‘Greatest Risk to Civilization’ Coming Sooner Than Thought, Scientists Warn
The number that demographers have cited for decades as the minimum required to sustain a human population is 2.1 children per woman. Researchers at Shizuoka University in Japan ran that assumption through mathematical models that account for the real, messy behavior of populations, and found it falls well short. Their conclusion points to a number considerably higher, and the gap between that threshold and where most of the world currently sits is larger than almost any government has acknowledged.
That gap is the core of what demographers are now calling a civilization collapse risk. The US, with a total fertility rate of 1.599 in 2024, is not an outlier – it is representative of a pattern spreading across the developed world. The question the new research forces is deceptively blunt: if 2.1 was always a simplification, how far below the real threshold are we?
The answer emerges from a study published April 30, 2025 in PLOS One by Takuya Okabe of Shizuoka University and colleagues. Its implications are stark enough that they have since been cited in policy discussions from Washington to Brussels. The study asked what fertility rate populations actually need, not under ideal model conditions, but in the real world where people die unexpectedly, where some adults never have children, and where the sex ratio at birth is never perfectly balanced.
The Study That Moved the Goalposts
A study published in PLOS One by Takuya Okabe of Shizuoka University and colleagues found that human populations need at least 2.7 children per woman to reliably avoid long-term extinction. That figure is significantly higher than the 2.1 replacement level that policymakers have used as a benchmark for generations.
The conventional 2.1 figure doesn’t account for random differences in how many children people have, as well as mortality rates, sex ratios, and the probability that some adults never have children. The technical term for these random population-level fluctuations is demographic stochasticity – the natural, chance variations in births, deaths, and sex ratios that no statistical average can smooth away. The study’s limitation is worth stating plainly: its findings are based on mathematical modeling, not empirical observation of actual populations. That does not undermine the conclusions, but it means the 2.7 figure represents a theoretical threshold derived from probabilistic models, not a directly measured rate.
Using those models, the research team examined how demographic variability affects population survival over many generations and found that a fertility rate of at least 2.7 children per woman is needed to reliably avoid eventual extinction.
Co-author Diane Carmeliza N. Cuaresma, whose affiliation and role in the study are verified in the published PLOS One paper, stated: “Considering stochasticity in fertility and mortality rates, and sex ratios, a fertility rate higher than the standard replacement level is necessary to ensure sustainability of our population.”
Since fertility rates in developed countries are already below this threshold, the family lineages of almost all individuals are destined to go extinct eventually. The study acknowledges that this condition has already been met in developed countries, and that extinction is not an immediate issue owing to the large population size in these countries. The risk plays out across generations, not news cycles.
Why the 2.1 Rule Was Always a Simplification
The replacement fertility rate of 2.1 assumes a stable world – one with low child mortality, equal numbers of boys and girls, and large, steady populations. These assumptions work in clean models, but reality is messier.
In small populations, chance variations in births and deaths can wipe out entire family lineages. When a population is large, these chance fluctuations average out. When it shrinks – as many developed-world subpopulations already have – random events carry disproportionate weight. A community, a cultural group, a linguistic minority: each faces extinction risk at fertility levels that would appear safe by the old standard.
The study draws a pointed conclusion: sustainability requires consistently meeting fertility targets each generation, not just approaching them on average. For languages, cultural traditions, and family lineages, missing the threshold even occasionally compounds risk over time.
Where the World Actually Stands
The data on current fertility rates make the 2.7 threshold feel less like a distant theoretical concern and more like a description of a crisis already underway.
According to the United Nations World Population Prospects 2024, the global fertility rate has fallen to 2.3 live births per woman as of 2024, down from approximately 5 in the early 1960s. Two-thirds of the world’s population now lives in areas where the total fertility rate is below the replacement level of 2.1.
In the United States, according to a 2026 Johns Hopkins analysis, achieving population replacement requires fertility rates of at least 2.1 children per woman – a threshold the US has not reached since 2007. The 2.1 figure doesn’t account for random differences in how many children people have, as well as mortality rates, sex ratios, and the probability that some adults never have children. In small populations, these chance variations can wipe out entire family lineages.
