Lynch, Harper, and Davey Smiths metaphor of the SS Income Inequality1 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 mortality2 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 disappeared6 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.
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On not adjusting for average state income |
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all we can conclude is that health effects of income inequality remain after adjustment for per cent black, but that this income inequality effect was unadjusted for mean income.1
Since it is a straightforward matter to add state-level income in our multilevel regression model, we now present a comparison of Model 4 estimates that appeared in our paper8 with and without adjustment for 1990 US Census median income for the states (Table 1).
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On the charge that individual income explains the contextual effect of income inequality |
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On not adjusting for educational attainment |
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controlling for education attenuated but did not completely explain the relation between levels of state income inequality and self-rated health. Our results do not support the contention that education at the individual level fully confounds or mediates the association of income inequality with health.10
We remain mystified by the apparent weight that prosecutors continue to give to ecological evidence when everyone in the courtroom agrees that this is severely problematic and that multilevel evidence is undoubtedly more reliable.
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On testing the income inequality hypothesis within the US |
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On not adjusting for regional fixed effects in US data |
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On testing the income inequality hypothesis elsewhere |
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The ability of income inequality to explain health variations between rich countries was just one of the original predictions of the theory. In ecological data, it may seem that income inequality does not explain variations in life expectancy between rich countries.19 However, such a conclusion may be erroneous since it fails to account for the within-country variations (both at the individual as well as at the area level within each country), not to mention that the cited evidence is again based on ecological cross-sectional data. Failure to corroborate one prediction should not result in a death penalty for the whole theory. Nor does it mean that income inequality does not exist in non-OECD countries, or that it is unimportant for the health of people in less economically developed nations.
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On the choice of the outcome |
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On national time trends in income inequality and population health |
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First, we must recognize that it is often very difficult to tease out causality from ecological time trend data, especially in the absence of knowledge about relevant induction times and lag periods. We have previously pointed out that a naïve analysis of smoking trends and lung cancer rates among US women risks the erroneous interpretation that quitting smoking (as US women have been doing since 1964) increases the risk of lung cancer.21 Such an interpretation is erroneous, because the rising rates of lung cancer in US women actually reflect their increased uptake of smoking 2030 years earlier. If income inequality takes 1015 years to affect population health,11,22 then it may still be too soon to detect an adverse impact of the surge in income inequality that occurred in the US in the 1980s and especially the 1990s. We concur with Lynch and colleagues that a life-course perspective would ideally take into consideration such lag effects, and as such, less reliance ought to be placed on crude visual inspection of time trend data to dismiss a theory.
Secondly, although much of the multilevel evidence on income inequality and health has been cross-sectional to date, this does not mean that the investigators were assuming an instantaneous effect of income inequality on health outcomes. The same US states that were unequal in 2000 were also unequal in the 1980s and 1990s. The state rankings of income inequality do not change a whole lot. Armed with this observation, it is still possible that cross-sectional data yields the correct answer, and as such capture the cumulative damage to health wrought by decades of living under conditions of inequality.
Lastly, ecological time trend data of the sort that Lynch and colleagues produce says little about what is happening to the health of sub-groups, such as the poor. Average life expectancy can improve for a nation, even as the health of disadvantaged groups stagnates or even deteriorates. Repeatedly adducing observations of questionable qualitywhether from New Zealand, South Korea, or the USdoes not amount to convincing epidemiological evidence to refute the income inequality theory. What would be helpful first steps are detailed examinations of the time trends in health of different (and potentially vulnerable) sub-groups. Indeed, no such evidence has been produced to date (and in the US would be very difficult to produce, given the lack of socioeconomic information on official vital records).
To summarize, we hope to have clarified our stand on the defence of the thesis that income inequality is a public health concern. No doubt our arguments will not be the last word on each of the issues we have raised. But further debate and discussion, based upon fresh multilevel empirical evidence (incorporating time as well as place dimensions), would be far preferable to hasty judgements formed on the basis of less than complete information and analysis.
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References |
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