An actress tosses her hair and lets drop the catchphrase, "Because I'm worth it"; by her own estimation, she is worth a million dollars per episode of the sitcom in which she stars, but settles for $750,000.

A pop star spends £15,000 a month on flowers. A top footballer is said to be looking at earnings of £100,000 a week, the rate phrased as if he was still a worker on a wage for all that.

People raise their eyebrows at sums like these, but they rarely raise their voices. Celebrities' excesses are, at worst, regarded as displays of poor taste - but the more tasteless the display, the more entertaining it is. The public feels resentment when boardrooms seem to be appropriating wealth whose source is public; in utility businesses, or High Street banks which hold substantial fractions of the public's money. It is largely indifferent to executive pay in companies whose business is perceived as private. And if anything it likes celebrities the better for their wealth. A hint of disapproval is part of the fun.

As far as entertainers are concerned, this is nothing new. Stars have always been felt to deserve their wealth for the pleasure they give. For those who emerged from poverty, a few nods in the direction of their origins would suffice to quell any resentment from below. But entertainers were treated as an exception to a general notion of fair shares, which was sustained in large measure by the conviction that the rich were rich at the poor's expense.

We now have half a century's experience of mass prosperity, but our understanding of the value of money is still based on scarcity. Once all but a few - however visible - are secure in the essentials, the force drains out of arguments about inequality. What would it matter if David Beckham were paid £100,000 a week, if those watching him earn £20,000 a year? If his anticipated 'wages' were redistributed around the stadium, they would stretch to a round of drinks; but the spectators don't need an extra pint any more than he needs the money. In any case, celebrities' whims are seen to redeem themselves through their redistributive effects. If Sir Elton John requires blooms in industrial quantities, he is sustaining an industry.

Wealth is presumed to be good in itself, however it is distributed. Even if it only trickles down, a trickle is better than nothing. But a growing body of data suggests that this common-sense assumption may be unwise. Inequality - not just the difference between comfort and want, but inequality per se - appears to do consistent and profound damage to health. Richard G. Wilkinson, one of the leading interpreters of inequality research, puts it bluntly: "Inequality kills".

One of the most remarkable findings in this field concerns the effect of infant mortality, surveyed across 70 countries by Robert J. Waldmann. Take two countries in which the bottom 20 per cent are equally poor (the effect can be achieved by making statistical adjustments) but the top five per cent in one are richer than their counterparts in the other. Common sense would suggest that the country with more resources at the top would have the lower infant mortality rate. But the reverse is the case: the babies of the poor are more likely to die in the country whose rich are wealthier.

Around the world, the figures affirm that unequal societies are unhealthy societies. Most of these data concern mortality statistics, as these are more reliable than other measures of health. The distinction between absolute and relative poverty corresponds to the 'epidemiological transition', from conditions in which infectious diseases are the major killers to ones in which the so-called diseases of affluence are the most feared. Above this threshold, absolute wealth makes little difference to life expectancy. Greeks are healthier than Americans, although average American incomes are more than twice as high. The objection that this may represent some other environmental effect, such as the superiority of the Mediterranean diet over the hamburger, is countered by comparison among the United States. The states with larger gaps between rich and poor have higher death rates, even after controlling for factors as diverse as absolute poverty, race and tobacco. Within developed countries in general, death rates at the lowest levels of the social hierarchy are between two and four times higher than those at the top.

Though the differential is striking, its meaning may be somewhat opaque to non-specialists: everybody dies in the end, after all. To convey the implications, Richard Wilkinson provides an intuitive way of understanding death rates. Imagine two people, one living in a rich area and one in a poor area, with similarly sized circles of friends. The one in the poor area will have encountered two to four times as many deaths among his or her friends and acquaintances as the one in the wealthy neighbourhood.

