We used to imagine humanity populating the universe. In Isaac Asimov’s Foundation (1952), mankind has established a vast multi-planetary empire by the year 47000. “There were nearly twenty-five million inhabited planets in the Galaxy,” Asimov wrote. “The population of Trantor [the imperial capital] … was well in excess of forty billions.”
In Liu Cixin’s Three-Body Problem (2006), by contrast, we’re a cosmic rounding error, bracing ourselves for the terrifying Trisolaran invasion. As the trailer for the new Netflix series puts it: “They are coming, and there is nothing you can do to stop them.”
When Asimov was born in 1920, the global population was around 1.9 billion. When he published Foundation, it was 2.64 billion. By the time of his death in 1992, it was 5.5 billion, nearly three times what it had been at his birth. Considering that there had been a mere 500 million humans when Christopher Columbus landed on the New World, the proliferation of the species homo sapiens in the modern era had been an astonishing feat.
Small wonder some members of Asimov’s generation came to dread overpopulation and fret about an impending Malthusian disaster. This led to all kinds of efforts to promote contraception and abortion, as described in Matt Connelly’s Fatal Misconception: The Struggle to Control World Population (2008). Among these was China’s one-child policy, the harshest ever government intervention in human reproductive behavior.
Superficially, these efforts were a complete failure. Frank Notestein, the Princeton demographer who became the founding director of the United Nations Population Division (UNPD), estimated in 1945 that the world’s population would be 3.3 billion by the year 2000. In fact, it exceeded 6.1 billion. Today it is estimated to be more than 8 billion. In its most recent projection, the UNPD’s median estimate is that the global population will reach 10.4 billion by the mid 2080s, with an upper bound of more than 12 billion by the end of the century.
Yet that seems rather a low-probability scenario. The European Commission’s Centre of Expertise on Population and Migration projects that the global population will peak at 9.8 billion in the 2070s. According to the Institute for Health Metrics and Evaluation, an independent research organization, it will peak at a lower level and earlier still, at 9.7 billion in 2064.
The key word is “peak.” Nearly all demographers now appreciate that we shall likely reach peak humanity this century. This is not because a lethal pandemic will drive up mortality far more than Covid-19 did, though that possibility should never be ruled out. Nor is it because the UNPD incorporates into its population model any other apocalyptic scenario, whether disastrous climate change or nuclear war.
It is simply because, all over the world, the total fertility rate (TFR) — the number of live children the average woman bears in her lifetime — has been falling since the 1970s. In one country after another, it has dropped under the 2.1 threshold (the “replacement rate,” allowing for childhood deaths and sex imbalances), below which the population is bound to decline. This fertility slump is in many ways the most remarkable trend of our era. And it is not only Elon Musk who worries that “population collapse is potentially the greatest risk to the future of civilization.”
Our species is not done multiplying, to be sure. But, to quote the UNPD, “More than half of the projected increase in the global population between 2022 and 2050 is expected to be concentrated in just eight countries: the Democratic Republic of the Congo [DRC], Egypt, Ethiopia, India, Nigeria, Pakistan, the Philippines and the United Republic of Tanzania.” That is because already “close to half of the global population lives in a country or area where lifetime fertility is below 2.1 births per woman.”
Not many people foresaw the global fertility collapse. Nor did just about anyone expect it to happen everywhere. And I can’t recall a single pundit predicting just how low it would go in some countries. In South Korea the total fertility rate in 2023 is estimated to have been 0.72. In Europe there is no longer a difference between Roman Catholic and Protestant countries. Italy’s current TFR (1.21) is lower than England’s (1.44). Nor is there a difference between Christian and Islamic civilizations — those great historical entities whose clashes the historian Samuel Huntington worried about. The US total fertility rate is now 1.62. The figure for the Islamic Republic of Iran is 1.54.
Among selected countries, only Democratic Republic of Congo had a birthrate above replacement level of 2.1 live births per woman in 2023
The timing of this huge demographic transition has varied, to be sure. In the US, the TFR fell below 2.0 in 1973. In the UK, it happened a year later; in Italy in 1977. The East Asian countries were not far behind: In South Korea TFR was above 2.0 until 1984; in China until 1991. Fertility remained higher for longer in the Muslim world, but it fell below 2.0 in Iran as early as 2001. Even in India the TFR has now fallen below 2.0.
