Demographics prompted relaxation of Beijing's one-child policy where decades of international criticism made no difference. A defining moment came early last year, with news that the percentage of the population of working age had fallen from 75.5 to 74.4. That tiny reverse marked the beginning of the end of what economists refer to as China's population dividend - an abundant supply of cheap labour to help drive economic growth. It was a sign that the ageing of the generation that wrought the country's economic miracle had brought forward the one-child policy's use-by date. Significantly, within months, it became evident that public discussion of the policy was no longer taboo. Academics from a State Council think tank called for couples to be allowed to have a second child to counter population ageing and a labour shortage, and 15 scholars joined the debate in an open letter to the National People's Congress calling for change. This reflected a widening consensus among officials on the need to revamp population policy, culminating in last week's decision by the third plenary session of the Central Committee to open the way to gradual change. As a start, families in which either parent is a single child will be allowed to have a second child. This will mainly affect urban residents. Rural residents can already have a second child if their first is a girl. Despite a likely initial surge driven by pent-up demand, the policy change is unlikely to lead to a significant rise in births, given that couples will take into account increasingly expensive housing and schooling in major urban areas. A demographer involved in drafting the new policy said it might be 2020 before every couple could have a second child. The Communist Party can claim that the one-child policy has lifted living standards and helped hundreds of millions escape poverty. But apart from population ageing, it has left a painful social legacy of abortion in favour of males, and the prospect of millions of men, without women, likely to remain single and to be a potential threat to social stability. The exhaustion of the population dividend, coming on top of the number of city dwellers exceeding the rural population - which defines an industrial economy - finally undermined economic and population-control arguments against relaxation. The change underscores the policy challenges if growth is to be sustained, such as restructuring the economy to reduce reliance on exports and promote domestic consumption.