This is a story of two Britains and two futures. In the first Britain, the work culture dominates; the talk is of economic growth and dynamism and competing with the rest of the world. Labour is young, cheap and biddable, and driven by the urge to "succeed" - to make it in material and career terms, with the consumer goods and lifestyles to match. In the cities of this 24/7 society, population densities rise: so do crime, violence and antisocial behaviour. Outside the cities, urbanisation spreads, along with noise, congestion, the creep of human clutter and development. Unspoilt places are increasingly hard to find. Pollution gets steadily worse.
The second Britain is a quieter place. The age profile is older, the values less strident and materialistic. People work longer - they are not pensioned off in their fifties - but they save more and spend less, at least on ephemera and gadgets. They drink much less, too, and don't get involved in fights. Work is important but so are hobbies, family and community life. Cities are more spacious, roads emptier, the countryside more rural. The air and water are cleaner and there is hope of getting the weather back to normal because the planet is no longer warming so rapidly.
Which future would we prefer? The first - let us call it "UK plc" - with its economic engine revving at full speed? Or the second, where quality of life matters more: not so much a plc as a community enterprise, with the emphasis on community rather than enterprise? Most people would plump for the second. Yet we seem to be heading for the first.
What we do not admit is that the difference between the two futures is largely one of human numbers. Population is a subject we don't like to mention. In September Michael Howard, the Conservative leader, pointed out that, over the next 30 years, Britain's population would grow by 5.6 million - an increase of nearly 10 per cent on the current 59 million. Immigration, running at an average net inflow of 158,000 a year in the past five years, accounts for 85 per cent of this increase. Because population is forecast to rise, the government plans an extra 3.8 million houses in England over the next 20 years. But that plan is based on net immigration of 65,000 a year. If it continues at 158,000 a year, we will need 4.85 million new homes.
Howard went on to quote, with approval, the conclusions of the government's Community Cohesion Panel, which said in July that people "need sufficient time to come to terms with and accommodate incoming groups, regardless of their ethnic origin. The 'pace of change' . . . is simply too great . . . at present."
Alarmist? Electioneering? Playing the race card? In so far as these parts of Howard's speech were reported at all, that was how the left-liberal media interpreted them. Yet his figures understate the contribution of immigration to housing forecasts, because they ignore the changes in fertility and household formation resulting from a younger population. According to the Optimum Population Trust, a continuation of the 2001-2003 growth rate of 0.4 per cent would result in a UK population of 71 million in 2050 and 100 million by the end of the century.
The implications of this bear examination. Given a population of "only" 65-66 million by mid-century, for example, we would need an extra nine or ten million houses by 2050 - more than twice the numbers Howard was talking about, and an increase of nearly 50 per cent on the current English housing stock. Should this worry us? Clearly many people think so; the government's housing plans have been a source of controversy ever since they were published. Examine this controversy in greater depth, and you will find a developing awareness of what ecologists call "carrying capacity": the balance (or lack of it) between a physical environment and the numbers it can support.
About all this, the environmental lobby is now silent. The last time such issues were deemed fit for public debate was in 1973, when a government population panel said Britain must accept "that its population cannot go on increasing indefinitely". The progressive-minded believe, on the one hand, in liberal multiculturalism and, on the other, in sustainability. They cannot resolve the conflict. The field has thus been abandoned to the political right.
The demographic facts are undeniable, however. Before the start of the current immigration surge in the 1990s, Britain's population, like that of many other developed countries, was heading for decline - as early as 2013, according to some forecasts. British women are having 1.7 children each, on average, above Germany (1.4) and Japan (1.3) but below the replacement level of 2.1. If this had been allowed to continue, with no immigration, we would be down to 30 million by 2120.
What would it be like to live in a country where population halved in the space of three or four generations? Environmentally, the case for population decline is unanswerable - less pollution, less strain on natural systems, greater national self-sufficiency, a reduction in fossil-fuel emissions, the freeing up of land for other species and higher-order human uses, such as wilderness. Psychologically, what the economist Fred Hirsch called "positional goods" - a view, an unspoilt beach, a piece of heritage - would be freer of the crowds and queues that now, for most people, mar them. Applied to social and economic life, this might reduce the awful sense of competitiveness that is a relatively recent feature of cultural life, for jobs, places at school or university, or entry to prized social institutions or niches.
