The 20th century saw the greatest population surge in human history, rising globally from 1.6 billion in 1900 to 6 billion in 2000. That trend is over. The majority of demographic data suggest that, despite previous concerns about overpopulation crises, the bigger problem for most parts of the planet will be too few babies.
Data clearly reflects this phenomenon. In Japan, people buy more diapers for the elderly than babies. China, which long enforced a one-child policy, recently raised its child limit to three; the nation expects its population to peak and then decline in 2030. And the population growth rate in the U.S. is at historic lows, reminiscent of the Great Depression era.
A new study published in npj Urban Sustainability explores the future of underpopulation and how it's likely to affect sustainability goals. Using demographic data from United Nations reports, the study argues that the underpopulation problem is dynamic and twofold: Populations are simultaneously shrinking and ageing.
"Globally, people above 65 years old are the fastest-growing segments of the population and in 2019, for the first time in human history, they outnumbered children younger than 5 years old," the researchers wrote. "In 2020, 9% of the global population was above 65 years old, accounting for 728 million people. This population is projected to increase more than twofold, reaching 1.55 billion in 2050 and accounting to 16% of global population, at medium fertility rates."
These changes won't spread evenly across the globe. By 2050, the regions set to see the biggest increases in elderly populations include Europe, Asia, and North America, while most nations in Africa will continue to have a relatively young population.
The enormous impact of urbanization
A key metric for understanding population shifts is replacement level fertility, which is the average number of children women need to have to keep the population constant. This rate is roughly 2.1 — two children to replace the mother and father, with 0.1 added on because not all children survive to adulthood.
In dozens of nations, the replacement rate has fallen below 1.5, especially in Europe and East Asia. One reason for the drop is rapid urbanization. In 1950, about one-third of humans lived in urban areas, but that ratio is projected to double by 2050 with about 7 billion people living in cities, many of whom will do so for employment opportunities in our increasingly industrial- and technology-focused global economy.
Proportion of aged population (in 2020 and 2050) and urban population (in 2018).Jarzebski et al., npj Urban Sustainability, 2021.
Urbanization affects the population in two key ways. One is that city-dwellers tend to have fewer babies for reasons such as higher cost of living, easier access to contraception, and career-focused urban women choosing to forgo or delay having children, the study noted. Urban life also offers different incentives: Families may benefit from having more children in rural areas, but the same is not true in cities. This explains, in part, why China chose to relax its one-child policy for rural families in the 1980s.
Urbanization also tends to lower mortality rates due to increased wealth and access to healthcare. So, adults have fewer babies while also living longer. The researchers noted that "there may be strong interactions in that increases in the proportion of elderly in a country can put more economic and social pressure on working age population, further decreasing birth rates and/or postponing child births, thus driving fertility rates even lower."
Evolution of the relative levels of mortality and fertility rates over time.Jarzebski et al., npj Urban Sustainability, 2021.
Baby stimulus
Facing ageing and shrinking populations, some nations are already passing or exploring policies to boost fertility rates, including "baby bonuses," subsidized child care, and paid paternity and maternity leave.
If successful, these interventions could usher in a new demographic phase which the study calls the "vulnerable hourglass," characterized by low mortality but recently high fertility. This could result in a population with many young and elderly people, but relatively few working-age adults, who could become overburdened.
Stylised population pyramid transition.Jarzebski et al., npj Urban Sustainability, 2021.
The researchers noted that demographic shifts are complex, and much remains uncertain about how factors like urbanization will affect not only population levels but also the environment and socioeconomic conditions worldwide.
"Considering the quick pace of these changes, especially as the rate of ageing and population shrinking might be underestimated in official statistics, there is a need for urgent action," the study concluded.
Consequential feature
A bather in Blagoveshchensk, on the Russian bank of the Amur. Across the river: the Chinese city of Heihe.
Credit: Dimitar Dilkoff/AFP via Getty Images
The Hu Line is arguably the most consequential feature of China's geography, with demographic, economic, cultural, and political implications for the country's past, present, and future. Yet you won't find it on any official map of China, nor on the actual terrain of the People's Republic itself.
