There is a debate over at Prospect on the Spirit Level thesis that a more equal society is better for everyone. W&P are the proponents, Christopher Snowdon is one of the opponents.
Snowdon objects that W&P's method "relies on comparing whole countries, a notoriously unreliable method which leaves unlimited scope for misinterpretation". However, he uses this method in his book, adding in three countries, the Czech Republic, Slovenia and Hungary, which has the desired effect of reducing or inverting the gradient in W&Ps results.
Clearly, there are many factors which go to generate the data that any one country will display. Overall wealth, culture, traditional social bonds, climate, ethnic mix, pollution levels and politics; these are a few that come to mind. Considering these many factors makes it all the more remarkable that W&P have produced so many clear correlations between inequality and health. They mention in TSL that the multifactorial nature generates the wide scatter on so many graphs. If we had a reliable means of weighting these factors, producing an index of income equality modified by other variables, the scatter might be reduced.
The effect of Snowdon's introducing the trio of ex-Soviet countries is interesting. Although more equal, they tend to score higher with some social problems, which suggests that there are special factors associated with the rigid political system that they are emerging from. Communism may have produced income equality, but there was a huge inequality of political power between the people and the Party, and Marmot's work suggests that this would produce health problems.
Having read W&P and Snowdon, I am confident that W&P's thesis is robust, but capable of adapting to and incorporating some of the dimensions that Snowdon's book suggests - once they have been supported by serious research.