Given the close association between crowding, densities, congestion and stress, and the greater distances available between people, we would also probably see less casual public aggression: less of the "rage" that emerged in the late 1980s. And, because young people are more likely to commit crimes, the ageing of society that would result from population decline would reinforce these trends. A Britain of 30 million people would almost certainly be a kindlier, more easygoing, more socially concerned place - exactly the sort of Britain that many readers of the New Statesman would like to see.
Most of the argument so far, however, has focused on the perils of decline: economic and social stagnation, the decrease in the support ratio (of workers to pensioners), emerging labour shortages and so on. Given that all of these "problems" are either illusory, fantastical or soluble (see below) it is instructive to ask why they obsess us. Why were the Tories, for example, thinking until recently about encouraging people to have babies and why does the government still envisage no upper limit on immigration? There are two answers. First, population growth is such a feature of the past two centuries - although not of preceding ages - that it has become synonymous in our minds with progress. Second, economic growth is how politicians and economists measure national success. And having more people is the quickest and easiest way to boost gross domestic product.
Much is made, therefore, of the impact of immigration on economic growth. Yet the growth comes almost entirely from additions to the national headcount. The increased wealth per person may be as little as 0.1 per cent a year, according to US research.
More important is what happens when "immigrants" become "natives". This is the central fallacy of the demographic "time-bomb" argument. Immigrants eventually become pensioners, and pensioners keep living longer. The only way to preserve a support ratio regarded as optimal is thus to have permanently high levels of immigration - and a population permanently, indeed infinitely, growing. David Coleman, professor of demography at Oxford University, has calculated that to keep the support ratio between pensioners and those of working age at roughly current levels would require a UK population in 2100 of approximately 300 million and rising. He calls it "the incredible in pursuit of the implausible".
And what about the world as a whole? How are developing countries, presumably expected to provide the young immigrants to the UK and other western countries, supposed to support their own old people?
New figures from the US Population Reference Bureau suggest a world population of roughly 9.3 billion in 2050, against 6.4 billion now. Studies such as the WWF's Living Planet Report say that by that time, humanity's footprint will be up to 220 per cent of the earth's biological capacity. We would need, in other words, another couple of planets to survive. But if we manage to control global population (and it looks increasingly likely that we can) numbers will start to decline, possibly around 2070. What is the world supposed to do then? Import extra-planetary aliens to maintain the support ratio?
Even within Britain, it is hard to make a case for labour shortages when unemployment is three times the number of vacancies and economic inactivity, notably among the over-fifties, is at an all-time high. It is also hard, morally at least, to argue that we should deliberately cream off the skilled and educated workers of poorer countries - little different from people-trafficking, according to a National Health Service overseas recruiter addressing this year's Royal College of Nursing conference - or that we should bring people in because there is nobody else to sweep our streets and clean our toilets.
The solutions to the "problems" of population decline, in fact, lie safely within the range of realistic policy options. They include: people saving more and consuming less; governments investing more in preventative health measures, to lengthen illness-free old age; better labour productivity; a higher retirement age; drawing the economically inactive back into economic activity (with penalties for ageism); and restructuring hard-to-fill jobs to make them more attractive. Population decline creates a (relative) shortage of workers and therefore shifts power from capital to labour and raises pay rates generally, as happened after the Black Death. Isn't the left supposed to be in favour of such an outcome and against the use of immigrants to create a US-style low-wage economy?
Yet the argument is not primarily about economics. Those who advocate increases in immigration and population do so largely on the grounds that they are good for GDP. They forget, as most economists do, that they are often bad for the environment and society. Economic growth, after all, is ethically undiscriminating: the wages earned from clearing up the effects of a car crash or a pollution mishap count towards GDP in the same way as those earned from making a loaf of bread. All over the world, Britain included, population growth is generating an extraordinary range of negative effects, from climate change and resource exhaustion to the destruction of species and habitats and the poisoning of the biosphere. Deliberately boosting Britain's population, either through large-scale net immigration or by telling people to have more babies, will ultimately make it a much worse place to live in.
How big should Britain's population be? That depends which sums you do - but some calculations from the Optimum Population Trust suggest 20 million or fewer.
Before you throw up your hands in disbelief at this idea, consider the view of a liberal from another generation, John Stuart Mill, who in his Principles of Political Economy (1848) acknowledged the economic potential for a "great increase in population" but confessed he could see little reason for desiring it. "The density of population necessary to enable mankind to obtain . . . all the advantages both of co-operation and of social intercourse," he wrote, "has, in all the most populous countries, been attained." In 1848, the world contained just over a billion people and the population of Britain was 21 million.