There are no monuments at its endpoints: not in Heihe in the north, just an icy swim across the Amur from Blagoveshchensk, in Russia's Far East; nor in Tengchong, the subtropical southern city set among the hills rolling into Myanmar. Nor indeed anywhere on the 2,330-mile (3,750-km) diagonal that connects both dots. The Hu Line is as invisible as it is imaginary.
Yet the point that the Hu Line makes is as relevant as when it was first imagined. Back in 1935, a Chinese demographer called Hu Huanyong used a hand-drawn map of the line to illustrate his article on 'The Distribution of China's Population' in the Chinese Journal of Geography.
The point of the article, and of the map: China's population is distributed unevenly, and not just a little, but a lot. Like, a lot.
- The area to the west of the line comprised 64 percent of China's territory but contained only 4 percent of the country's population.
- Inversely, 96 percent of the Chinese lived east of the 'geo-demographic demarcation line', as Hu called it, on just 36 percent of the land.
Much has changed in China in the intervening near-century. The weak post-imperial republic is now a highly centralized world power. Its population has nearly tripled, from around 500 million to almost 1.4 billion. But the fundamentals of the imbalance have remained virtually the same.
Even if China's territory has not: in 1946, China recognized the independence of Mongolia, shrinking the area west of the Hu Line. Still, in 2015, the distribution was as follows:
- West of the line, 6 percent of the population on 57 percent of the territory (average population density: 39.6 inhabitants per square mile (15.3/km2).
- East of the line, 94 percent of the population on 43 percent of the territory (average population density: 815.3 inhabitants per square mile (314.8/km2).
Persistent dichotomy
Hu Huanyong's original hand-drawn map of China, showing population density and the now-famous line (enhanced for visibility).
Credit: Chinese Journal of Geography (1935) – public domain.
Why is this demographic dichotomy so persistent? In two words: climate and terrain. East of the line, the land is flatter and wetter, meaning it's easier to farm, hence easier to produce enough food for an ever-larger population. West of the line: deserts, mountains, and plateaus. Much harsher terrain with a drier climate to boot, making it much harder to sustain large amounts of people.
And where the people are, all the rest follows. East of the line is virtually all of China's infrastructure and economy. At night, satellites see the area to the east twinkle with lantern-like strings of light, while the west is a blanket of near total darkness, only occasionally pierced by signs of life. In China's 'Wild West', per-capita GDP is 15 percent lower on average than in the industrious east.
An additional factor typifies China's population divide: while the country overall is ethnically very homogenous – 92 percent are Han Chinese – most of the 8 percent that make up China's ethnic minorities live west of the line. This is notably the case in Tibet and Xinjiang, two nominally autonomous regions with non-Han ethnic majorities.
This combination of economic and ethnic imbalances means the Hu Line is not just a persistent quirk, but a potential problem – at least from Beijing's perspective. Culturally and geographically distant from the country's east, Tibetans and Uyghurs have registered strong opposition to China's centralizing tendencies, often resulting in heavy-handed repression.
Long-term strategy
Street view in Tengchong, on China's border with Myanmar.
Credit: China Photos/Getty Images
But repression is not the central government's long-term strategy. Its plan is to pacify by progress. China's 'Manifest Destiny' has a name. In 1999, Jiang Zemin, then Secretary-General of the Chinese Communist Party, launched the 'Develop the West' campaign. The idea behind the slogan retains its political currency. In the last decade, Chinese Premier Li Keqiang has repeatedly urged the country to "break through" the Hu Line, in order to modernize China's western half.
The development strategy has an economic angle – adding industry and infrastructure to raise the region's per-capita GDP to the nation's average. But the locals fear that progress will bring population change: an influx of enough internal migrants from the east to tip the local ethnic balance to their disadvantage.
China's ethnic minorities are officially recognized and enjoy certain rights; however, if they become minorities in their own regions, those will mean little more than the right to perform folklore songs and dances. The Soviets were past masters in this technique.
Will China follow the same path? That question will be answered if and when the Hu Line fades from relevance, by how much of the west's ethnic diversity will have been sacrificed for economic progress.
Strange Maps #1071
Got a strange map? Let me know at strangemaps@gmail.com.
