Part one presents their basic argument. Above a certain level, further increases in material wealth — income per person — are no longer correlated with improvements in child well-being, health, social trust, and so forth. Instead, these are strongly correlated with income equality, while inequality is correlated with crime levels, teenage births, obesity, and other social problems. This holds in comparisons across some two dozen of the world's wealthiest countries, and across the fifty states of the United States.
A possible causal mechanism lies in the psychological effects of social inequality. It is notable that even the wealthy are better-off on health and social metrics in more equal countries.
The nine chapters in part two examine this correlation in a broad range of different areas: social trust and the status of women, mental health and drug abuse, health and life expectancy, obesity, educational performance, teenage births, violence, prisons and punishment, and social mobility and equality of opportunity. Charts plot key measures of these against inequality, as measured among wealthy countries and the states of the United States, with lines of best fit drawn where significant.
Part three steps back to provide some context for all this data. One chapter considers alternative causal explanations, such as race, single parents and different cultural histories. None of these are convincing, with the correlations apparently robust against, for example, the exclusion of the Scandinavian countries or the lumping together of the Anglo-Saxon countries. The possibility that causality goes the other way, with social problems causing inequality, is also considered.
Turning to our social inheritance, Wilkinson and Pickett offer a bit of evolutionary psychology, but emphasize human flexibility in coping with different kinds of social structures, conditioned on our experiences in early childhood. They also consider the relationship between inequality and sustainability, looking at consumerism, greenhouse emissions, and so forth.
A final chapter on policy recommendations is perhaps the weakest part of The Spirit Level. The ideas Wilkinson and Pickett present are interesting, but it seems unlikely that employee ownership of businesses, the idea they spend the most time on here, will be more than a small part of a solution. There's also the question of finding a negotiable political path towards greater equality, though that may take different forms in different countries and is a topic which really deserves a book — or a political manifesto — of its own.
There's a lot in The Spirit Level, but it offers no big surprises. With so many different areas covered, the individual analyses can't go into much detail either in the data analysis or the evaluation of possible causes. Nor is the overall thesis novel. It is, however, valuable to have such a broad range of data brought together, and the analysis and interpretation is compelling.