War, famine and pestilence emphatically are not the solutions. Indeed the only strategies of population control that are remotely liable to work are benign; far more benign than the policies now commonly practised in the world The statistics show how huge the problem is. According to the United Nations the population will reach 6 billion by 2000; Homo sapiens has long since broken the ecological law which says that "big, fierce animals are rare". More to the point: numbers are increasing by 1.6 per cent per year. That may seem modest; but because of the principle of compound interest, a population increasing at 1.6 per cent per year will double every 40 years. So at the present rate we would reach 12 billion by 2040, 24 billion by 2080, 48 billion by 2120, and about 100 billion within two centuries. In the field of agriculture there aresome optimists who, over the past 25 years, have felt that a human population of about 15 or even 20 billion could be sustained - if we managed to triple the yields here and there, and correct economic imbalances I know no one who thinks that 30 billion is sustainable. Yet such a figure could be reached in 100 years - within the lifetimes of millions of babies already lying in their cots.

In short, something big has got to happen in the 21st century. Either there could be the most extraordinary collapse - for populations do not generally level out in anticipation of disaster, but only after disaster has occurred; or human beings have got to change their ways.In principle - still dealing, for the minute, only in statistics - there are two ways to curtail population growth: increase the death rate, or reduce the birth rate. For some reason - perversity? - most people seem to have focused on the first of these: and hence Keynes's advocacy of war and pestilence. But in practice history shows that such measures do not work. After catastrophic set-backs, populations bounce back, provided the environment is not already too devastated. There have been baby booms after major wars, and after the Black Death of the 14th century.

Ireland remains pro-natalist not expressly because it is Catholic, but because of the famine and emigrations of the 1840s; Catholic Italy, by contrast, has one of the lowest birth rates in Europe. In the poorest countries, where infant mortality is high, women have huge families in the hope that a few will live. But a biologist would observe that human beings reproductively speaking are 'K' strategists, like elephants and whales: our nature is to have only a few offspring, expecting that most will thrive. High infant mortality represents a system in biological disarray; not only inhumane, but hugely wasteful.Curtailment of birth rate tends also to be presented in an unpleasant light: coercive vasectomies in 1970s India; the draconian one-child-per- family law of modern China.

For such reasons population was not mentioned at all in the latest Indian elections, although no Indian can doubt its pertinence.Yet the same demographic statistics that reveal the magnitude of our present plight also demonstrate that it can be solved. If couples produce an average of, say, 2.4 children, then human population will inexorably double every few decades. But if they average, say, 1.9 children - anything less than two - then overall population will stabilise within decades, and will eventually start to go down, leaving future generations to decide how small they would like human numbers to be.So what inducements lead couples to have two children or fewer? History shows that the answers are: wealth, security, opportunity, fulfilment in areas other than child-bearing, and knowledge that any baby born has a good chance of reaching adulthood. People in affluent countries almost invariably have fewer children.Wealth cannot be supplied instantly; but a reduction in infant mortality helps. In brutal terms, a succession of child deaths is a drain on resources.