First, our numbers fell to as low as 18,500, and our ancestors were more endangered than chimpanzees and gorillas. Then we bounced back to extraordinary levels, far surpassing the other great apes.
Today the total population of gorillas, chimpanzees, bonobos, and orangutans is estimated to be only around 500,000, according to the World Wildlife Fund. Many species are critically endangered. Meanwhile, the human population has surged to 7.7 billion. And the irony is: Our astonishing ability to multiply now threatens the long-term sustainability of many species, including ours.
What happened? Why do we live in the Anthropocene and not a world resembling Planet of the Apes? We share around 99 percent of our DNA with our great ape cousins, chimpanzees and bonobos. So, what makes us different from our closest relatives that gives us our staggering capacity for reproducing and surviving?
As an evolutionary anthropologist, these questions have led me to live and study among the Yucatec Maya of Mexico, the Pumé hunter-gatherers of Venezuela, and the Tanala agriculturalists of Madagascar. My research,* combined with genetic data and other studies, offers clues to what developed in the deep past that has made humans so successful—for better or for worse.
In the 1970s, the isolated village of Xculoc, in Mexico's Yucatan Peninsula, was home to about 300 Maya people. The maize-farming residents had no electricity or running water. Women hauled water from a 50-meter-deep well using ropes and buckets. They ground maize—the mainstay of their diet—in hand-cranked grinders.
Then two technologies were introduced that changed these Maya's lives and, ultimately, their population: a gas-powered water pump and two gas-powered maize grinders.
Using these devices, young women saved about two and a half hours of labor and 325 calories a day. In addition, younger siblings could more easily fetch water and crush maize, freeing up their older sisters' time and literally decreasing their daily grind. That's important because studies have found that heavy subsistence work suppresses ovarian function, whereas reducing labor and raising women's energy balance is associated with a bump in fertility.
Subsequently, the age at which women in Xculoc first gave birth dropped by two years. And according to my long-term research, women who started childbearing after these machines arrived produced significantly larger families than prior generations. By 2003, women who started reproducing in the 1970s had eight to 12 children.
Saving women time and energy is central to increasing the population. And humans have developed numerous technological and social ways of accomplishing this that differ from our great ape relatives.
It's important to note that scientists must be cautious about drawing direct analogies between contemporary people or apes and our ancient ancestors. But modern humans and primates are our best tools for inferring how the underpinnings of our numerical success may have evolved.
Somewhere along the evolutionary road, humans started to favor new ways of having and raising their young. Mothers began weaning their infants earlier. In modern societies where infants rely on their mother's milk and not bottle feeding, babies nurse for two to three years. By contrast, great ape mothers nurse their young for four to six years.
Breastfeeding is calorically expensive. It takes a mother about 600 extra calories a day to produce milk. So, the sooner she stops nursing, the sooner she can biologically support another pregnancy. In modern societies without contraception, mothers give birth on average every three years. Other great apes may wait as many as six to eight years between births.
Our ancient ancestors also fed, sheltered, and cared for youngsters who were weaned but still growing. This gave them a better chance at surviving than nonhuman great ape young, which fend for themselves after they're weaned. Today a child living in a hunter-gatherer society is twice as likely as a wild chimpanzee to survive to age 15.
Novel ways of parenting, compared to earlier hominins, meant human mothers were in the unique situation of having multiple dependents of different ages to care for at the same time. I cannot underscore enough how much this sets human mothers and children apart from the other great apes.
Having lots of kids is great for the success of the species. But there's a hitch. Mothers don't have enough hours in the day to care for their babies full time while providing for their older offspring. That's especially true because the unique aspects of the human diet give mothers a lot of tasks to juggle
When these ancient life history traits were evolving, our ancestors made their living as hunter-gatherers, who typically eat diverse fare, including fruits, nuts, tubers, roots, large and small game, birds, reptiles, eggs, insects, fish, and shellfish. Cobbling together such a menu requires modern hunter-gatherers to travel, on average, 13 kilometers per day. By contrast, chimpanzees and gorillas roam, on average, 2 kilometers per day.
What's more, hunter-gatherers process most of their food to make it more digestible or to boost the bioavailability of nutrients. And as everyone who prepares food knows, that takes a significant amount of time.
