One thing to be said in the Millennium's favour was that it provided the perfect opportunity to discharge the head of irrationalist steam which had been building up in the West over the last quarter of the old century.

Come January 1, with the bats all out of the belfry, we could perhaps look forward to a new era of common sense and reason. We certainly seem to be entering in a period where reason in its most productive form, science, will enjoy an intellectual ascendancy, thanks both to its own spectacular dynamism and to the lack of conviction among other ways of changing the world. But reason and good sense are not the same thing. Maybe science will turn out to be the enduring home for undissipated millennial fervour.

Charles Murray's essay, published in Prospect last month under the title 'Genetics of the Right', illustrates what may be in store. It is anything but millennial in tone, yet it makes an arrestingly millennial claim about our understanding of human nature. By the end of this century, predicts Murray, we will pretty much know it all. As he puts it, "we will be approaching biological truth" about "many, many aspects of human nature and their social implications". Referring to the biologist E.O. Wilson's book Consilience, he anticipates the dawn of joined-up knowledge, in which neuroscientists understand the brain, molecular biologists understand "which genes do what", and the scientists' understanding forms the basis of a new, scientifically rigorous analysis of human behaviour, which, in turn, will explain the shape of culture and society.

Wilson himself foresees in Consilience that the social sciences will split, one part fusing with the humanities and the other "folding into or becoming continuous with biology". In other words, the useful part will be absorbed by a major science, and the rest will be mere literary criticism. Science will take care of everything that really matters. And, as Murray makes clear when he looks forward to the time when "we finally learn for sure" what human nature is, science will make all other ways of knowledge redundant. The philosophers have interpreted the world; the scientists, however, will explain it.

To support his hundred-year timescale, Murray points to the pace at which scientific knowledge is being gathered. It is certainly true that knowledge is being generated in large quantities, and that tools now exist, for examining the genome and imaging the brain, whose potential is only beginning to be tapped. The impression that science is leaping beyond itself has been heightened by two dramatic developments: the increasing speed at which it is proving possible to sequence the human genome, and the cloning of mammals. But scientific progress can be very uneven. If fusion power research had gone as well as gene sequencing technologies, we would have electricity too cheap to meter after all. The 1990s, designated by the US Congress as the Decade of the Brain, did not produce a neuroscientific counterpart of cloning. And the thousands of published neuroscience research papers have to be set against the billions of neurons of the brain.

The data for the human genome have proved easy to gather. For Murray's confidence to be well placed, there will have to be steady progress on the difficult part, turning data into knowledge. So far, however, progress has been faltering. Murray obliquely acknowledges the history of "false pronouncements that will be revised a year later, as new data come to light". He also acknowledges that the neural and hormonal processes that affect behaviour are "unimaginably complicated". But one is left with the feeling that for Murray, as for many scientists engaged in the study of behaviour genetics, personality, and differences between individuals, the basic truth is simple and already clear. Human personality can be resolved into a few dimensions - some researchers speak of the 'Big Five' - such as extraversion or conscientiousness. Psychological traits generally show significant degrees of heritability. Intelligence is considered to be highly heritable, with estimates ranging up to eighty per cent. At the other end of the range, a Swedish study of sociability in twins raised apart found a correlation level of 0.2, on a scale from 0 to 1. Psychologists joke that "everything correlates at 0.3", meaning that moderate levels of correlation can usually be found among the measurements they make, and don't necessarily mean very much. To the scholars whose views inform Murray's, though, heritabilities in general are significant. Moderate or high, they add up to a picture of the mind in which the genes' contribution is what counts.

This picture has been drawn without direct access to the genes. But it has convinced a significant layer of scientific opinion that when - not if - the genes are revealed, the findings will confirm what they already believe. Many more scientists would probably accept, without turning into correlation fetishists, that whatever it is that the psychologists are measuring, it's substantially heritable. The question is not whether psychological qualities are affected by how each individual deck of genes has been shuffled, but how well the constructions placed upon the mind correspond to how it really is constructed. It is possible to define a quality called 'conscientiousness', and calculate how heritable it is, but this does not prove that one's concept of conscientiousness carves human nature at the joints. If one believes that the mind is the work of God, it might well have been equipped with a faculty of conscientiousness. But whether nature thinks that way is another matter.

