Response: In defence of the income inequality hypothesis

Abstract

Lynch, Harper, and Davey Smith’s metaphor of the SS Income Inequality 1 is amusing, but we think that a more accurate representation of the current debate in this area would be a kangaroo court, in which the defendant (viz. the hypothesis that income inequality is detrimental to population health) is in imminent danger of being summarily executed without the benefit of a fair hearing. Indeed, some jurors already seem to have decided that a relationship between income inequality and health does not exist. One recent assertion, for instance, was that ‘statistical adjustment for ethnicity statistically accounts for all of the association between income inequality and health within the US’. 2 Other assertions, based on an ecological analysis, 3 were that ‘adjustment for education … also accounted for all of the association between income inequality and mortality‘ 2 and that the ‘evidence for the income inequality hypothesis is weak, beyond its important mechanical effects on individual income’, 1 also based on ecological evidence. 4,5 Examples of other claims include: ‘the evidence favoring a negative correlation between income inequality and life expectancy has disappeared’ 6 and that ‘we can muster little evidence to show that the extent of income inequality, per se, affects population health’. 7 These are strong claims which, taken at face value, imply that income inequality is not a public health concern and the public health community has no cause to be alarmed about the sharp increase in income inequality that has occurred in the last two decades both within and between countries. However, we are not so confident that the income inequality story can be so hastily dismissed. In particular, several key accusations levelled by the prosecutors in this case can be tested with new evidence and better-designed studies. As witnesses for the defence, we would like to draw the attention of the jurors to evidence based on the more appropriate multilevel methods.