Among the Pumé hunter-gatherers from the savannas of Venezuela, women spend about three hours a day cracking, mashing, grinding, pounding, sifting, winnowing, butchering, and cooking food. The same is true of Efe women—hunter-gatherers living in the Ituri forest of Central Africa.
That prep time is in addition to the hours the Pumé and Efe spend foraging and then carrying ingredients back to camp. Furthermore, each processing task requires a specialized technology, which means someone has to collect raw materials and make tools. !Kung women and men in the Kalahari Desert of Southern Africa spend about an hour each day making and repairing tools. Savanna Pumé women devote nearly two hours to toolmaking—twice as much as the men.
Hunter-gatherers also build shelters and hearths to provide a safe place to process ingredients, to store food and tools, and to leave children who may be too young to accompany others on long foraging trips. Plus, they must haul water, chop firewood, fashion clothing, and maintain the social and information networks needed to access geographically dispersed resources.
There are simply not enough hours in the day for any one person to accomplish all this. So, our ancestors came up with a solution.
That solution was cooperation—but not the kind of task-sharing many species engage in. Hunter-gatherers developed a distinct feature called intergenerational cooperation: Parents help kids, and children help parents.
This is not a trait we share with the other great apes, who aren't particularly good at sharing food, helping mothers or offspring who aren't their own, or even supporting their own children after they reach a certain age. Nonhuman great ape mothers rarely share meals with their juvenile offspring once they're weaned, and juvenile apes don't offer food to their moms.
But among humans, intergenerational cooperation means it really does take a village to raise a child. Across cultures, mothers in hunter-gatherer and agricultural societies offer only about half of the direct care an infant receives. Savanna Pumé infants, for example, have an average of nine caretakers besides their mother. Efe infants have an average of 11.
Fathers and grandparents certainly play important roles in supporting their families. But it's not enough. An average Maya mother is 60 by the time her last child leaves home, so she has very few years after that to be a babysitting or food-collecting grandmother.
My research suggests a much more obvious source of help has been overlooked: kids. Other than mothers, children provide most of the child care in many cultures. And 7- to 10-year-olds do the bulk of the babysitting.
Children are also responsible for processing much of the food and running the household. A Pumé boy carries home an average of 4.5 kilograms of wild fruit on days he forages. That's the equivalent of 3,200 calories—enough to feed himself and at least some of his family. (And that's in addition to any snacking he does in the field.) His sister can bring home more than a kilogram of roots (worth about 4,000 calories)—some of which she will eat, but most of which she shares. Among the Hadza hunter-gatherers of East Africa, children forage for five to six hours a day. By age 5, they can supply about 50 percent of their own calories during some seasons.
Children in agricultural communities are also hard workers. Yucatec Maya between the ages of 7 and 14 devote two to five hours a day to domestic and field work. Teens between the ages of 15 and 18 labor about 6.5 hours a day—as much as their parents.
By the time a Maya mother is 40, she has an average of seven children at home. These children contribute a combined 20 hours of work a day and supply 60 percent of what the family consumes.
Thanks to this multigenerational help, a woman can spend time doing what only she can do: have more children. So, children increase the population, but their labor is also a built-in engine to fuel their community's fertility and speed up reproduction.
With intergenerational cooperation and a diversity of dietary strategies, our ancestors multiplied and weathered population bottlenecks. Just after 1800, the human population hit 1 billion.
The global population then expanded exponentially, largely due to the enhanced survival of both infants and older people. It reached 2 billion in 1927, 3 billion in 1960, 4 billion in 1974, 5 billion in 1987, 6 billion in 1999, 7 billion in 2011, and today is at over 7.7 billion.
These figures intrigue me as an evolutionary enigma and deeply concern me as a contemporary issue. There is no question, though, that humans have been incredibly successful. The question is: How long can we maintain that success and still be sustainable? The answer, like our secret to growth in the past, stands on the shoulders of cooperation.
* Editor's note: The author derived much of this essay from her 47th JAR Distinguished Lecture, "How There Got to Be So Many of Us," which was published in the Winter 2019 issue of the Journal of Anthropological Research.
This work first appeared on SAPIENS under a CC BY-ND 4.0 license. Read the original here.