It is quite plausible that natural selection has produced mechanisms underpinning what we call conscientiousness. The propensity to persist with an activity, and to review it to make sure that all its elements have been completed, would seem to be an evolutionarily adaptive one. It might have to be balanced, however, by mechanisms deterring an individual from persisting in hopeless tasks, or allocating excessive effort to fulfilling them. Randolph M. Nesse, an evolutionarily-minded psychiatrist, has speculated that there may be a connection between such mechanisms and the phenomenon of depression.

Some people will feel that we can do without that kind of speculation. But the trouble with behaviour genetics and its allied disciplines in this respect is not that they are excessively concerned with evolution. It is that they do not take evolution seriously enough. If you start from social science data, showing that Americans of African descent have an average IQ score substantially lower than that of whites, and you discount environmental differences while laying emphasis on heritabilities, you are likely to arrive at a racial explanation for IQ differences in a divided society. If, like most scientists interested in human evolution, you consider that the driving force behind intelligence was the selective pressure of life in groups of intelligent hominids, you will accept that these pressures were similar in all groups. You may therefore find it hard to see how differences in intelligence could have arisen between populations. This is a perspective which those who argue for racial differences, and for any major claims about human nature, need to recognise. You cannot say what human nature is without saying how it came to be.

Evolutionary psychologists, who like to make out that they are not at the same party as the behaviour geneticists, have been criticised for their claim that "our modern skulls house a stone age mind". The mind, in other words, is a system of adaptations to ancient circumstances, which are significantly different from modern ones. Just how different they are can be debated, but at least the evolutionary psychologists have reminded us that to understand human nature, we must try to start at the beginning. If we try to start from here, tangled up in racial politics, ideological transitions, the Western obsession with individuals and their psyches, and the stout exertions of a Christian morality trying to reassert itself, we are likely to remain where we are.

We can also thank evolutionary psychology for promoting two other important ideas about our innate nature. One is that the universal elements in human nature are extensive. Scholars who call themselves evolutionary psychologists generally define their project as the study of these universal traits, and have thereby counterbalanced the behaviour geneticists' emphasis on differences. Another is that the mind's flexibility arises from the possession of large numbers of specialised systems, a mosaic of instincts, which allows an appropriate mental tool to be selected for a particular situation. This model supports the view that people behave differently in different circumstances. Charles Murray notes a selection of traits associated with social problems: "low IQ, impulsiveness, short time-horizons, sociopathy, indolence". Nobody who has ever observed cats, large or small, can be in any doubt that there is a gene for indolence. But from an evolutionary psychologist's perspective, some of these human traits might be seen as 'conditional strategies', which could be adaptive under certain circumstances. Inactivity may be an appropriate response if none of the immediately possible activities are worth doing.

Hereditarians think statistically. They do not imagine that the combination of qualities Murray mentions will invariably result in crime or other social problems. On the other hand, they are not particularly concerned with how impulsiveness or indolence might have played in the Stone Age. Drawing data from contemporary social science and psychology, they are preoccupied with how their clusters of putative inherited traits are expressed in contemporary industrialised societies. The background assumption, shared far beyond the circles in which correlations are measured and heritabilities calculated, is that the environment is basically constant. It is generally accepted that however prosperous a society becomes, there will remain an element which continues to live in varying degrees of wretchedness, violence and poverty. For much of the twentieth century, this was seen as a problem that would be solved by social engineering. Today, social engineering has vaguely sinister overtones, and the preferred solution is to provide opportunities for those who wish to escape by bettering themselves. We are back to the great Victorian distinction between the deserving and the undeserving poor.

Murray echoes this in his confident prediction that the new sciences of behaviour will reveal human nature to be conservative in its political shape. At the same time, he admits that no Republican presidential candidate would declare in public that one of the reasons for poverty in the US is that "a lot of poor people are born lazy". This may reflect, as he suggests, the continuing power of the idea that all people are born equal. But conservatives have not generally been confounded by the idea that some people are born with flaws of temperament. On the contrary, they have taken it for granted. In the past, however, they tended to favour vigorous environmental interventions to sort the slackers out. Military service was one possibility; muscular Christian evangelism another. These were solutions for mass societies, depending on the acceptance of state coercion in one case and of a dominant religion in the other. They also require a sense of the collective that is problematic in American traditions, and is declining in this country. Continental European countries retain conscription, together with a belief in social partnership that now seems foreign here. The latter is widely seen as out of date, and is being challenged in the name of competitiveness.

