中美对比反映出中国人口增长模式的深层缺陷。
正如英雄伊利亚特，中国看起来无敌于世界。在2010年，中国在商品制造输出，能源使用及车辆销售方面赶上了美国，在过去20年，它的军事开支名义上以每年16%的速度增长。根据国际货币基金组织预计，中国将在2017年代替美国成为世界最大经济体（以购买力平价衡量）。但当西提斯，伊利亚特的母亲把他浸泡在冥河使他拥有刀枪不入的能力时，她必须抓住他的脚。伴随着其他问题的出现，中国也不得不面临其致命弱点--人口结构问题。
在过去的30年，中国的总生育率（即一名妇女一生的生育数）已经从2.6降为1.56，从超过维持人口稳定的基数降为低于这个基数。因为低生育率能够自我强化，独生子女背景的人更愿意只生一个孩子。因此中国现在可能将面临一段长时间的低生育率时期，不顾计划生育政策所带来的后果。
中国政府对于这个政策做了细微的调整，比较显著的是两个独生子女结婚可以生育超过一个以上的孩子，并且可能进一步调整。但目前，中国的计划生育政策依然坚定，并且低生育率盛行，特别是在一些富裕地区。根据联合国人口分部预计，全世界的生育率将持续性地降低，可能在2015到2020年降为1.51.相反的是，美国的生育率是2.08并且正在增长中。
1.58和2.08的差距看似不大。但长时间之后这种区别对社会有着巨大的影响。从现在到2050年，中国人口将降为13亿以下，这是在假设生育率恢复正常的前提下。但如果生育率一直持续降低的话，到2060年中国人口将降为10亿，相反，美国人口在未来40年预计增长40%，中国将在2026年到达它的人口高峰，但没有人知道美国什么时候会迎来它的人口高峰。
如果参照两个国家的人口平均年龄的话，两者的区别将使你更为震惊。在1980年，中国人口年龄中位数是22（即中位年龄），这是一个新兴发展中国家的特征，而现在这个数字是34.5，更像是一个和美国区别不大的富裕国家，美国人口年龄中位数为38.但现在中国正在以一种空前的步伐迈向老龄化。因为越来越多的成年人走向老龄化，而新生儿却越来越少。到2050年，中国的人口年龄中位数将攀升至49岁，与同时段的美国相比老了9岁。中国一些城市的人口年龄中位数将更大。上海人口和计划生育委员会预计三分之一的上海人口将在2020年超过60岁。
这种趋势将会对中国的经济和社会造成深远的影响。最为明显的是，未来当大批领养老金的人出现时，中国还未想好如何安置他们。不像其他发达国家，中国未富先老。当前中国8.2%的人口超过65岁，而美国有13%。到2050年，中国的份额将达到26%，远远超过美国。
在中国的传统家庭中，子女，尤其是儿子，必须赡养父母（尽管这种现象现在正在发生改变，详情参照后文）。但快速老化意味着中国将面临所谓的4-2-1现象，即一个独生子女要负责赡养其父母和两对祖父母。尽管储蓄率很高，但年轻一代似乎难以承受这一重负。因此大多数老年人不得不严重依赖社保退休金。
中国在2000年建立了国家养老基金，但实际只有3.65亿人口能正常领到退休金。并且这个系统现在正处于危机中。这个国家未备基金几乎是GDP的1.5倍。有一半省市运作的养老基金处于负债中，有些地方政府甚至拒绝发放养老金。
但这些问题只是冰山一角。从2010到2050年，中国的劳动力将出现巨大的收缩，从72%降至61%，尽管目前中国的劳动力份额依然巨大。这意味着中国的养老抚养率----（已退休人员数量占在职人口数量的比例）将急速增长。目前中国的养老抚养率是11，是美国的一半，但到2050年，这个数字将增长4倍到42，远远超过美国。更让人吃惊的是，到2050年，中国的退休人员将超过人口的10%，而刚开始工作的人（那些20几岁，受过高等教育的社会人才）将减半。
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这种变化使中国不复成为世界工厂。原有的大量廉价劳动力资源开始走向枯竭。尽管有很多未充分就业的农村居民，中国仍将面临体力劳动者的短缺。到2013年劳动人口下降之后，这些问题会更加地严峻。牛津大学研究所所长萨拉·哈珀指出中国已经知道了职业年龄构成，并且知道何时会出现技术劳动力的短缺。中国很可能通过雇佣外来劳动力来抵消这些影响。人力资源公司万宝盛华，预计到2030年中国将从外国进口大批劳动力，而不是出口。
大规模的外来移民带来其自身的问题。美国是少数能够通过复杂的外来移民建立本国熟练劳动力的国家。但美国是一个开放，多民族的，有长期移民史的国家，并有强大完善的法律和政治体系，而中国没有这些特点。
在缺乏透明机制的情况下，中国社会依赖“关系”，一个以大家庭血缘关系为中心的人际网。但当大家庭越来越少时，情况又会发生什么变化？一个可能性是中国走向更为透明化的法律体系和更加开放的政治文化。同时，由于中国的经济增长速度将放慢，中国领导人将面临一个两难选择，是买武器还是拐杖。
中国不是唯一一个面对这些问题的国家，所有富有的国家都面临不断增长的养老金支出。但中国在解决这些问题时还是有一定优势的。最为明显的是低税率（可上升空间大），公众对社会福利的期望值低。当然，中国在两个方面与其他老龄化国家不同。一个是它比其他国家要贫困，另一个是其人口转变比较突兀。中国似乎不可能跳出人口问题来实现经济增长。相反，这些问题会降低它的经济增长速度------更不必说所带来的巨大社会挑战。中国的软肋也许不会致命，但却足以使这个巨人跛足。
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.
In the traditional Chinese family, children, especially sons, look after their parents (though this is now changing—see story on next page). But rapid ageing also means China faces what is called the “4-2-1 phenomenon”: each only child is responsible for two parents and four grandparents. Even with high savings rates, it seems unlikely that the younger generation will be able or willing to afford such a burden. So most elderly Chinese will be obliged to rely heavily on social-security pensions.
China set up a national pensions fund in 2000, but only about 365m people have a formal pension. And the system is in crisis. The country's unfunded pension liability is roughly 150% of GDP. Almost half the (separate) pension funds run by provinces are in the red, and local governments have sometimes reneged on payments.
