Christopher Snowdon blogs:
Five years ago, in May 2010, I published The Spirit Level Delusion in response to Richard Wilkinson and Kate Pickett’s book The Spirit Level which was being widely read at the time. Using similar scatter-plots to those used in The Spirit Level, I showed that most of the statistical associations between inequality and social problems that were reported by the authors were the result of selection bias. This was certainly true of the key assertion that income inequality causes poor health and shorter life expectancies (leading to the claim that “inequality kills”).
As shown in The Spirit Level, this is the relationship between income inequality and life expectancy. There is a modest (r2=0.20) correlation, with the less equal countries appearing to have shorter life expectancies.
But, as I said in my rebuttal, the data used by Wilkinson and Pickett for this graph were peculiarly old. Indeed, I got my first hint that all was not as it seemed in The Spirit Level when I turned to the references and noticed that the authors used life expectancy figures from a 2006 report for one graph but used figures from a 2004 report for the graph shown above. Using the more recent figures weakens an already weak relationship, but a more fundamental problem was the exclusion of several countries from their analysis. Wilkinson and Pickett provide a justification for only studying rich countries in The Spirit Level, but there are a number of rich countries that are needlessly excluded. When those countries are added and the data from the 2006 report used, a rather different picture emerges, as the graph below shows:
This them shows a correlation in the opposite direction with r2=0.26. So the authors cherry picked their countries to get the result they wanted. However what if you use up to date data with their original cherry picked countries:
Since it’s been five years since my rebuttal was published, I decided to look at the most recent life expectancy statsand see how The Spirit Level was holding up. The results are interesting. Even if you limit the analysis to The Spirit Level’s questionable group of countries, the association with inequality has completely disappeared (r2=0.02). This remains true if you include the countries added above and if you use different measures of inequality.
That is coefficient of determination very close to zero.
Will the authors publish a revised edition?
It seems that the relationship between inequality and life expectancy only holds when we use data from early in the last decade and arbitrarily exclude a number of countries. It fails the basic scientific test of reproducibility. A law that only works under certain circumstances and in certain years is no law at all.
The fact that a correlation no longer exists even when we confine the analysis to the countries that were specially selected in The Spirit Level does not leave much wriggle room for the book’s authors. It is difficult to exaggerate to importance of the supposed link between life expectancy and inequality to The Spirit Level’s argument.