It isn't wealth that's good for our health but income equality and friendship, writes Richard Wilkinson.
Societies with a wider income gap between rich and poor tend to have lower standards of health and higher levels of violence. More than 20 studies show the links between income inequality and poor health, and more than 30 show the links with violence. The US is richer and spends more on medical care than other countries, yet its huge income inequalities mean its life expectancy lags behind almost all other rich countries. It is not the richest states, but rather the most egalitarian ones - such as Sweden and Norway - that enjoy the best health and lowest levels of violence.
Understanding how income inequality affects people means understanding the impact of social hierarchy. A crucial clue is that more unequal societies develop an anti-social culture: greater income inequality leads to a deterioration in the social environment marked not only by more violent crime, but also by more hostility, less trust and less involvement in community life.
Death rates are usually two or three times higher among people near the bottom of the social hierarchy than for people near the top. But rather than being the direct effect of lower material living standards alone, research from sources as varied as international comparisons and animal studies suggests the health damage may have more to do with subordination and inferior social status themselves. Move monkeys between social groups to alter their status while holding diet and the material environment constant and subordinate status produces similar health effects among them as among humans.
By contrast, almost any measure of friendship is associated with good health. Death rates among people with weak social affiliations are two or three times as high as among those who have strong social connections. Experiments in which people are exposed to the same risk of infection (from nasal drops containing cold viruses) showed that those with friends in few areas of life are over four times as likely to develop colds as people with more friends.
Why do social relations deteriorate as inequality increases, and why do they both exert such powerful influences on health? The answer seems to be that they are two sides of the same coin. Social ranking and pecking orders are about preferential access to resources based on power and coercion - regardless of others' needs. Friendship, on the other hand, is about sharing resources, social obligations and a recognising each others' needs. These are the two opposite bases on which human beings can associate: access to resources is either a matter of power and strength, or of social reciprocity and sharing.
Because members of the same species have the same needs, they are potentially each other's worst competitors. Yet people can also be the best source of help, cooperation, love and learning. As the nature of social relations has always been crucial to human welfare, we have become highly attentive to these dimensions of social life - so attuned that social relations are powerful generators of either stress or mutual support.
Health suffers because chronic stress gets under the skin. Its effects are analogous to more rapid ageing and make us more vulnerable to a wide range of diseases.
This research highlights modern society's shortcomings. Powerful risk factors point to intensely social dimensions of health. What is new is to see that these dimensions of personal life have powerful structural determinants.
Richard Wilkinson is professor of social epidemiology, University of Sussex, and author of Mind the Gap: Hierarchies, Health and Human Evolution , published this week by Weidenfeld & Nicolson, £7.99.