IF DEMOGRAPHY is destiny, as Auguste Comte, a French philosopher, once said, then China has many destinies. As a result of 30 years of the now-relaxed one-child policy, the country has an exceptionally low overall fertility rate: 1.2 according to the census of 2010. (The fertility rate is the number of children an average woman can expect to bear during her lifetime. If it is less than 2.1 a population will shrink in the long run, unless immigration makes up for the dearth of babies.) What is almost never recognised, however, is that this is not a uniform problem. Just as China has richer and poorer regions, so it has areas of higher and lower fertility—or, to be more precise, of low and lower fertility.

As a whole, China has too few young adults relative to the size of older generations, meaning it will not have enough workers to support its pensioners (or children) properly in the future. But some areas will hit demographic trouble earlier and harder than others, with serious implications for economic growth and regional stability. Wang Feng, of the University of California, Irvine, dubs the problem “the Balkanisation of Chinese demography”.

The place with the lowest fertility is Beijing, where the rate was 0.71 in 2010. The highest rate that year was in Guangxi, a province in the south bordering Vietnam, where the fertility rate was 1.79. Both rates are below the replacement level. But Guangxi’s fertility is two-and-a-half times higher than Beijing’s, which is a wider spread than the one separating the states with the highest and lowest fertility in Brazil, and only a little less than the equivalent gap in India.

Degrees of dwindling

The main reason is that, in practice, the one-child policy was never uniform. Ethnic minorities, such as Tibetans or Uighurs (the largest group in the western province of Xinjiang), were never subject to it. Minorities, who account for 8% of the population nationwide, were usually allowed two children in urban areas and three or four in rural ones. In addition, in most rural areas, everyone, including the majority Han group, was allowed two children.

As a result China has four categories of fertility, not one (see map):

1. Areas of ultra-low fertility (rates of less than 1). These are three mega-cities, Beijing, Shanghai and Tianjin, and three provinces in the north-east, sometimes called Manchuria, where the one-child policy was applied most strictly. They have a total population of 170m.

2. Areas where fertility is between 1 and 1.29. These include provinces on China’s populous coastline, as well as the huge Sichuan basin in western China. They are overwhelmingly Han areas, so had few exceptions to the one-child policy. They were also the places where China’s growth and urbanisation took off quickest after 1980, so have relatively few rural dwellers. This is the largest category, with 600m people.

3. Provinces with fertility rates between 1.3 and 1.49. Many, such as Henan, Hunan and Anhui, are just inland from the coast. They, too, are populous (460m in total) and mostly Han but have fewer city-dwellers: more than half of the populations of Hunan and Anhui is rural. This group also includes several provinces with lots of members of minorities, such as Ningxia, in the north-west, which is a third Muslim.

4. Areas with rates above 1.5, which tend both to be more rural and to have big minority populations, such as Guangxi. These have a total population of 116m.

Since the one-child policy was in force so long, differences in fertility have become entrenched and their impact profound. To take one example, provinces with relatively low fertility tend to have an even bigger excess of boys over girls than is the norm. Nationally, the imbalance has ebbed somewhat since 2000, with the sex ratio at birth falling from 121 boys for every 100 girls in 2005 to 114 in 2015. But in the north-east there has been little or no improvement—a worry considering the high levels of crime associated with large numbers of unmarried men (called “bare branches” in China).

Fertility is not the only force pushing provincial demography in different directions. The migration of more than 245m workers from poor, rural areas to booming cities amplifies the difference in fertility in some places and counteracts it in others.

In the decade before 2010 the population of Chongqing, a large urban province in the west, fell by 2m (or 6%); in neighbouring Sichuan it fell by 3m. Births exceeded deaths in both places over the period, so the population should have grown. But this was offset by the outflow of migrants. Cai Yong of the University of North Carolina calculates that more than 10m people left Sichuan and nearby Hubei.

The combination of migration and varying fertility means that provinces are ageing at different speeds. The median age nationally rose from 25 in 1990 to 35 in 2010; it had increased to 37 by 2016. But the three north-eastern provinces all aged by even more than average. In Liaoning the median age reached 39.2 in 2010, about the same as Russia. In contrast, the median age in Tibet, the youngest province, is 27.8, about the same as India.

