post by Paul Kelleher
Last week, Kay H. Smith had a short op-ed in the Birmingham News wherein she says promoting population health...
...will require greater diligence as a citizen in demanding social policies that significantly address income inequality more than greater diligence in exercise and nutrition.
Smith is an MPH student, and she reports surprise when she learned in school that income inequality may have a greater adverse effect on population health than some other well-known culprits (e.g. unhealthful behaviors). I teach in an MPH program, and I also find that students are amazed to learn that income inequality itself causes poor health.
So you can imagine their skepticism when I--the program's link to the humanities--tell them it's not true. Ok, what I tell them is that there is very very little evidence that it's true. This is from a recent survey (pdf) on the issue written by Andrew Leigh, Christopher Jencks, and Timothy Smeeding, and which appears in the Oxford Handbook of Economic Inequality:
[W]e argue that although there are plausible reasons for anticipating a relationship between inequality and health (in either direction), the empirical evidence for such a relationship in rich countries is weak. A few high quality studies find that inequality is negatively correlated with population health, but the preponderance of evidence suggests that the relationship between income inequality and health is either non-existent or too fragile to show up in a robustly estimated panel specification. The best cross-nation studies now uniformly fail to find a statistically reliable relationship between economic inequality and longevity. Comparisons of American states yield more equivocal evidence.
The "more equivocal" evidence referred to in the last sentence is actually utterly fascinating. It turns out that while the international correlation between income inequality and lower life-expectancy was an artifact of poor data (discussed further by Leigh, Jencks, and Smeeding, as well as by Angus Deaton here), the correlation remains for U.S. states and cities. However, according to two papers by Deaton and Darren Lubotsky, the correlation between income inequality and mortality disappears when controlling for the fraction of a metropolitan statistical area that is black:
White incomes are higher and black incomes are lower in places where there is a high fraction of blacks, and this between-race difference induces a strong positive correlation between income inequality and fraction black. Our regressions showed that, once the fraction black was controlled, income inequality as measured by the gini coefficient was no longer a risk factor for mortality. We regard this result as showing that there is no direct effect of income inequality on health.
Deaton and Lubotsky do not conclude that inequality does not have an adverse effect on population health. They deny only that income inequality is bad for health. They therefore do not take their findings to be at odds with the conclusions about status inequalities that emerge from Michael Marmot's Whitehall studies and studies on non-human primates. (Although Deaton has in the past questioned the conclusions Marmot draws from Whitehall, and at least one recent study suggests that the variations in health uncovered by Whitehall may have more to do with health behaviors than was previously thought.)
Finally, Deaton and Lubotsky offer a tentative conjecture as to why the racial composition of a city should have an effect on its population's health:
We are more sympathetic to the idea that racial politics are important...In recent years, work by Bach, Pham, Schrag, Tate, and Hargraves (2004), as well as by the Dartmouth group (Skinner, Chandra, Staiger, Lee, & McClellan, 2005) has shown that America is running something of an apartheid healthcare system, in which most blacks are treated in hospitals or by primary care physicians that treat few or no white patients, and where most whites are treated in hospitals or by primary care physicians that treat few or no black patients. Bach et al. show that the physicians that primarily serve blacks--who may or may not be black themselves--have fewer resources and are less well-qualified. Skinner et al. show that mortality rates after an MCI are higher for all patients in hospitals that treat mostly blacks. In line with this work, our leading hypothesis is that blacks receive worse healthcare than whites, and that this spills over into mortality among whites who live in cities with a large black population and who share, at least in part, their inferior healthcare. This is indeed an important inequality, but it is not an income inequality.
If Deaton and Lubotsky are right, this could call into question two other common pieces of the MPH curriculum. First, it may be that the real racial problem in our health care system is not racial bias by providers, but rather black Americans' confinement to a health care system that's just not very good. Second, it seems that health care does make a difference to population health, contrary to some claims about the superlative importance of the social determinants of health.
(Lane Kenworthy has a related post on Wilkinson and Pickett's The Spirit Level here.)