Nevertheless, Murray feels it to be "obvious" that the new neurogenetic paradigm may become a cause for the Left. Casting a selective eye over the British eugenics of the earlier twentieth century, he recalls its popularity among socialist intellectuals. But for these thinkers, socialism and eugenics were both forms of progress, based on reason and science. They were not exclusive partners. No Briton was more passionate about eugenics than Ronald Fisher, a geneticist and statistician who helped lay the foundations of modern evolutionary theory, yet his belief co-existed with a conservative political outlook and a conventional religious faith.

Behaviour genetics and allied disciplines, might appeal to a new Left, argues Murray, by offering to replace a generalised target, "'the lower classes'", with a precisely specified subset, "'people with the following genetic profiles'". He might also have noted that the new science would be able to replace Victorian moral conviction, putting the deserving poor on an objective scientific footing. Then the undeserving poor could be modified, to make their children deserving. Where social engineering failed, genetic engineering will succeed.

It is far from obvious, though, how this Left might spring up. After all, Murray believes that the progressive discovery of scientific truth will affirm the Right's understanding of human nature. Having been deprived of its economic leg by the triumph of the market, the Left will then lose its social leg to the sciences of brain and genes. It is hard to see what the new Left will stand on. But of course the Right is looking forward to its advent, in the same way that it can't wait till China replaces the Soviet Union as a superpower-class enemy.

Murray suggests that this Left of the future will warm to a eugenics which requires the lower classes not "to stop having children, only start having better children". This underlines the question of where the Left will find its means of support. Better children sound like more expensive children. Their parents will still need state assistance to rear them, until the investment matures. And the eugenic interventions themselves will not come cheap, festooned as they will be with royalties and patent rights established by the kind of scientific entrepreneurs who are even now laying claim to swathes of the human genome. They will also be expensively inefficient, since they will in most cases only promise a statistical likelihood of improvement. (Affirmations of the power of the environment will live on in the manufacturers' liability disclaimers.) On the other hand, the public authorities should enjoy savings in the judicial area. Either the new improved offspring will not be inclined to crime, or they will be clever enough to avoid getting caught.

Maybe the Left will have been reborn, due to unforeseen circumstances, by the time science reaches the degree of understanding Murray anticipates. But consistency requires that, if scientific progress is predicted from extrapolation, the society in which the science will be applied should likewise be imagined by projecting today's dominant themes into the future. Murray's hypothetical Left would be taking up a notion of eugenics favoured in the 1930s by the Eugenics Review, which defined its subject as "the study of agencies under social control that may improve or impair the racial qualities of future generations either physically or mentally". Today the interest in social control seems as archaic, and in some quarters as disagreeable, as the idea of "racial quality". It belongs to the era of the big state, of corporatism, of progress through planning. We are in the era of the outsourcing state, of consumer choice, and of progress through competition. These are the forces that will shape eugenics in the foreseeable future.

Just as the invisible hand of a myriad individual decisions is believed to produce the most efficient kind of economy, the invisible hand of decisions made by individuals and families is felt to be a better means of shaping society than command from above. Charles Murray is optimistic about the individual decisions that will be made over genetic manipulation. He thinks that parents will not want to interfere very much with their children's genes, since after all those are their genes too.

This is the most doubtful of all his claims. Some parents will certainly opt for their children to remain 'naturals', in the term coined by the biologist Lee M. Silver, author of Remaking Eden: Cloning And Beyond In A Brave New World. In some cultures, perhaps ones in which grandparents and other family members make decisions about marriage partners, parents may have the choice made for them. Others, as Lee Silver emphasises, will not have the choice because they don't have the money.

It is true that parents want their children to be like them. But there is no reason to suppose they will be any less anxious to improve their children than they are to improve themselves. Murray suggests that parents will opt for manipulations that prevent congenital defects. He adds that they will also seek ones which promote physical and mental abilities, as if these were merely touching up the paintwork on the genome. But if it ever comes about, genetic enhancement will be far more than taking the edge off nature. It will become the core technology of personal competition, and the most valuable of inheritances.