Only in the countries of sub-Saharan Africa does fertility remain well above the replacement rate. In the DRC, for example, the average woman still bears more than 6 children. But there, too, fertility is expected to plummet in the coming decades. The global TFR, according to the UNPD’s medium-variant projection, will fall from 2.3 in 2021 to 1.8 in 2100. The differences in estimates of when we reach peak humanity largely hinge on how quickly demographers think family size will shrink in Africa.
Total fertility for selected countries, in average live births per woman
Note: Projections start in 2023
What are the drivers of the great fertility slump? One theory, according to a thought-provoking 2006 paper by Wolfgang Lutz, Vegard Skirbekk and Maria Rita Testa, is that “societies progress up the hierarchy of needs from physical survival to emotional self-actualization, and as they do so, rearing children gets short shrift because people pursue other, more individualist aims. … People find other ways to find meaning in life.” Another interpretation (see for example this paper by Ron Lesthaeghe) gives the agency to women, emphasizing that fertility drops as female education and employment rise.
Over the past century, beginning in Western Europe and North America, a rising proportion of women have entered higher education and the skilled labor force. Improved education has also given women greater autonomy within relationships, a better understanding of contraception, and greater input into family planning. Many have opted to delay becoming mothers in order to pursue their careers. And the opportunity cost of having children increases as women’s wages rise relative to their male partners.
Another way of looking at the problem is that, after its initial kids-in-cotton-mills phase, the industrial revolution reduced the importance of children as a source of unskilled labor. As countries develop economically, families invest more in their children, providing them with better education, which increases the cost of raising each individual child.
Cultural change has also played a part. One study estimated that roughly a third of the decline in fertility in the US between 2007 and 2016 was due to the decline in unintended births. My generation — the baby boomers — were more impulsive and indeed reckless about sex. By contrast, according to the psychologists Brooke Wells and Jean Twenge, millennials have fewer sex partners on average than we did. A 2020 analysis of responses to the General Social Survey revealed higher rates of sexual inactivity among the most recent cohort of 20- to 24-year-olds than among their predecessors born in the 1970s and ‘80s. Between 2000-02 and 2016–18, the proportion of 18- to 24-year-old men who reported having no sexual activity in the past year increased from 19% to 31%.
The fact that the declines in sexual activity were most pronounced among students and men with lower incomes and with part-time or no employment suggests that declining sexual activity is economically determined. However, other possible explanations include the “stress and busyness of modern life,” the supply of “online entertainment that may compete with sexual activity,” elevated rates of depression and anxiety among young adults, the detrimental effect of smartphones on real-world human interactions, and the lack of appeal to women of “hooking up.”
The most recent version of the UK National Survey of Sexual Attitudes and Lifestyles revealed a similar marked decline in the frequency of sex in Britain. The return of the “No sex please, we’re British” ethos mainly affects married or cohabiting couples and — according to a careful analysis in The British Medical Journal — is most likely due to “the introduction of the iPhone in 2007 and the global recession of 2008.”
Another key driver of declining fertility has been declining religiosity. Using data in the World Values Survey, we can identify a clear correlation between the rise of secularization and the fall of family size. A fascinating historical anomaly, the early decline of fertility in late-18th century France — described by the French demographer Alfred Sauvy as “the most important fact” of his country’s history — has been plausibly explained by the advance of secular thinking, and therefore of contraceptive practices, in the wake of the religious strife of the previous two centuries.
Fertility can sometimes go back up — witness the Covid baby “bump.” Moreover, according to survey data, many women would like to have more children. In low-fertility countries, according to a 2019 study for the UN Population Fund, there is “a wide gap between fertility aspirations at younger ages and achieved fertility later in life, signaling that many women, men and couples face obstacles in realizing their fertility plans.”
That the main obstacles are the perceived economic costs of a larger family is borne out by the fact that many of the most successful professional women have more than two children. In the words of Moshe Hazan and Hosny Zoabi, “the cross‐sectional relationship between fertility and women’s education in the US has recently become U‐shaped. … By substituting their own time for market services to raise children and run their households, highly educated women are able to have more children and work longer hours.”