The global TFR has plummeted from 5 in 1950 to approximately 2.3 in 2024, and the UN projects global population will peak around 10.3 billion in the mid-2080s before declining – meaning that even the global aggregate is trending toward the edge of the old threshold, while most of the developed world crossed that line years ago.
The United States in Detail
US fertility data tracked by World Data shows the US total fertility rate fell to 1.599 births per woman in 2024, dipping below 1.6 for the first time in recorded American history. That figure sits roughly 41% below the new 2.7 threshold identified by the PLOS One study.
Since 2007, the general fertility rate in the US has fallen by 23%, according to Brady Hamilton, a demographer with the CDC’s National Center for Health Statistics. The decline is not evenly distributed. Fertility among women in their teens and twenties has collapsed to record lows, while women in their 30s and 40s are having more children than before.
Half of American women now reach age 30 without having at least one child, according to NPR’s reporting in 2025. The demographic math behind the broader trend is unambiguous: in 2024, there were four million more women aged 20 to 39 than in 2006, but seven million fewer had given birth, as Newsweek reported in 2025.
The Congressional Budget Office projects the US fertility rate will remain near 1.6 through 2055 if current patterns hold. The Institute for Family Studies projects that, if trends continue, the US population will likely peak during the 2050s before entering a prolonged decline.
The United Kingdom and Europe
The UK fertility rate for 2025 was 1.76, according to Macrotrends data – also well below both the conventional 2.1 benchmark and the new 2.7 threshold. The number of children aged 0 to 15 in the UK is projected to decline by 1.6 million, or 12.7%, between mid-2024 and mid-2034, according to analysis from the UK House of Lords Library.
Europe as a whole is in a structurally similar position. The total fertility rate across Europe has already reached 1.46, significantly below the replacement rate of 2.1, according to a March 2025 IMF working paper on fertility and economic growth.
East Asia: The Most Extreme Cases
If the US and UK represent the broad trend, East Asia illustrates where it leads. Among OECD countries, the average fertility rate has fallen to 1.5 births per woman. Japan, often cited as an example of demographic risk, stands at 1.2 births per woman. South Korea’s rate has dropped to 0.75, the lowest in the world.
South Korea’s fertility rate remains far below the 2.1 children needed to maintain its population of 51 million. At current rates, the population will nearly halve to 26.8 million by 2100, according to the Institute for Health Metrics and Evaluation at the University of Washington. Despite the South Korean government offering cash subsidies, babysitting services, and infertility treatment support, and spending over $200 billion on pronatalist incentives since 2006, the fertility rate dropped more than 25% over that same period, according to Newsweek. Long-term shifts in education, employment, housing costs, and gender norms remain the primary structural drivers of that collapse.
These population projections are estimates, not certainties. They carry inherent uncertainty, and actual outcomes will depend on migration patterns, policy changes, and behavioral shifts that no model can fully anticipate.
Over the coming quarter century, 38 nations with populations over one million will likely experience population declines, up from 21 in the previous 25 years. Population loss will be largest in China, projected to drop by 155.8 million people, followed by Japan at 18 million and Russia at 7.9 million – all figures drawn from projections in the IMF’s Finance & Development journal.
Demographers frame fertility decline as a population problem. Economists frame it as a fiscal one.
Variations in fertility, mortality, and migration affect both population growth rates and age structures, with a direct bearing on growth through a declining share of the working-age population. Falling birth rates result in a smaller workforce, which affects economic productivity and increases the burden on social support systems. The ratio of retirees to workers is rising, leading to higher costs for pensions and healthcare and putting pressure on public finances, according to the IMF’s demographic analysis.
In the US, at around 1.6, without immigration the population would have already begun to decline. If the fertility crisis goes unaddressed, the US faces an existential economic crisis driven by steep demographic decline, according to analysis from the American Enterprise Institute.