The people of the late 21st and the 22nd centuries will be different from us in many respects, but you don't have to be a paid-up evolutionary psychologist to agree that they will feel the same as we do about many aspects of their lives. They will, above all, want the best sexual partners they can find, especially for relationships that lead to children. Many of the qualities they find attractive in potential sexual partners will be those that they hope to encounter in friends and colleagues. They will like good-looking, happy, pleasant people. They may admire assertive, dominant or aggressive people, within limits set by their culture and by the need for go-getters to be team players. As for intelligence, it will command its own premium in a knowledge economy.

Even today, personal qualities are not optional extras. Several of the major developments in economic and social life are increasing the importance of appearance and personality. Where employees interact with the public, intense competition between consumer products favours individuals who find it easy to look pleasant and behave agreeably. Within organisations, the flattening of hierarchies both increases competitive pressure, since Buggins no longer gets promotion by waiting his turn, and heightens the importance of demeanour, since interactions are less formal. And a world based on information will be a world ever more full of images. By the nature of the market, these will be dominated by images relating to physical attractiveness in various ways; athletic perfection, sexual allure, wholesome vigour, or pleasant smiles.

For many professional people, qualifications are not sufficient for success. They need looks and personality to thrive in the social networks that underpin their status. A telegenic appearance is now deemed vital to the achievement of political power through elected office; so much so that it has been suggested, not entirely in jest, that bald men like Churchill or Attlee could not become Prime Minister today. In Brazil, plastic surgery is said to be as essential an investment for politicians as it is for starlets. Brazil's enhanced politicians may not set an example for their counterparts in other countries, though many will discover the trick for themselves.

Other competitive enhancements have a global reach. Cosmetic surgery for women has become a key tool for advancement in the American entertainment industry. The result is that surgically enhanced looks set the standard for female appearance around the world. Within the United States, the numbers of cosmetic surgical procedures performed last year (on both sexes) were an order of magnitude higher than in 1990. As the industry expands, it will develop new markets overseas, trading on the norms established by American entertainment products.

As it does so, it will change our understanding of what surgery is for. Ironically, the idea of surgery to enhance rather than to cure resonates with the message of 'alternative' medicine; that orthodox medicine is too negative in its emphasis on illness, and should treat the whole person. It also mirrors a tendency which is likely to develop in medicine, as new psychoactive drugs are marketed as mood or personality enhancers. There were early ventures in that direction during the Prozac and Valium eras, but they sat uneasily with an institutional framework that expected drugs to cure diseases. One way of getting round this might be to exploit the grey area between drugs and food supplements.

Another would be to insist that cosmetics are not trivial. The stakes are high in a competitive economy with large income differentials, flexible labour and spartan social security arrangements. In a global economy, the global becomes personal. Individuals must practice the same total commitment to competitiveness that governs their employers' business strategies. Enhancements that improve their personal presentation are as important as their company's brand image. Brands are particularly important when products are similar, or indeed indistinguishable, like petrol or cola drinks. Intangible qualities are likewise important when employees are trained and qualified to the same standard.

Not many companies adhere to the McKinsey management consultants' philosophy of "up or out", at least not explicitly, but life in the modern economy has generally come to resemble that of the Red Queen, who had to keep running just to stay in the same place. This is true for the captain of industry and for the call centre telephonist. For the call centre employee, the penalties of failure are higher than the rewards of success. For the captain of industry, the rewards may be astronomical. At many levels in between, both the penalties and the rewards may be considerable. For many middle-class people, the pay can be very good, but career failure can entail major loss of status, and possibly socioeconomic collapse. One of the distinctive features of the new business architecture is that, whatever floor you are on, you can see through the boards to the inferno of the underclass down below.

Such a fate, or the fear of it, or the efforts to avoid it, can cause ill-health; stress symptoms, emotional disorders that may reach grave proportions, physical illnesses that threaten life. And then there is the properly personal, partner-seeking side of life. Here too, the stakes are being raised by the various pressures to compete on appearance and personality. All in all, there is a reasonable case to be made that, in a market-based society that is highly competitive and getting more so, personal enhancements may be critically important to personal well-being.