But that is only part of a wider problem. Between 2010 and 2050 China's workforce will shrink as a share of the population by 11 percentage points, from 72% to 61%—a huge contraction, even allowing for the fact that the workforce share is exceptionally large now. That means China's old-age dependency ratio (which compares the number of people over 65 with those aged 15 to 64) will soar. At the moment the ratio is 11—roughly half America's level of 20. But by 2050, China's old-age ratio will have risen fourfold to 42, surpassing America's. Even more strikingly, by 2050, the number of people coming towards the end of their working lives (ie, those in their 50s) will have risen by more than 10%. The number of those just setting out (those in their early 20s, who are usually the best educated and most productive members of society) will have halved.
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The shift spells the end of China as the world's factory. The apparently endless stream of cheap labour is starting to run dry. Despite pools of underemployed country-dwellers, China already faces shortages of manual workers. As the workforce starts to shrink after 2013, these problems will worsen. Sarah Harper of the Oxford Institute of Population Ageing points out that China has mapped out the age structure of its jobs, and knows for each occupation when the skills shortage will hit. It is likely to try to offset the impact by looking for workers abroad. Manpower, a business-recruitment firm, says that by 2030 China will be importing workers from outside, rather than exporting them.
Large-scale immigration poses problems of its own. America is one of the rare examples of a country that has managed to use mass immigration to build a skilled labour force. But America is an open, multi-ethnic society with a long history of immigration and strong legal and political institutions. China has none of these features.
In the absence of predictable institutions, all areas of Chinese society have relied on guanxi, the web of connections that often has extended family relations at the centre. But what happens when there are fewer extended families? One result could be a move towards a more predictable legal system and (possibly) a more open political culture. And, as shifts in China's economy lead to lower growth, Chinese leaders will have to make difficult spending choices; they will have to decide whether to buy “guns or walking sticks”.
China is not unique in facing these problems. All rich countries have rising pension costs. And China has some advantages in dealing with them, notably low tax rates (giving room for future increases) and low public expectations of welfare. Still, China is also unusual in two respects. It is much poorer than other ageing countries, and its demographic transition has been much more abrupt. It seems highly unlikely that China will be able to grow its way economically out of its population problems. Instead, those problems will weigh down its growth rate—to say nothing of the immense social challenges they will bring. China's Achilles heel will not be fatal. But it will hobble the hero.
