and mortality and self-rated health.MethodsWe will identify relevant studies using search terms relating to income inequality, mortality, and self-rated health (SRH). Four databases will be searched: MEDLINE, ISI Web of Science, EMBASE, and the National Bureau of Economic Research. The inclusion criteria have been developed to identify the study designs best suited to assess causality: multilevel studies that have conditioned upon individual income (or a comparable measure, such as socioeconomic position) and natural experiment studies. Risk of bias assessment of included studies will be conducted using ROBINS-I. Where possible, we will convert all measures of income inequality into Gini coefficients and standardize the effect estimate of income inequality on mortality/SRH. We will conduct random-effects meta-analysis to estimate pooled effect estimates when possible. We will assess causality using modified Bradford Hill viewpoints and assess certainty using GRADE.DiscussionThis systematic review protocol lays out the complexity of the relationship between income inequality and individual health, as well as our approach for assessing causality. Understanding whether income inequality impacts the health of individuals within a population has major policy implications. By setting out our methods and approach as transparently as we can, we hope this systematic review can provide clarity to an important topic for public policy and public health, as well as acting as an exemplar for other “causal reviews”.
Highlights
Income inequality refers to the uneven distribution of income between people, assessed across regions, states, or countries [1, 2]
A rigorous, systematic, and transparent evaluation of the evidence and causality is necessary to be certain of the relationship between income inequality and health, which can clarify the need for policymakers to intervene on the exposure [11, 12]
Pickett and Wilkinson [14] incorporated Bradford Hill viewpoints to their review assessing the relationship between income inequality and mortality
Summary
Income inequality refers to the uneven distribution of income between people, assessed across regions, states, or countries [1, 2]. A rigorous, systematic, and transparent evaluation of the evidence and causality is necessary to be certain of the relationship between income inequality and health, which can clarify the need for policymakers to intervene on the exposure [11, 12]. Pickett and Wilkinson [14] ( included in the review of reviews [2]) incorporated Bradford Hill viewpoints to their review assessing the relationship between income inequality and mortality. They did not use a systematic approach to searching, identifying, analysing, evaluating, and synthesising evidence. We describe the methods for a systematic review that will transparently assess whether a causal relationship exists between income inequality and mortality and self-rated health
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