Two British academics, Richard Wilkinson and Kate Pickett, set out to find why health within a population gets progressively worse the further down the social scale you go. What they found instead is stunning in its implications: almost every modern social and environmental problem — ill-health, lack of community life, violence, drugs, obesity, teen pregnancy, mental illness, long working hours, big prison populations — is more likely to occur in a less equal society.

They have published their observations in the book, The Spirit Level: why more equal societies almost always do better. From the Times Online:

Their eureka moment came when they thought of putting the medical data alongside figures showing the extent of economic inequality within each country. They say modestly that since dependable statistics both on health and on income distribution are internationally available, it was only a matter of time before someone put the two together. All the same, they are the first to have done so.

Their book charts the level of health and social problems — as many as they could find reliable figures for — against the level of income inequality in 20 of the world’s richest nations, and in each of the 50 United States. They allocate a brief chapter to each problem, supplying graphs that display the evidence starkly and unarguably. What they find is that, in states and countries where there is a big gap between the incomes of rich and poor, mental illness, drug and alcohol abuse, obesity and teenage pregnancy are more common, the homicide rate is higher, life expectancy is shorter, and children’s educational performance and literacy scores are worse. The Scandinavian countries and Japan consistently come at the positive end of this spectrum. They have the smallest differences between higher and lower incomes, and the best record of psycho-social health. The countries with the widest gulf between rich and poor, and the highest incidence of most health and social problems, are Britain, America and Portugal.

Richard Wilkinson, a professor of medical epidemiology at Nottingham University, and Kate Pickett, a lecturer in epidemiology at York University, emphasise that it is not only the poor who suffer from the effects of inequality, but the majority of the population. For example, rates of mental illness are five times higher across the whole population in the most unequal than in the least unequal societies in their survey. One explanation, they suggest, is that inequality increases stress right across society, not just among the least advantaged.

So why?

Societies where incomes are relatively equal have low levels of stress and high levels of trust, so that people feel secure and see others as co-operative. In unequal societies, by contrast, the rich suffer from fear of the poor, while those lower down the social order experience status anxiety, looking upon those who are more successful with bitterness and upon themselves with shame. In the 1980s and 1990s, when inequality was rapidly rising in Britain and America, the rich bought homesecurity systems, and started to drive 4x4s with names such as Defender and Crossfire, reflecting a need to intimidate attackers. Meanwhile the poor grew obese on comfort foods and took more legal and illegal drugs. In 2005, doctors in England alone wrote 29m prescriptions for antidepressants, costing the NHS £400m.

Provocative, no?

The implications for public policy are obvious. As is the inevitable skepticism. The book will be released in the U.S. on December 22.

RELATED: In this lecture Wilkinson and Pickett discuss the book.