In rich countries, death rates appear to be related to the differences between incomes, rather than to absolute income levels.

This article first appeared in the New Statesman's 'Big Ideas' feature, 26 July 2004.

Among those committed to understanding the mind as the work of natural selection, there is a sense that the time has come: we are now beginning to see what we really are. Two major propositions have emerged, sustained by a construction boom in Darwinian theory and the confidence that supporting data will increasingly be delivered in hard genetic currency. One is that human nature is evolved and universal; the other is that variations in personality and mental capabilities are substantially inherited. The first speaks of the species and the second about individuals. That leaves society - and here a third big idea is taking shape. In two words, inequality kills.

The phrase (which is that of Richard Wilkinson, one of the leading researchers in the field) sticks out from current consensus like a sore thumb. For the most part, the major biological ideas concerning human nature and mental capabilities are seen to confirm the way the world has turned out. In a world so seemingly short of serious alternatives to the way it is currently arranged, that is only as expected. But what might be the biggest biological idea of all, in terms of its implications for human health and happiness, shows the world in a very different light. It finds that society has a profound influence over the length and quality of individuals' lives. The data are legion and the message from them is clear: unequal societies are unhealthy societies. They are unhealthy not just in the strict sense but also in the wider one, that they are hostile, suspicious, antagonistic societies.

The most celebrated studies in this school of thought are those conducted among Whitehall civil servants by Michael Marmot, whose recent book Status Syndrome presents his ideas in popular form. He and his colleagues found a steady gradient in rates of death between the lowest and the highest ranks of the civil service hierarchy. Top civil servants were less likely to die of heart disease than their immediate subordinates, and so on down the ladder; at the bottom, the lowest grades were four times more likely to die than the uppermost. The key features of these findings were that the gradient was continuous, and that only about a third of the effect vanished when account was taken of the usual lifestyle suspects such as smoking and fatty food. This influence upon life and death affected everybody in the hierarchy, according to their position in it. Differences in wealth were an implausible cause in themselves, for most of the civil servants were comfortably off and even the lowest paid were not poor. The fatal differences were in status.

What goes for Whitehall seems to go for the world. In rich countries, death rates appear to be related to the differences between incomes, rather than to absolute income levels. The more unequally wealth is distributed, the higher homicide rates are likely to be. Although the findings about income inequality are controversial, the broad picture is consistent; and remains so if softer criteria than death are measured, like trust or social cohesion. Inequality promotes hostility, frustrates trust and damages health.

It is hard to make sense of these findings outside a framework based on the idea of an evolved psychology. Understanding humans as evolved social beings, however, made what we are by the selective pressures of life in groups of intelligent beings, it is easy to see that our minds and bodies depend upon our relations with our kind. These relations assume central importance for our health once economic development has minimised the dangers of infectious disease and relegated starvation to history.

Studies of baboons, social primates obliged by their nature to form hierarchies, tell the same story. A state of subordination is stressful; such stress may put the body into a mode that is vital in emergencies but corrosive as a permanent condition, interfering with the immune system and increasing the risk of heart disease. Conversely, human relationships formed on a broadly equal basis may support the immune system and promote health. An American researcher, Sheldon Cohen, demonstrated this by dripping cold viruses into volunteers -- noses, and then asking them about the range and frequency of their social relationships. The more connections they had -- with acquaintances, colleagues, neighbours and fellow club members as well as with nearest and dearest -- the less likely they were to develop colds.

The relationship between the length of life and its everyday quality is the relationship between its biological and social dimensions, which demands an evolutionary explanation; and the findings seem to demand egalitarian measures. It's an unfamiliar combination. But Darwinian readings of the data on health and equality are not incompatible with claims that humans are innately unequal. They do, however, lead to markedly different views of how to make the best of people.

So do the prior ethical commitments that evolutionary thinkers bring to their projects. In his book The Blank Slate, having stated the case for the substantial innateness of all human characteristics and their imperviousness to parental influence, the psychologist Steven Pinker devotes a chapter to denouncing the past century's art and its associated discourses. Folk wisdom and popular taste are right, he affirms; "elite art" is perverse and wrong. The argument is built upon the idea that we all share an evolved human nature, but it would not be terribly difficult to remove the Darwinian passages and produce a standard-issue comment piece for those pages of right-leaning newspapers that are devoted to castigating the liberal elite.

Pinker turns his moral compass to take bearings on literary reference points such as 1984, that affirm the individual and condemn attempts to impose equality upon humankind's natural inequality. At a fundamental level, modern Darwinism encourages individualism, for it holds that evolutionary processes act on individual organisms rather than upon groups of organisms. It makes no particularly strong predictions about variations among individual human minds. That part of the picture comes from the behaviour geneticists, who compare identical twins with fraternal twins (or study their prize specimens, identical twins who have been reared apart) and conclude that a large proportion of the variation between individuals' personality traits, temperaments and intelligence is due to inherited differences. Such findings readily lend themselves to a view of the world which attaches great importance to allowing individuals to fulfil their potential, while regarding social programmes to reduce inequalities as vain at best. Equality of opportunity is a fundamental principle; equality of outcome is a pernicious fantasy.

The result is an upbeat fatalism; upbeat about the prospects for scientific understanding of human psychology, fatalistic about the prospects that society might be improved by such understanding -- and upbeat, also, in the confidence that society needs no radical alteration. Many of those who dislike such visions collude in them, by acquiescing in the assumption that the effects of environments can be altered but those of genes cannot, and by failing to recognise the words "tend to". The big idea that provides much of the driving force for evolutionary psychology, the project to describe a universal human nature, is that the sexes have different reproductive interests. The sex which invests the most in reproduction will be the one which takes more care in its choice of mates. Among humans, this implies that women will tend to be more discriminating than males in their choice of partners. It also implies that men and women will have different emotional propensities -- as Stephen Jay Gould put it, conceding the central principle of evolutionary psychology in the very act of deploring the neo-Darwinian school. It does not imply that every woman will be more circumspect in choice of partners than every man, or that every man will be readier to take risks than every woman, any more than the tendency for men to be taller than women means that all men are taller than all women. Through the widespread failure to recognise that evolved behaviours and ways of thinking are tendencies, evolutionary psychology has determinism thrust upon it.

In the application of evolutionary perspectives to health and equality, however, the prospect of a better society -- or at least of better communities or workplaces -- is unmistakeable. This way of understanding human nature has the qualities that have marked great Darwinian ideas since the Origin of Species: it is profound in its implications, potentially transformative, and challenges existing wisdom. On one hand, it calls into question the idea that equality of opportunity should be pursued without regard for equality of outcome. On the other, it goes beyond the mechanistic assumption that the task of "progressive" politics is to ensure that the least well off have enough, emphasising that how much is enough depends on how much others have. It replaces vestigial sentiments about the abstract virtue of co-ops and community spirit with data about life and death, implying that we would all (or almost all) be healthier and happier if we were prepared to share more of what we have. It speaks to the world we live in, where want is marginal but trust is precarious.

In Richard Wilkinson's words, it is "the science of social justice". Like other big evolutionary ideas, though, it may be honoured more by denial than by engagement.