Mortality gradients can be as steep even where those at the bottom are not poor by any serious standard. A study of 17,000 Whitehall civil servants found death rates three times higher among junior grades than at the mandarin levels. Deaths from heart disease were four times more frequent at the lower end of the hierarchy. Most of this excess could not be attributed to poor discipline in the lower ranks. Only about a quarter of the effect is likely to have been due to unhealthy behaviour, such as smoking. Another obvious possibility is that poor health impedes promotion, while vigour is needed to reach the top. But analysis indicates that this is no more than a minor factor. In short, the gradient in deaths arises from the hierarchy itself.

The ideological vision that this provokes is a mixture of the familiar and the unexpected. Its immediate effect is to reinvent the world on which the Left used to insist. Yes, the poor really do pay a price for the luxuries the rich enjoy - a price that can already be measured in dead babies. Yes, there really is something wrong with a society which is merely amused by personal extravagance that would make Marie Antoinette blush. Yes, inequality is oppressive. And yes; under the circumstances, envy is good.

Yet although the sentiments are familiar, they rely upon ideas from a quarter long regarded as beyond the Left's pale. The new critics of inequality have not reinvented the cake. They are not talking about how to slice up a given quantity of material wealth more equitably. Their focus is upon the relations within a society, and the effects of these relations upon the well-being of individuals. Above the threshold of want, they believe, health and happiness are affected more by psychological processes than by improvements in material standards of living. This realisation has drawn thinkers like Richard Wilkinson and the American economist Robert H. Frank towards biology, and modern evolutionary thought in particular. Wilkinson's latest account of his views is set out in Mind the Gap: Hierarchies, Health and Human Evolution, a pocket-book in the Darwinism Today series. As he points out, "it is difficult to make sense of what appear to be the environmental causes of population health without evolutionary theory".

There are plenty who say that it couldn't or shouldn't be done. Generations of students have learned that mixing biology with sociology produces a toxic substance called sociobiology, which is inherently reactionary. Like those who declare that charity should begin at home, meaning that it should end there too, opponents of discipline-mixing aver that we must never forget the lessons of Nazism, implying that we need never learn anything else about biological perspectives on human society. Views like these are so ingrained that they have turned into reflexes.

Yet these attitudes are being modified; and the most striking evidence of change can be found in what should be the most implacable statement of opposition to the most prominent current form of sociobiological thought, Steven and Hilary Rose's compilation Alas, Poor Darwin: Arguments against Evolutionary Psychology. In Stephen Jay Gould's contribution, which originally appeared in the New York Review of Books, Gould refers to the Darwinian recognition that males and females have differing reproductive interests. Female mammals have to incubate their young, and may support them after birth. Males may succeed in fathering offspring even if they walk away from their future progeny the moment that mating is finished. Male reproductive success therefore tends to be determined by opportunities to mate, whereas for females, access to resources is more important. Each sex is therefore likely to have evolved different reproductive strategies; and the sex which invests more in reproduction will be more selective in its choice of mates.

Gould doesn't bother to quibble over the bourgeois phrase "differential parental investment". He agrees that the idea "makes Darwinian sense and probably does underlie some different, and broadly general, emotional propensities of human males and females". Don't be misled by that offhand "probably". In one sentence, Gould endorses the basic theoretical foundation of the school he is excoriating. He accepts the central insight of evolutionary psychology, and agrees that men's and women's psyches have been shaped differently by evolution. The only question is that of degree. It is like reading a pamphlet from the Adam Smith Institute and coming across the observation that "the history of all hitherto existing society is probably the history of class struggle".

With enemies like these, left-wing Darwinians might not seem to need friends; but with friends like Peter Singer, they don't need enemies. Singer raised the idea of "a Darwinian Left" in the Darwinism Today booklet of that title (and, in edited form, in the June 1998 Prospect). "The Left needs a new paradigm," he remarked, in comradely fashion. But in the subsequent pages he conducted a rather short march straight to the centre ground on which the world's social democratic parties have been camped for some time.