But not everyone can be a supermom with a crew of house managers and nannies. Can governments do anything to push back up fertility across the board? They are certainly trying. Since the 1970s, the number of countries aiming to raise fertility with a variety of government incentives has risen roughly fivefold. But there are no examples I know of in which pro-natal policies have really worked. For years, President Vladimir Putin has urged Russians to have more babies in order to prevent the depopulation of the vast federation he governs. Though Russian fertility rose in the decade after 2000, the TFR never even got close to 2, and has slumped back to 1.5.
What Mussolini called “the battle for births” is a losing proposition. The global trend is to make abortion easier. (In the past 30 years, more than 60 countries have altered their abortion laws. All but four — the US, El Salvador, Nicaragua and Poland — eased access to abortion). A growing number of countries permit euthanasia and/or assisted suicide. Average sperm counts have fallen by more than 50% in 50 years. No one knows exactly why, but bad food, bad air and bad lifestyle are the contenders. How Mankind Chose Extinction will be an interesting read if anyone is left to write it.
Half a century ago, we worried about The Population Bomb (the title of Paul Ehrlich’s 1968 bestseller). Now that we can see “peak humanity” within our children’s lifetimes — conceivably in the 2060s — why isn’t everyone breathing a sigh of relief? I can think of three reasons.
First, the advanced countries that already have declining populations find the consequences of fertility restriction rather melancholy: low economic growth, empty schools, crowded retirement homes, a general lack of youthful vitality.
Second, because the fertility drop came later in the Middle East and North Africa and has barely begun in sub-Saharan Africa, we are seeing a dramatic shift in the global demographic balance in favor of people with darker pigmentation — as a Scotsman married to a Somali, I am doing my part for this trend — many of them Muslims. This worries many of the mostly white and mostly Christian peoples who were globally dominant from around 1750 to 2000.
Third, the peoples with the highest fertility mostly live in poor places that climate change and armed conflict are making even less appealing. So they move if they can — through North Africa or Western Asia toward Europe, or via Mexico to the US — or, to a significant extent, get involved in violent activities (crime or terrorism) where they can’t escape.
All this drives up the probability of right-wing politics in the developed world (old people vote for this and they outnumber the young), more conflict (borders can’t seriously be defended without at least the threat of violence), the more rapid spread of infectious pathogens, and no effective attempt to address the climate issue.
Yet immigration still seems to North American and European elites to be the simplest solution to the problem of falling fertility. That is why, in high-income countries between 2000 and 2020, the contribution of net international migration to population growth exceeded the balance of births over deaths. What the geopolitical consequences of mass migration will be is anyone’s guess. Some Russians worry that the Chinese have designs on their vast Eurasian empire east of the Urals. That seems unlikely if China’s population is set to halve between now and 2100. China’s problem is not a shortage of space; it is a surplus of empty apartment blocks.
In contemplating these and other scenarios, most pundits struggle to grasp that, when the human population begins to fall, it will do so not gradually, but almost as steeply as it once rose. “Humanity will not reach a plateau and then stabilize,” writes Dean Spears in the New York Times. “It will begin an unprecedented decline … If the world’s fertility rate [after 2100] were the same as in the United States today, then the global population would fall from a peak of around 10 billion to [less than] 2 billion about 300 years later, over perhaps 10 generations. And if family sizes remained small, we would continue declining.”
The problem is that this precipitous decline will come a century too late to avert the disastrous consequences of climate change that many today fear — and which are another reason why people will flee Africa, and another reason why young people in Europe say they will have few or no children.
The appropriate science fiction to read is therefore neither Asimov nor Liu Cixin. Begin, instead, with Mary Shelley’s The Last Man (1826), in which a new Black Death wipes out all but one forlorn specimen of humanity. Then turn to Margaret Atwood’s Oryx and Crake (2003), in which the addled “Snow-man” is one of just a handful of survivors of a world ravaged by global warming, reckless genetic engineering, and a disastrous attempt at population reduction that resulted in a global plague.
For those, like Elon Musk, who still dream of building Asimov’s galactic empire, such visions of human extinction are hard to stomach. He and others swim against the tide, siring five or six times as many offspring as the average male. But the reality is that a sub-2.1 global TFR is a more powerful historical force than even the fecund Mr. Musk. It is coming. And there is nothing we can do to stop it.
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