It's been a part of the ebb and flow of human society since we raised ourselves up on our two hind legs. But now, after an almost total absence since the industrial revolution, it's threatening to come back with a vengeance across the western world. And we don't like it one little bit.
After 200 years of continuous rapid population growth, there is little that inspires as much panic from political leaders, big business and right-wing populists as the prospect of population decline - which is imminent, according to the UN, in more than 60 countries.
Some countries, such as Japan, Russia and the Baltic states, have already fallen into the abyss. Italy's population and Germany's are shored up only by immigration. The recent British census showed population decline in Scotland and parts of northern England. Across the UK as a whole, it could start as soon as 2020.
In Scotland, as elsewhere, population decline prompted two predictable responses. On the one hand, the Scottish National Party MSP Alex Neil urged tax breaks to encourage couples to "conceive for Scotland". On the other, the Scottish Executive told people to prepare for more immigration. The First Minister, Jack McConnell, told the Institute of Directors: "For a growing economy, we need a growing population, and I am determined to see us focus policy and promote Scotland to meet that objective."
Yet the rational response is the one you never hear publicly: "Don't panic, let the numbers fall. It will be good for us."
Population decline drums up visions of collapsing markets, permanent recessions, devastated communities, bankrupt pension funds and decrepit wrinklies with no young to replenish and support them. All this might indeed come to pass if population decline were rapid. A gradual population decline would be a different matter. The environmental benefits are obvious - fewer cars, fewer houses, more wilderness. But population decline could also empower workers, raise the status of the socially marginalised, reduce inequalities and eradicate poverty. It will not make Britain poorer, as the politicians fear, but wealthier. From British universities to Japanese think-tanks, the benefits of slow population decline are being increasingly studied and promoted. But this new thinking has yet to reach the echelons of elected politicians.
Population decline is usually associated with economic decline, political turmoil, famine and disease - but that is not because it causes them, rather because it is caused by them. Declining economies lead people to leave in search of opportunities elsewhere - a quarter of the population of Europe's poorest country, Moldova, have emigrated since the collapse of communism. HIV in some African countries may throw previously prodigious population growth rates into reverse, just as the Black Death wiped out a third of the population of Britain. Devastating climate change eliminated the medieval Greenland colonies. Potato blight shrunk the population of Ireland from eight million to four million through famine and emigration.
For millennia, when humanity was not the author of its own destiny, population went up and down with the rise and fall of human fortunes. Good times led to a growing population, bad times to a declining one. Now, for the first time in history, we are faced with a decline caused not by bad times but by good times. Now it is affluence, not poverty, that leads to falling numbers.
But if the causes are benign, what about the consequences? If the decline in the number of people is slower than the natural growth in productivity (or output per person), then the economy will still grow. For example, a modest population decline of 0.25 per cent a year would reduce Britain's trend economic growth rate of 2.25 per cent to just 2 per cent a year. That's hardly a recession. The number of consumers may decline, but the growth in incomes - and export markets - will ensure that demand stays buoyant. Nor will there be a demographic crisis, with huge numbers of old people overburdening those of working age. Population decline also leaves fewer children to support, train and educate for the first 20 economically unproductive years of their lives. The dependency ratio of workers to non-workers is virtually unaffected whether the population is growing 0.25 per cent a year or falling 0.25 per cent. Adjustments to an ageing society - discouraging early retirement, moving from pay-as-you-go to funded pensions - will be necessary in any case.
However, a declining population - and this is why businesses fear it - will involve a gradual but significant redistribution of power from the owners of capital to the owners of labour. A declining workforce puts those who work in a far stronger position - and for those marginalised in the workforce, it can have a very dramatic effect. Companies will be forced to train the unskilled, provide family-friendly policies to retain women and to entice the elderly to stay on rather than forcing them out. People who own properties will have to rent them out at lower rates, while those who rent can choose bigger places to live.
The dramatic and beneficial effects of this transfer of power from the owners of productive assets to the owners of labour - from the employers to the employed - were seen after the Black Death, which cut the population by a third, led to the collapse of feudalism and heralded the "golden age of peasants". Landowners could no longer force the landless to work for them for free under the bonds of feudalism - the shortage of labour was such that the peasants could go elsewhere to get paid real wages. The deaths from the disease may have been devastating, but the lives of those left behind improved dramatically.
So what would life be like in a Britain with fewer people? Imagine the M25 without traffic jams, imagine trains where you could always get seats. Imagine all the postwar tower blocks being knocked down, and trees planted in their stead. Imagine large houses, now divided into flats, becoming proper homes again. Imagine low-income people learning the joys of spare bedrooms, playrooms and studies.
The Green Party has long championed a smaller population in Britain, one of the most crowded islands in the world. The Optimum Population Trust, chaired by John Guillebaud, professor of family planning at University College London, argues passionately for letting the population of Britain decline naturally over the next 150 years to the level it was 100 years ago - 30 million. "The case for lower populations both worldwide and in the UK is now irrefutable - the environment is suffering like there is no tomorrow," he said. "The prospect of population decline is a new feature which is worrying people, but it shouldn't."
Increasingly, economists and demographers agree. Bob Rowthorn, professor of economics at Cambridge University, said: "There are no credible arguments against gradual population decline." The Japan Centre for Economic Research, after an extensive study, concluded: "The negative consequences of population decline can be avoided. An increasing scarcity of labour would stimulate the incentives for more efficient utilisation of resources, shifting the economic growth pattern from the 'input-driven type' to that of 'gains in efficiency'."
In other words, instead of bluntly boosting the economy in a dumb, Jack McConnell way by boosting the population, you train everyone up more and mechanise more - fewer people working more smartly. Scotland's problems are not that its population is falling, but that good people are leaving because the good jobs aren't there. Simply bringing in people does nothing to address the underlying problems. All the arguments against gradual population decline are based on false assumptions or on 19th-century - or even totalitarian - ways of thinking. Arguments about collapsing markets assume we live in a closed economy, whereas economies depend increasingly on international trade.
Studies by the OECD show there is no correlation between population size and GDP per capita. If big populations create wealth, then the world's most populous countries, China and India, would be the richest, not among the poorest. Many low-population countries, such as Norway and Switzerland, are very wealthy. Ireland, with only four million people, has overtaken Britain's 60 million in GDP per capita. By far the smallest member of the EU, Luxembourg, is also by far the wealthiest.
Once, countries needed large populations for military strength in a hostile world: large numbers of people meant large armies. Women in Victorian Britain were urged to lie back and think of England so that they could help sustain an overstretched empire. Women in the Soviet Union and Nazi Germany were urged to have babies to promote the power of their country. But with kill-by-satellite, large armies don't matter; with international peace treaties, being small no longer means being vulnerable.
Political leaders still like large populations because it enhances their prestige, and their negotiating power. Nowhere is this better seen than on a local level - for example, Quebec is promoting population growth as a form of demographic warfare against Anglophone Canada. Leaders of Midwestern US states with falling populations want to reverse the trend so they can become more important on the national stage.
The same is true on the international level. Canada has an explicit programme of rapid population growth so that it can hold its own against its domineering neighbour. Australian leaders want more citizens so they can hold their own against the vast populations of India, China, Indonesia and Malaysia.
Will a Britain with just 30 million people be able to maintain its seat on the UN Security Council, or its influence in the EU? British politicians may care, but their voters would probably prefer larger houses, empty roads and more wilderness.
From Hitler and Stalin to Jack McConnell, you should never trust a politician who tries demographic engineering on his people. Instead of lying back and thinking of their country, women should think of their country and pop the Pill.
Anthony Browne is environment editor of the Times