Singer's Left would recognise that human nature is neither inherently good nor infinitely malleable. It would accept that some inequalities are not caused by "discrimination, prejudice, oppression or social conditioning". It would expect people to compete in pursuit of their own interests, or those of their kin, but also to respond positively to offers of co-operation, when satisfied that these are genuine. It would stand up for the weak, the poor and oppressed, "but think very carefully about what social and economic changes will really work to benefit them". By the sound of it, Singer's Left is already in power.

Speaking as he does from a position of sympathy, Singer conveys the impression that for a Darwinian Left, there is no alternative. "In some ways," he concludes, "this is a sharply deflated vision of the left, its utopian ideas replaced by a coolly realistic view of what can be achieved. That is, I think, the best we can do today - and it is still a much more positive view than many on the left have assumed to be implied in a Darwinian understanding of human nature." In other words, things could be worse. It's not much of a rallying cry, let alone a campaign slogan.

Socialists a century ago were understandably wary about a scientific principle expressed as "survival of the fittest". It was hard to see how such a doctrine, inspired after all by Malthus, could be any help to a movement based on the co-operative spirit. Since the 1960s, however, modern Darwinists have become fascinated by the conditions that induce individuals to help each other. They assume selfishness in order to make sense of altruism, and use a theory based on competition to understand co-operation. Any left-wing tendency today must do the same, if it aspires to more than sentiment.

The broad picture that emerges from the current evolutionary paradigm is of a human nature that is universal, shaped by Darwinian selection pressures, and differently accented in each sex. Racial differences, the undead of human biology, are not ruled out, but they are not expected and not sought. Innate inequalities among individuals are accepted, while remaining peripheral to the interests of evolutionary psychology; especially to the program defined by that name by John Tooby, Leda Cosmides and like-minded thinkers. They are the peculiar enthusiasm of psychologists of a different stripe. While they are also grist to the mill of some conservatives, notably Charles Murray, they should present no insuperable difficulties to any Left with a modicum of self-confidence. Inequality was implicit in the formula for communism: from each according to their ability; to each according to their needs.

In the era of labour, even the most overweening snobs recognised deep down that they needed the workers. Today the working class has been eradicated; not in actual fact, but as a political force and, in this country at least, as a recognised estate. The trees are still there, but they are no longer seen as a wood. When the working class was respected, it underwrote the self-respect of individual workers. Although the workers' social status was low in rank, it was accorded a significant value.

Membership of a class was also regarded as a matter of fortune rather than a measure of ability. Nowadays, though, individuals are presumed to determine their own rank, according to their capacities and their application. Low-ranking jobs do not enjoy compensatory status. Heavy industrial jobs, earning respect for the strength and commitment they demanded, have either disappeared or seem obsolete. Automation has eroded the sense that society depends on labour. Without the acknowledgement that even humble jobs matter, there is nothing to check or balance meritocracy.

A particularly cruel twist in a world of winner-take-all celebrity is the suggestion that ordinary young people of low status should take the most extraordinary individuals, typically sportspeople, as 'role models'. Since by definition there is only room for a handful of champions, almost everybody who sets them as their standard is doomed to failure. The idea of the role model requires a rose-tinted view of competition, in which the winners are there for the losers. It looks somewhat different from a perspective informed by Darwinism, primatology and anthropology. Prehistoric people measured themselves against the small numbers of peers with whom they lived; as did most people in historic times, until the recent onrush of urbanisation. They never encountered a world champion, so they never realised just how high the ladder of status could go. World champion sportspeople are the de facto winners of a status competition that includes spectators as well as those who actually take the field. The same is also true of beauty, with more deeply pervasive consequences. Supermodels and movie stars are the virtual winners of a globalised sexual selection process, triumphing over their audiences as well as their peers.

The pertinence of evolutionary thought lies in its facility for interpreting a series of phenomena that have become salient since the developed market economies passed the threshold of affluence and accelerated away. It is attuned to stress, sexual display, conspicuous consumption, and a Red Queen world in which everybody has to run faster and faster just to stay in the same place.

