There are quite a few interesting comments on my earlier post on The Spirit Level on Economist's View. I can't respond in detail to all of them, but here are a few additional thoughts.
A few commentators seem to think I'm unsympathetic to the book, which isn't accurate. So let me be more direct in my assessment of The Spirit Level: Why Greater Equality Makes Societies Stronger. I find the empirical data presented to be highly suggestive and interesting. Pickett and Wilkinson demonstrate simple trend relationships between the income inequality score of a set of countries and states and a series of social problems. I myself do not have reason to doubt their statistical methods, though other critics have done so, including Lane Kenworthy in a balanced and reasonable critique in 2010. What troubles me about the central argument of the book is what I find to be a flatly unconvincing hypothesis, that the psychological states created in a country’s population by status conflict and income inequality suffice to explain the variation in health outcomes for individuals in these societies. There are many determinants of a person’s satisfaction with his/her life besides income inequality; and there are many social factors that influence health outcomes for individuals besides their perception of income inequalities and the level of stress that those inequalities create in them.
In my view, a good causal explanation requires an analysis of the mechanisms that bring about the relation between cause and effect. I find only one mechanism underlying their accounts of this relationship – essentially a psychological mechanism linking the individual’s perception of income inequalities to health and behavioral outcomes. They flesh this out with a description of the physiology of stress induced by rising income inequalities leading to rising mental illness; perceived status differences leading to decline in trust; stress based on perceived inequalities leading to increased “comfort eating” and obesity; status competition leads to increased violence; etc. This is what I mean by monocausal; there is just one causal process at the heart of their analysis.
The variety of causal mechanisms that seem pertinent to me in consideration of their data include several different kinds. First, as I mentioned in my post, there are serious and systemic sources of separation between individuals within societies that have nothing to do with income inequalities and that might be expected to have the same kinds of psychological and physiological effects on individuals: racism, ethnic hatred, sexism, economic conflicts that do not have to do with inequalities, and so on. For example, in the United States there are credible efforts to explain racial disparities in health outcomes on the basis of the effects on individuals living in circumstances of endemic racial discrimination and prejudice. (See, for example, Arline T. Geronimus, ScD, Margaret Hicken, MPH, Danya Keene, MAT, and John Bound, PhD, "“Weathering” and Age Patterns of Allostatic Load Scores Among Blacks and Whites in the United States"; link.) These effects are unrelated to income inequalities and derive instead from another aspect of social separation. So race rather than income appears to explain this set of health disparities. So perhaps this implies that if we had a way of measuring a “composite index of non-economic social separation” we might have a similar set of graphs, with societies with a lot of racism and ethnic hatred at the upper right quadrant of the graph. I don’t suppose that this index would be highly correlated with the quintile-quintile ratio that the authors use as their proxy for economic inequality.
Second, there are structural influences on a population’s health outcomes that do not derive from the degree of income inequalities that exist in a society but are nonetheless highly influential on health status of the population. For example, the quality and accessibility of rural healthcare has a large influence on the health status of rural people; the accessibility of health insurance to inner city residents has a large influence; the targeting of food marketing in favor of “comfort foods” has a large potential influence on the whole population; and differential treatments and discrimination within health systems for different populations of patients have large potential effects. None of these factors derive from the ratio of quintile incomes – the degree of income inequality – in the society; they are independent causes of differential health outcomes across sub-populations.
I’m not an expert on statistical reasoning; but of course every reader knows that evidence of correlation between two variables A and B is not conclusive evidence of a causal relation existing between those variables. There may be a common factor that causes variation in both A and B, or the correlation may be flatly spurious. It is necessary to frame a credible and testable hypothesis about how the putative causal condition leads to the putative effect. On the other hand, finding that a proposed mechanism is not in fact valid does not establish that there is not a causal connection between the variables; it only mandates that we then search further for a mechanism that does in fact obtain. Only if we are ultimately convinced that there is no possible mechanism would we come to the conclusion that the correlation is spurious. So my position in the posting under discussion is simply that the authors have not established a mechanism linking income inequalities to bad health outcomes that satisfies me.
One can be opposed to extreme and rising income inequalities in one’s society – as many of the readers of The Spirit Level are – without being convinced of the particular causal connections that the authors assert. And this matters quite a bit; if we want to reform society, we need to have well grounded theories of the causal processes that lead to the outcomes we want to reduce or avoid.