RICHARD Wilkinson and Kate Pickett, the authors of Spirit Level, which is a bestseller, argue that most of the important health and social problems of the rich world are more common in unequal societies. Using data from 23 rich countries and 50 US states, they found problems are anything from three to 10 times as common in more unequal societies. Again and again, the Scandinavian countries and Japan are at one end of the scale as the most equal, while the US, UK and Australia are at the other.

A key explanation is the psychological impact of inequality which, they say, causes stress and anxiety. For example, maths and literacy scores are lower in more unequal countries, affected by the issues of health, anxiety and depression and consequent drug and alcohol use. The way parents react to relative poverty also affects the way they treat their children, affecting education.

Violence rises in more unequal societies too. Following psychological studies that say men have an incentive to achieve as high a status as possible because their sexual competitiveness depends on it, the authors explain that men use violence when their status is threatened, and more so when there is little status to defend.

“The association between inequality and violence is strong and consistent. The evolutionary importance of shame and humiliation provides a plausible explanation of why more unequal societies suffer more violence.”

Suicide is the only social ill that increases in more equal societies, they say.

Crucially, the authors argue that the evidence shows that all levels of society benefit from more equality, not just the poorest. On health, “at almost any level of income, it's better to live in a more equal place”. Whether rich or poor, inequality causes stress, which causes biological reactions that put pressure on the body and increase illness.

Arguably the most profound conclusion is that economic growth among rich countries has finished its work because it is no longer increasing life expectancy and the only way to do that is to better share the wealth we have.

In its most simple terms, the book yearns for society to celebrate humankind's ability to cooperate and support one another. Are we fighters — which increases inequality? Or are we lovers? The authors say we don't have to see society, as the philosopher Hobbes saw it, as naturally in conflict — “every man against every man” — owing to rivalry for scarce resources.

Instead, “human beings have a unique potential to be each other's best source of cooperation, learning, love and assistance of every kind”.

— The Guardian, London