Commentary: Income inequality and health: The end of the story?

Over the last 10 years, there are few issues that have captured the imagination of public health researchers and advocates, as has the question, whether income inequality drives population health. This was indeed a ‘big idea‘1 that attracted contributions from scholars motivated by the humanitarian potential of showing how health could be improved through greater equity and social justice.2 The question facing us now is whether this idea has had its 15 minutes of academic fame? Our own work3 and several papers recently published in the British Medical Journal prompted an editorial comment by Johan Mackenbach that ‘… evidence for a correlation between income inequality and the health of the population is slowly dissipating‘.4 It is therefore timely, that the International Journal of Epidemiology has revisited Rodgers' study5 as it was the first to directly examine links between income distribution and health, although these issues had been raised in Preston's seminal paper 4 years earlier.6 Rodgers' overall concern was to try to understand the determinants of mortality change, especially in regard to developing countries, and he presaged many of the issues which have since occupied researchers in this field. He recognized that specific factors like clean water, sanitation, food supply and health care—aspects of social infrastructure investment—were important but empirically difficult to disentangle because they tend to be highly collinear with each other and with income. That realization certainly remains salient today.7 He noted that disentangling their specific contributions was important for policy formulation, but not ‘critical for a description of mortality changes in the process of development’ (p. 343). He thus set aside the messy issues of figuring out appropriate confounders and specifying causal pathways that might link aspects of development, income, inequality and population health. Rodgers also explicated what Gravelle later referred to as the ‘artefact’ explanation,8 and showed how the curvilinear individual association between income and health will produce an apparent effect of income inequality on health at the population level. However, he was unable to directly examine this because he only used aggregate data. The issue of control for individual income has been seen as crucial by several later researchers.9–11