Slower population growth also leads to fewer new ideas and inventions that power long-term productivity, the IMF noted. Smaller cohorts of young people generate fewer discoveries, fewer companies, and fewer patents – the mechanism through which demographic decline becomes an innovation problem as well as a labor one.
The Debate: Is This Actually a Crisis?
Not everyone accepts the framing of falling fertility as an unqualified catastrophe. Elon Musk has made the issue a recurring theme, calling declining birth rates “the biggest danger civilization faces by far.” The American Enterprise Institute has characterized the situation as humanity entering a new era of rapid population decline. Both views have attracted significant attention and significant pushback.
Some scholars argue that a smaller population could ease pressure on finite economic and environmental resources and may even lead to better, healthier lives in the long run – a perspective covered in Britannica’s 2026 demographic analysis. The counterargument is that the regions driving future population growth are the least equipped to absorb the economic and infrastructural demands that come with it.
The United Nations World Population Prospects 2024 report projects that global population will continue growing until the mid-2080s before peaking at around 10.3 billion, due to population momentum and continued growth in sub-Saharan Africa. Africa’s share of global population is likely to increase from 19% in 2025 to 26% in 2050 as the exceptions to the low-fertility trend – Africa and a number of low-income countries – maintain rates of 4 or higher. The divergence between the global aggregate and what is happening in developed economies is precisely why aggregate figures can be misleading.
The PLOS One study does not claim imminent species extinction. Its authors explicitly note that the condition of sub-2.7 fertility has already been met in developed countries, and that extinction is not an immediate issue owing to the large population size in these countries. What the research argues is that the lineage-level, cultural, and community-level consequences are already in motion, playing out quietly across generations rather than dramatically within decades.
Government Policy: What Has Worked
The policy record on reversing fertility decline is not encouraging. South Korea has spent over $200 billion in pronatalist incentives over the past two decades with minimal effect on its structural birth rate. Nordic countries offer some of the world’s most generous parental leave and subsidized childcare, yet fertility rates across Scandinavia remain well below the 2.1 replacement threshold – evidence that even the most supportive environments have not reversed the structural pressures discouraging childbearing.
Various measures announced by successive Japanese governments to support the birth rate have only slowed the decline, which in recent years has consistently hovered around a 5% annual fall. The structural drivers – housing costs, education costs, gender inequality in childcare responsibilities, and labor market inflexibility – have proven more resistant to policy intervention than any single government benefit program.
Research suggests that the most effective measures to increase fertility are reducing the cost of education and improving childcare provision, alongside labor market reforms that increase flexibility through remote work and flexible hours. Immigration can also play a decisive role in expanding the workforce and supporting economic growth in aging societies, especially when appropriate integration policies are in place, according to the IMF’s 2025 working paper.
A Number That Changes Everything
The fertility rate of 2.1 was a useful shorthand for decades. The PLOS One study’s recalibration to 2.7 doesn’t simply raise the bar – it retroactively reclassifies the demographic position of every developed nation. Countries that believed they were approaching replacement were already in deficit. Countries that thought they were managing a slow decline were already deeper into it than the standard models reflected.
For individuals, the decision to have children sits within a web of structural forces – housing affordability, workplace flexibility, access to childcare, and the unequal distribution of parenting responsibilities – that government policy has proven slow to reshape. Governments that treat low fertility as a cultural problem rather than a policy one have, so far, failed to move the numbers. Countries that have made the most progress, modest though it is, have done so by reducing the concrete economic costs of raising children: expanding subsidized childcare, offering paid parental leave for both parents, and reforming labor markets to make part-time and remote work genuinely viable for parents of young children.
The civilization collapse risk embedded in current fertility trends will not arrive as a singular event. It accumulates, generation by generation, in shrinking school cohorts, rising dependency ratios, and the quiet extinction of family lineages, languages, and cultural traditions. The most consequential shift in demographic science in 2025 was not a new crisis – it was the discovery that the old metric for managing an existing one was set too low all along.
AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.