In this respect the publicists of evolutionary psychology have spun their theory the wrong way. They have dwelt heavily on the idea that human nature is out of date, having evolved to meet conditions remote from our modern way of life. But if we do have an evolved nature, highly developed technological societies might actually render it more visible, by freeing it from physical burdens, than it has been since the advent of farming, towns and epidemic disease.

Evolutionary psychology understands humans as fundamentally social beings. It is unsurprised to hear that increments of wealth fail to bring corresponding increases in happiness. Instead, it expects to find the keys to happiness in the eternal verities of status and belonging. Status exists without hierarchy, through friendship, reputation, reciprocity and mutual obligation; but as a society becomes more hierarchical, status increasingly depends upon the ability to achieve visible superiority. It becomes less important to be well regarded, and more important to be looked up to.

The impression that we are now generally on first-name terms with each other is neither here nor there. Hierarchy is not the same as formality. At issue here is the balance within relationships between dominance and mutuality. The traditional Japanese company might be highly formal, but it is based on an ideology of mutual involvement, which in turn rests on the assumption that its relationship with an employee will be long-lasting. Nor do the Japanese seem especially concerned, by international standards, about asserting status through wealth. Japan has the lowest income differentials in the world - and the greatest life expectancy.

Over here, the call centre managers may teach their staff the company song, but they'll be ready to fire them the moment a bad quarter's results come in. Loyalties are as portable as pension plans. As individuals do not belong anywhere for very long, their status and reputations attach to themselves alone. Instead of building up a store of respect through living within a single community, you can take your accumulated material wealth and use it to assume status anywhere you choose to go. Despite the managerial talk of 'flattened hierarchies', the modern economy is as fluent in dominance as it is in English.

Inequality may be a step backwards, evolutionarily speaking. Our primate relatives live in hierarchies, and evolutionary logic suggests that this was also true of our distant predecessors, from the first upright apes to the people like us who appeared a hundred thousand years ago. Yet those peoples who still hunt and gather, as all humans did till ten thousand years ago, have a markedly egalitarian outlook. They recognise and admire exceptional individuals, but they do not permit them to acquire exceptional degrees of power. As the anthropologist Christophe Boehm points out, humans never lost their ancestors' instinct to dominate. But among hunters and gatherers, the instinct is collectivised. Individuals who seek to dominate are restrained by "counter-dominance" from the rest of the group. The politics of envy may be the oldest politics of all.

That doesn't make them any more attractive, of course. Few of us want to live hand to mouth, or without roofs, or writing. Equality seems to go with stagnation, inequality with progress. But despite "constant revolutionising of production, uninterrupted disturbance of all social conditions, and everlasting uncertainty and agitation", in the words of Marx and Engels that now loom so presciently out of the Communist Manifesto, there are aspects of the human environment which remain essentially the same. People live among friends, families, peers and collaborators, as they always have and always will, whatever their institutions and whatever their technology. In order to sustain a place in these webs of relationship, they need trust and they need respect.

Egalitarianism is a strategy through which these immaterial essentials can be secured. It should not be dismissed as an adaptation that has become a liability, like the appendix. We may need to get back in touch with it, since inequality seems to be running away with us. And it needs to be more than intuitive counter-dominance, recognising only obvious forms of domination. There is something of the hunter-gatherer in our attitude to sports stars, for example. We enthusiastically recognise and admire exceptional athletes, who do not attempt to translate their abilities into political power. But their hypertrophied incomes and their triumphant images are part of our problem.

The costs of inequality are not confined to health. Homicide and violent crime are also higher where the range of incomes is wider. At bottom, according to Richard Wilkinson, it is a matter of trust. Health and harmony are nurtured by trust, the thread of social cohesion, A society rich in social capital, measured by indices such as the strength of civic associations, is likely to be a healthy society. A society rich in material capital, but short of social capital, is likely to be unhealthy. And trust must be mutual. It is easier to achieve between equals, and strained by inequality.

