Although first published under a Labour government in 2009, this book is still highly relevant now we have a Conservative / Liberal Democrat coalition. In fact, it is even more relevant because the current political and economic circumstances are forcing politicians to think carefully about how much we are worried about inequality of outcome. Wilkinson and Pickett argue that widespread inequality helps increase a huge range of social ills, with the result that everyone suffers - even the most well off. Inequality in their view isn't just bad for the poor, it's also bad for the rich.
Analysing data primarily from 21 developed countries and also the different American states, they present evidence of a correlation between the level of inequality in each country (or state) and a range of outcomes: levels of trust, mental illness, life expectancy, infant mortality, obesity, children's educational performance, number of teenage births, murders, imprisonment rates and social mobility. More inequality goes with lower trust, more mental illness, higher murder rates and so on.
Within a particular society being richer may go with the problem being smaller for yourself, but across the society as a whole it is the level of inequality that, they say, determines the overall levels of the problem.
The authors therefore argue that rather than securing further economic growth, inequality is now the big challenge facing developed societies: "When the wolf was never far from the door, good times were times of plenty. But for the vast majority of people in affluent countries the difficulties of life are no longer about filling our stomachs, having clear water and keeping warm. Most of us now wish we could eat less rather than more. And, for the first time in history, the poor are - on average - fatter than the rich."
As statisticians everywhere says, correlation does not mean causation - so the authors go on from their presentation of the case that higher inequality goes with worse outcomes across their measures (and there is a debate over how significant a correlation their evidence shows) to present pieces of evidence that it is inequality which is actually causing those worse outcomes.
In particular, they pick from relatively recent medical advances showing how stress brings about chemical changes in the body that then has very tangible effects. Added to this is evidence that a person's sense of self-worth has an important impact on their ability to carry out tasks - so again more inequality leads to a worse outcome for individuals. Moreover, "the evolutionary importance of shame and humiliation provides a plausible explanation of why more unequal societies suffer more violence".
In a way this is very optimistic book, for if all these ills have a common factor - inequality - then in turn doing something about inequality could bring very widespread benefits. That clarity and simplicity of prognosis as to how to improve society makes the book far more optimistic than previous accounts of the ills of modern society such as JK Galbraith's The Culture of Contentment.
However, this optimistic logic highlights one of the book's weaknesses. Not only does it rely overwhelmingly on comparisons across countries at the same point in time, rather than in tracking ailments varying over the years, the limited amount of such evidence deployed is almost all of the `inequality increased and then things got worse' form. There is no automatic reason why, even if increasing inequality makes things worse, then decreasing it will make things better. The world is not always symmetrical. Moreover, even if the effect works strongly `in reverse', is it the most cost-effective route to take? If inequality causes stress which causes social ills, is targeting stress going to be more successful?
Despite these questions left unanswered, the book is an important contribution in urging politicians to see social problems as having social solutions; the focus needs to be on society and not on just individuals.