For most of my life, population growth felt like part of the background of the world. Cities spread, schools filled, new workers arrived, and every argument about the future began with the assumption that there would be more people living in it. That assumption is starting to weaken. The United Nations estimated the global fertility rate at 2.25 births per woman in 2024, down from 3.31 in 1990, with fertility below the long-run replacement level in more than half of all countries and areas. One quarter of humanity already lives in a country whose population has peaked.
Global fertility rate decline Chart of total fertility rates for six regions and the world from 1950 to 2100, based on United Nations World Population Prospects data, illustrating the long-term fall toward and below replacement levels. via Wikimedia Commons
The world as a whole is still growing. The same UN projection puts the global peak around 10.3 billion people in the mid-2080s, followed by a gradual decline to about 10.2 billion in 2100. Those numbers are large enough to make the language of population decline sound premature. But global totals can hide a change that is already well underway: in many countries, fewer children are entering schools, fewer young adults are entering work and family life, and a growing share of the population is moving into old age.
World population trajectory to 2100 Our World in Data chart showing historical growth and United Nations medium-fertility projections that peak around the mid-2080s before a slight decline. via Wikimedia Commons
This is a story about fewer births, and then about everything fewer births touch. It changes family life first. Later it reaches the workplace, the care of older people, and organizations trying to find successors. A birth today may become the person who carries some inherited craft into another generation. When births fall for many years, the effects arrive slowly and then appear almost everywhere at once.
World population pyramid, 2024 Age-sex structure of the global population in 2024, reflecting the ongoing shift toward a larger share of older people amid falling fertility. via Wikimedia Commons
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Changing age structure of the world, 1950–2100 Series of population pyramids illustrating the transition from a youthful base in the mid-20th century toward a top-heavy, aging structure by the end of the century. via Wikimedia Commons
The arithmetic becomes human
Population aging comes from something very good: people live longer. Sustained low fertility shapes it too. The UN Population Division describes the sequence plainly. As fertility first falls, the working-age share of a population can rise and create a demographic dividend. Later, the age distribution shifts upward. By 2050, the UN expects roughly one person in six worldwide to be over 65.
Ratios do not care for anyone, though people do. A rising old-age dependency ratio eventually means more retirees relative to conventional working ages, but it does not tell us how healthy those retirees will be, how long people will work, how productive each worker will become, or how families and communities will share care. The UN’s own work on aging cautions that traditional dependency measures can overstate the burden when longer lives change what a fixed age threshold means. Demography sets the conditions; families and governments decide how to respond.
Empty classroom A vacant schoolroom that visualizes the first, most visible effect of sustained below-replacement fertility: fewer children in successive generations. via Wikimedia Commons
Even so, conditions matter. Pension systems need contributors. Hospitals and nursing homes need staff. Schools, firms, and public agencies need successors who can understand what they have inherited before deciding what to change. Automation can help with some of this. Immigration can change the scale and age structure of a country. Healthier old age can extend working lives. None of them makes the underlying generational question disappear.
That question is whether a society can continue to flourish when each generation is smaller than the one before it. I mean flourishing in a fairly ordinary sense: people able to form the families they want, care for those who depend on them, and leave their children more possibility than they inherited. For one generation, a society can draw down inherited abundance. Across five or ten, the people available to renew that inheritance keep thinning. A civilization has to be able to think past the first comfortable decade.
A real benefit, and a real trade-off
A slower-growing population can ease pressure on habitat and the climate. The UN has said that an earlier and lower population peak could reduce environmental pressure through lower aggregate consumption. The Intergovernmental Panel on Climate Change likewise identifies both rising income and population growth as drivers of emissions, while finding that growth in consumption associated with rising incomes has had the larger effect in recent decades.
It would be strange to write about fertility decline without taking that benefit seriously. A smaller population may make some environmental goals easier. It may also allow crowded places to become less expensive or give damaged ecosystems room to recover. Consumption can outweigh population as a driver of environmental harm. A smaller affluent population can consume more than a larger poor one. Energy technology and consumption determine whether fewer people actually produce a lighter footprint.
The trade-off is not between a crowded planet and an abstract desire for more births. It is between goods that can pull in different directions. Lower aggregate consumption may relieve pressure on nature. Fewer young people may also reduce the human capacity available to repair ecological damage, maintain complex infrastructure, discover cleaner technologies, and respond to risks we cannot yet see. I don’t assume that more people automatically solve those problems. I do think human beings are the source of every solution we have found so far.
Freedom and continuity
Any serious discussion of fertility has to begin with the dignity and freedom of the people whose lives are being counted. A government may want more births, an economist may want more workers, and a pension system may need more contributors. None of those needs gives the state ownership of a person’s body or family. People are not inputs into a demographic plan.
They are also not isolated choices floating outside every institution. The United Nations Population Fund’s 2025 survey, conducted in 14 countries containing more than a third of the world’s population, found that nearly one in five reproductive-age adults expected not to have the number of children they desired. Money and difficulty finding a suitable partner were common barriers. Fear about the future also appeared. The survey cannot speak for every person in every country, but it complicates the easy claim that low fertility is simply modern freedom expressing itself. Sometimes people are choosing smaller families. Sometimes the life they hoped to build has become difficult to reach.
I want to hold both truths together. Reproductive freedom deserves consideration, including the freedom not to have children ethically. A society also has reason to care when many people cannot form the families they want, and when the accumulated result is fewer future parents.
The question becomes larger as fertility decline spreads. A country can draw workers from younger places. That helps the receiving country, sometimes enormously, but it moves a person rather than creating one and may take talent from the country that raised her. The OECD’s review of migrant family formation finds that immigrant fertility tends to move toward native-born patterns with time and across generations. In France, for example, completed fertility among daughters of immigrants was 1.9, the same as among women with native-born parents, compared with 2.3 among immigrant women. Immigration props up a national population while leaving the host country’s low-fertility conditions in place. At the global level, it moves people rather than creating them.
At a hundred-year horizon, a country can still rely on technology, accumulated capital, and institutions built by larger generations. At a thousand-year horizon—roughly forty generations—the question looks different. If human beings are to maintain a capable civilization on Earth, protect it from rare catastrophes, and establish self-sustaining communities among the stars, they will need demographic renewal over and over again. A population can be smaller and still do magnificent things. A population that contracts every generation eventually loses the scale to attempt them.
I don’t know where that threshold lies. There probably isn’t one clean number. The practical question is whether we can make family formation compatible with modern freedom and prosperity while learning to live well through the demographic transition already underway. I think we have to reverse the decline. The long future contains too much that is good, strange, and still unbuilt to meet it with fewer people in every generation.
Citations
- World Population Prospects 2024: Summary of Results, United Nations Department of Economic and Social Affairs, Population Division, 2024.
- Ageing, United Nations Department of Economic and Social Affairs, Population Division.
- Ageing, United Nations.
- World Social Report 2023: Leaving No One Behind in an Ageing World, United Nations Department of Economic and Social Affairs, 2023.
- Climate Change 2022: Mitigation of Climate Change, Chapter 2, Intergovernmental Panel on Climate Change, 2022.
- State of World Population 2025: The Real Fertility Crisis, United Nations Population Fund, 2025.
- “Migrant family building: Recent evidence and implications”, in International Migration Outlook 2023, Organisation for Economic Co-operation and Development, 2023.