Ageing matters because pension provision is partly a provincial responsibility in China. The value of the basic state pension is fixed nationally, but provinces set their own contribution rates, administer the money collected and distribute the pensions. How heavy a burden this is depends on a province’s demography. As a rule, the lower the fertility rate, the faster the rise in the dependency ratio (the number of pensioners relative to the number of working people). In relatively fecund Guizhou and Yunnan, the ratio is still falling. In Beijing and Shanghai, it rose by more than four percentage points between 2010 and 2015, more than the national average.

Giant cities such as Beijing, Shanghai and Tianjin have ultra-low fertility and fast-rising dependency ratios yet are still able to attract young workers because China’s highest-paying jobs are clustered there. As a result, their demographic profile is healthier than you would expect. The three cities, which have provincial-level status, are China’s fastest-growing provinces by population, increasing by around 3% a year in 2000-10, thanks largely to migration. Since the migrants are mostly young, the cities’ median ages rose much more slowly than the national average and their dependency ratios remain relatively low. And since the cities are also rich, they have hospitals, social services and schools to cope with their demographic problems.

Provinces with high fertility and outward migration are the opposite. Take Hainan, a tropical island in the far south. It has high fertility (by Chinese standards) and stable dependency. It ought to be doing well. Yet it is one of China’s poorest provinces (23rd out of 31) and is ageing fast, mainly because hundreds of thousands of workers from the freezing north-east are spending their retirement there. Its medical services are collapsing under the strain.

To see the convergence of all these trends, compare two regions, the north-east and Guangdong. The north-east is China’s rust belt, a place of depleted coal mines and decayed steel mills. It has had low fertility for decades, falling below replacement levels as long ago as 1982, much earlier than elsewhere (and before the one-child policy even began). It also implemented the policy especially strictly because it is dominated by state-owned industries which decreed that people who had a second child would lose their jobs. “Who would risk it?” asks a former steel worker. The area’s high wages used to attract migrants from elsewhere in China. But since 2000, when heavy industry ran into trouble, it has suffered a net outflow of over 2m people. Hotels near the Harbin Institute of Technology (in the region’s largest city) are packed around graduation day with recruiters from southern firms.

Last year a series of articles in China Business News, a state-run newspaper, revealed the extent of the region’s demographic problems. In China as a whole, it said, there were 2.9 people paying into provincial pension schemes for every person drawing a pension. In Liaoning, there was only 1.8; in Jilin, 1.5; and in Heilongjiang, just 1.3. The region’s share of China’s young workers (20 to 39 years of age) fell from 10% in 1982 to 8% in 2010. Zhou Tianyong of the Central Party School in Beijing says the region’s lack of young workers is his biggest worry. The national government has a grand policy to help the region called “the north-east revitalisation plan”, but as one of the articles noted, the region’s demographic crisis is never discussed.

Now compare that with Guangdong at the other end of the country, next to Hong Kong. On the face of it, China’s largest province, with a population of 108m, also faces severe problems. Its fertility rate was reported to be 1 in 2010, more than in the north-east but still alarmingly low. Yet its population rose more quickly in 2000-10 than any other province except the three huge cities. Its median age is five years below that in the north-east. It has 9.7 workers per pensioner, three times the national average, which has helped it to stash more money in its pension fund than any other province.

Whereas Beijing and Shanghai are attempting, misguidedly, to curb migration, Guangdong is trying to attract new arrivals. It has made it easier for their children to enroll in local schools (elsewhere the household-registration, or hukou, system, raises barriers to this). It also encourages everyone, including migrants, to join local social-insurance schemes. In mid-July, the province’s capital, Guangzhou, said it would allow the children of better-off migrants who rent property the same access to schools as local home-owners. This is significant since almost all migrants rent, not own, their houses.

Unlike in Guangzhou, the national authorities have been slow to recognise the problems of demographic decline. As a result, low fertility, ageing, labour shortages and dependency have all taken on a provincial aspect. The three great cities look relatively healthy, as do Guangdong and Zhejiang, a nearby province that shares some of its features. But provinces with low fertility, declining or ageing populations, and rising dependency are in deep trouble. These include the north-east, Sichuan and Chongqing in the west and several provinces in the third category in terms of fertility, such as Anhui.

The result is a big problem for the national government. Even now, it is having to bail out some provincial pension funds. But the threat is also philosophical. The Communist Party has long sought to narrow economic differences and erase local political distinctions because it is terrified of regional challenges. It thinks the only way to keep China together is to impose strong central control. If it is right, its failure to deal with demographic problems is setting back that cause.