Wilkinson's equations offer a pleasing reconciliation. between the metaphorical and the literal senses of health. They express data which suggest that moral health and medical health are interwoven. They also give a copper bottom to feelings, providing figures and theory to anchor otherwise free-floating moral objections to neo-liberalism.

Other evolutionarily-minded thinkers have also sounded anti-materialistic notes. Regarding winner-take-all markets as pernicious, Robert Frank and his colleague Philip J. Cook suggest that the exponential trend in top salaries could be curbed by taxes of up to 90 per cent - not on income, but on spending. They reason that the pleasure a man obtains from owning a £50,000 sports car does not reside principally in the absolute value of £50,000, which would be equally pleasing if it were realised in the form of a country cottage or a savings account, but arises from seeing that his neighbours' cars cost less. What matters is not wealth but status. Geoffrey Miller, author of The Mating Mind, commends song and conversation, rather than designer labels, as indicators of a suitor's worth than designer labels.

 Both of these arguments take human nature to be fundamentally competitive. Frank and Cook assume that the pursuit of happiness entails the pursuit of superiority. Miller's vision is based on the premise that human minds are unequal. Yet they lead to proposals for taxes to make the pips squeak, and praise for "hippie" values - Miller's own term - which evoke the ideal of discovering the real person behind the consumer. Premises that look like pillars of conservative wisdom lead straight through consensus into left field: each seeks ways to curb the ability of consumer capitalism to exploit human nature. This is not the maintenance of traditional values in a modern setting, as John Prescott used to put it. It is the rediscovery of left-wing ideas in response to analyses of distinctively contemporary phenomena.

 Ideas like these, flowering from heterogeneous ideological stocks, suggest the possibility of a Darwinian left which could take a sober view of the role of self-interest in human affairs, and use it as the platform for an array of radical ideas and policies. The common theme in them would be the drawing together of society, by reducing disparities of wealth and enriching the fabric of community, for the common good. A Darwinian left would take heart from its new perspectives, which suggest that the left's instincts about solidarity and equality were right after all.

 The immediate difficulty is that the people in whom such feelings endure are the least likely to appreciate the suggestion that these actually are instincts. And besides the left's traditional dislike of sociobiology, there is the question of sex differences. Evolutionary psychologists expect - together with Stephen Jay Gould - different emotional propensities in males and females. They are unlikely to see fifty-fifty shares in boardrooms or childcare as the measure of fairness in the division of labour. But divisions of labour do not simply reflect how individual men and women choose to invest their efforts. Evolutionarily-minded thinkers should appreciate better than anyone how males will tend to form coalitions, which exclude and subjugate females.

 Supposing, however, that an evolutionarily-minded left did succeed in establishing itself, could an electoral majority be persuaded to support its vision? The slope of the heart disease graph, from the Whitehall civil servants' study, hints that it might. Although the excess in mortality declines steadily as the grade ladder is ascended, it persists right to the top. If they could vote on measures to counter it, individuals at most levels of the hierarchy might readily be persuaded that they shared a common interest with those at the bottom. Death might prove a great leveller.

 Within the United Kingdom - where income inequalities have widened during the period of the Labour government - an evolutionarily-minded Left would be more likely to find sympathetic policies north of the border. Scotland is considered to be a less individualistic, more mutually oriented society than England, or at any rate the dominant south of England. Its legislators have already affirmed this perception by diverging from English policy in two areas, agreeing to meet students' tuition fees and nursing care for the elderly from public funds. If Scotland's Parliament continues to support collective values - and if it finds ways to do so without robbing a mutual Peter to pay a social Paul - the cohesion hypothesis would predict that the Scots' health will show the benefit.

 Pointing out the diminishing psychological returns on consumption, an evolutionary Left would affirm that we have less to lose from environmental politics than we think. It would offer an alternative to the rambling protest and resigned acquiescence into which the left is currently divided. It would show why equality and solidarity are as necessary in the world of images and information as they were in the world of industrial labour.And it would discover a new version of an old maxim, in the recognition that unity is health.