A comparison with America reveals a deep flaw in China�s model of growth

LIKE the hero of �The Iliad�, China can seem invincible. In 2010 it overtook America in terms of manufactured output, energy use and car sales. Its military spending has been growing in nominal terms by an average of 16% each year for the past 20 years. According to the IMF, China will overtake America as the world�s largest economy (at purchasing-power parity) in 2017. But when Thetis, Achilles�s mother, dipped her baby in the river Styx to give him the gift of invulnerability, she had to hold him somewhere. Alongside the other many problems it faces, China too has its deadly point of unseen weakness: demography.

Over the past 30 years, China�s total fertility rate�the number of children a woman can expect to have during her lifetime�has fallen from 2.6, well above the rate needed to hold a population steady, to 1.56, well below that rate (see table). Because very low fertility can become self-reinforcing, with children of one-child families wanting only one child themselves, China now probably faces a long period of ultra-low fertility, regardless of what happens to its one-child policy.

The government has made small adjustments to the policy (notably by allowing an only child who is married to another only child to have more than one child) and may adapt it further. But for now it is firmly in place, and very low fertility rates still prevail, especially in the richest parts of the country. Shanghai reported fertility of just 0.6 in 2010�probably the lowest level anywhere in the world. According to the UN�s population division, the nationwide fertility rate will continue to decline, reaching 1.51 in 2015-20. In contrast, America�s fertility rate is 2.08 and rising.

The difference between 1.56 and 2.08 does not sound large. But over the long term it has a huge impact on society. Between now and 2050 China�s population will fall slightly, from 1.34 billion in 2010 to just under 1.3 billion in 2050. This assumes that fertility starts to recover. If it stays low, the population will dip below 1 billion by 2060. In contrast, America�s population is set to rise by 30% in the next 40 years. China will hit its peak population in 2026. No one knows when America will hit its population peak.

The differences between the two countries are even more striking if you look at their average ages. In 1980 China�s median (the age at which half the population is younger, half older) was 22. That is characteristic of a young developing country. It is now 34.5, more like a rich country and not very different from America�s, which is 37. But China is ageing at an unprecedented pace. Because fewer children are being born as larger generations of adults are getting older, its median age will rise to 49 by 2050, nearly nine years more than America at that point. Some cities will be older still. The Shanghai Population and Family Planning Committee says that more than a third of the city�s population will be over 60 by 2020.

This trend will have profound financial and social consequences. Most obviously, it means China will have a bulge of pensioners before it has developed the means of looking after them. Unlike the rest of the developed world, China will grow old before it gets rich. Currently, 8.2% of China�s total population is over 65. The equivalent figure in America is 13%. By 2050, China�s share will be 26%, higher than in America.