Review Income inequality and health: A causal review Kate E. Pickett a, * , Richard G. Wilkinson b a Department of Health Sciences, University of York, York, UK b Division of Epidemiology and Public Health, University of Nottingham, Nottingham, UK article info Article history: Available online 30 December 2014 Keywords: Income distribution Review Population health Causality abstract There is a very large literature examining income inequality in relation to health. Early reviews came to different interpretations of the evidence, though a large majority of studies reported that health tended to be worse in more unequal societies. More recent studies, not included in those reviews, provide substantial new evidence. Our purpose in this paper is to assess whether or not wider income differences play a causal role leading to worse health. We conducted a literature review within an epidemiological causal framework and inferred the likelihood of a causal relationship between income inequality and health (including violence) by considering the evidence as a whole. The body of evidence strongly suggests that income inequality affects population health and wellbeing. The major causal 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 periods which are too short. The evidence that large income differences have damaging health and social consequences is strong and in most countries inequality is increasing. Narrowing the gap will improve the health and wellbeing of populations. © 2015 Elsevier Ltd. All rights reserved. Key points  Evidence that income inequality is associated with worse health is reviewed.  It meets established epidemiological and other scientific criteria for causality.  The causal processes may extend to violence and other problems with social gradients.  Reducing income inequality will improve population health and wellbeing. 1. Introduction World leaders, including the US President, the UK Prime Min- ister, 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 in- come 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 re- views, provide substantial new evidence. There is also growing evidence that a wide range of social out- comes, 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. * Corresponding author. Department of Health Sciences, Seebohm Rowntree Building, Area 3, University of York, Heslington, York, YO10 5DD, UK. E-mail address: kate.pickett@york.ac.uk (K.E. Pickett). Contents lists available at ScienceDirect Social Science & Medicine journal homepage: www.elsevier.com/locate/socscimed http://dx.doi.org/10.1016/j.socscimed.2014.12.031 0277-9536/© 2015 Elsevier Ltd. All rights reserved. Social Science & Medicine 128 (2015) 316e326