ReviewIncome inequality and health: A causal review
Introduction
World leaders, including the US President, the UK Prime Minister, the Pope and leaders at the International Monetary Fund, the United Nations, World Bank and the World Economic Forum have all described income inequality as one of the most important problems of our time and several have emphasized its social costs (Cameron, 2009, Elliott, 2014, Lagarde, 2013, Moon, 2013, Obama, 2014, Pope Francis, 2013, World Economic Forum, 2014). Inequality is increasing in most regions of the world, rapidly in most rich countries over the past three decades (OECD, 2011, Ortiz and Cummins, 2011). There is a very large literature examining income inequality in relation to health. Early reviews came to different interpretations of the evidence, though a majority of studies reported that health tended to be worse in more unequal societies (Lynch et al., 2004, Macinko et al., 2003, Subramanian and Kawachi, 2004, Wagstaff and van Doorslaer, 2000, Wilkinson and Pickett, 2006). More recent studies, not included in those reviews, provide substantial new evidence.
There is also growing evidence that a wide range of social outcomes, associated with disadvantage within societies, are more common in societies with bigger income differences between rich and poor. Although our objective in this paper is to assess whether or not wider income differences play a causal role leading to worse health (including the public health issue of violence), we consider studies of other social outcomes where they affect interpretation of the health data.
The first task is to clarify the causal hypothesis, and how it has developed as research has progressed. Research was initially focused simply on whether health was worse in more unequal societies, but there is now growing evidence to suggest that this should be seen as part of a wider tendency for a broad range of outcomes with negative social gradients (i.e. more prevalent where social status is lower) to be more common in societies with bigger income differences between rich and poor. Rather than this pattern being confined to physical health, it may apply also to mental health, and public health issues such as violence, teenage births, child wellbeing, obesity, and more.
Whether causality is tested in relation to a hypothesis confined to a relationship between inequality and physical health, or whether the hypothesis extends to problems with social gradients more generally, has important implications for understanding possible causal mechanisms, mediators and confounders.
In this paper, we will focus on the strongest and most important claim underpinning an effect of inequality on health: that large income differences between rich and poor lead to an increasing frequency of most of the problems associated with low social status within societies. Fig. 1 provides an illustration of the relationships with which this paper is concerned. It shows a cross sectional association between income inequality in developed countries and an index which combines data on: life expectancy, mental illness, obesity, infant mortality, teenage births, homicides, imprisonment, educational attainment, distrust and social mobility. (Raw scores for each variable were converted to z-scores and each country given its average z-score (Wilkinson and Pickett, 2009).)
Section snippets
History
The hypothesis that problems (including poor health) associated with low social status are more common in more unequal societies can be traced back to independent roots in papers on homicide rates and on mortality rates. The research literature on homicide and inequality goes back at least 40 years, to a demonstration that they were positively associated among states in the USA (Loftin and Hill, 1974). The earliest paper on mortality and income inequality – some 35 years ago – showed a
Popperian theory testing
The philosopher of science, Sir Karl Popper, taught that the best evidence of the value of a theory was provided by testing its novel predictions (Popper, 2002, Popper, 2014). A successful theory was ‘corroborated’ (but could never be finally proven true) if it accurately predicted the results of scientific observations which had not previously been expected. The initial evidence of a relation between income inequality and population health using international data was first explicitly tested
Epidemiological criteria for causality
In observational epidemiology, causality cannot be proven or disproven by any single study – there are no ‘black swans’ – just because income inequality might not affect some health outcomes, or not in some times or places or for some populations, does not mean that it isn't a causal relationship in other contexts. Instead, in epidemiology, a body of evidence needs to be considered, usually including non-epidemiological studies, to judge whether or not an exposure–outcome relationship is
Discussion and conclusions
The body of evidence on income inequality and health points strongly to a causal connection. The major criteria of temporality, biological plausibility, consistency and lack of alternative explanations are well supported. Of the small minority of studies which find no association, most can be explained by income inequality being measured at an inappropriate scale, the inclusion of mediating variables as controls, the use of subjective rather than objective measures of health, or follow up
Acknowledgments
The authors are grateful to Robert Kaplan, Daryn David, and Michael Spittel of the US National Institutes of Health, Office of Behavioral and Social Sciences Research, who first invited us to write a paper on causality for NIH OBSSR. We would also like to thank Hector Rufrancos for collating the time-series literature.
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