China's Fiscal Policy Report 2010/2011, published by the Chinese Academy of Social Sciences, forecast that, by 2030, the ratio of over-65s in the population will overtake that of Japan and become the highest in the world. Figures from the UN show that China's population will peak at 1.446 billion in 2030 and start a long-term decline.

'The negative growth rate of China's population is set,' said Gu Baochang, a professor of the population and development research centre of Renmin University and advocate of a two-child policy.

'The greatest risk China faces in the 21st century is the risk resulting from this negative growth rate. In many places, schools are being turned into old people's homes and new schools are empty, for want of students. Private universities are closing and the pass rate for entering university has fallen sharply. In its history, China has never had such a low birth rate.'

In proposing a change to such a strategic policy, Gu and his fellow scholars prepared their arguments carefully. In 2005 and 2006, they conducted surveys in districts in four provinces where a two-child policy was allowed in the 1980s; they found the birth rate was not high, with fewer and fewer two-child families.

They found the same results in a recent survey of 18,000 women in six counties in Jiangsu; 4,000 were eligible to have a second child, because they and their husbands were both single children, but fewer than 10 per cent actually had one.

China's economy and social structure have changed drastically from 50 years ago, when large families were seen as desirable. Women have more control over their bodies, easy access to contraception and more economic independence. In a country that is increasingly urban, couples need money to buy a home, car, pensions and health insurance and the best education for their child.

So, Gu and his colleagues argue, a two-child policy does not mean that most couples would have two. A gradual relaxation would lead to a slight increase in the birth rate to bring it up to the replacement level and save China from the 'Japanese disease' - a shrinking pool of young people paying for an increasing number of elderly and a ballooning government debt.

Another argument is economic. China's competitive advantage in the global economy is its enormous pool of cheap labour; in the short term, it will be unable to replace this with superior technology and mechanisation. Already, factories in the Pearl River Delta complain that they cannot find the workers they need.

Another issue is the burden on a single-child couple of looking after four elderly parents: if they cannot do it, who will provide the care and who will pay?

But changing the one-child policy is a decision of enormous significance and one that cannot be taken lightly. Despite the extraordinary economic successes of the past 30 years, China remains a country of shortages, feeding 22 per cent of the world's population from 8 per cent of its arable land. In per capita terms, it is poorly endowed when it comes to minerals and natural resources.

So the national policy will continue to limit the population, by administrative means as well as persuasion and easy access to contraception. Any relaxation of the policy will be step by step, limited to couples in a particular situation.

Over the past 30 years, the government has invested so much manpower, money and resources into changing the centuries-old preference of its people for large families that it cannot be seen to be wavering. The national policy will remain in place but with adjustments on the margin.

Mark O'Neill worked as a Post correspondent in Beijing and Shanghai from 1997 to 2006 and is now an author, lecturer and journalist based in Hong Kong