Once the roles of medicine and surgery have been amended to incorporate personal enhancement, the cultural and institutional framework will be in place for any genetic enhancement procedures that may then be developed. They will also raise expectations for genetic intervention. Smart drugs and plastic surgery will seem like crude interim fixes pending the ultimate enhancement technology.

Genetic intervention is likely to begin as an unequivocally medical procedure, and work outwards. The first targets will be genes associated with rare hereditary diseases, because these are the simplest ones scientifically and ethically. Although exercises like these will initially be carried out on families with a history of inherited illness, they could in principle be applied to the entire population, since we all carry harmful genes buried in our genomes. Attempts may also be made to develop 'genetic vaccines', transferring genes which confer natural resistance to pathogens such as HIV.

Next may be genes influencing mood, already the subject of extensive research efforts. These have unarguable medical legitimacy, since the effects of serious mood disorders can be incapacitating or even fatal. Interventions would have a powerful appeal. No parent who has suffered severe depression would wish their child to do so. But the implications of work on mood genes may be far wider. It may become possible not just to reduce the likelihood of severe emotional disorder, but to increase the chances that a child will meet life's challenges in a positive, happy frame of mind. And what does a loving parent want, if not that their child should be happy?

Some aspects of appearance, such as height or fat distribution, might prove easier to manipulate than others, such as facial symmetry, which is thought to reflect a generally healthy constitution. The visible signs of a modified genotype may turn out to be a healthy glow rather than any designed features. Greater efforts are likely to be expended in the search for genes affecting intelligence. The reception for The Bell Curve, the book in which Richard Herrnstein and Charles Murray aired the argument that the IQ gap between blacks and whites may be partly genetic in origin, shows that such projects are likely to be hotly controversial. It also shows that the controversy will probably not be so great as to stifle the research. In its early stages, studies may concentrate on severe deficits, thus securing a base for themselves within medicine.

Sceptics will argue that, except in cases where a single gene causes a single disease, these ventures are doomed to fail because of the complexity of the relationship between genes and environment. Results cannot be guaranteed, and even the statistical likelihood of success may fluctuate wildly according to the path the modified individual takes through life. Whether this uncertainty spells failure depends, however, on who is paying. It would probably be unacceptable for a publicly-funded health service, should such a thing exist by the time the brave new world arrives. For individuals in a market economy, though, the uncertainty might add to the value of the investment. The proof of wealth is the ability to spend money profligately, whether on luxury goods or on charitable donations. For this reason, prices of luxury items tend to increase exponentially as the scale is ascended. The most expensive eugenic interventions would be those with the most slender prospects of success.

By virtue of their cost and associated status, such procedures would create an image of an ultimate eugenic goal, raising aspirations across the social scale in the same way that Ferraris shape the aspirations of Mondeo Man. They would also act to maximise the gap between the genetic haves and have-nots. If market eugenics became a reality, the lower classes would be more visibly different from the wealthy than they are today, more unhealthy, less intelligent and even less likely to escape from their situation, since they would be less able to compete with their modified betters.

And the results of eugenic choice among the upper classes might not be quite so attractive as they appeared at first glance. There would be more than an overtone of Stepford about them, with the lower registers of the mood scale muffled, and a somewhat creepy air of emotional uniformity. Dispensing with the usual polite qualifications about how bell curves overlap and people should be treated as individuals, Charles Murray predicts "we will learn for certain such things as that women innately make better nurturers of small children than do men and that men innately make better soldiers". In our present world, some men care for children better than some women, and some women make better warriors than some men. In a eugenically manipulative society, the manipulators might regard androgyny as a deviation from a design ideal. Procedures offering to reduce the likelihood of homosexuality would certainly prove popular. Even if parents of future generations are free from homophobic prejudice, they are still going to want grandchildren.

Speculations like these, or fantasies, cannot tell us anything for sure about the future. The idea that the truth about human nature is just over the horizon, along with the prospect of altering it, does tell us something about the present. The less we believe in the power of politics, the more we believe in the power of science. We seem to be approaching the point where altering the genes of the poor looks like a more realistic project than transforming the environments in which they live.