What does the UN say about Earth's future population?

The UN expects a peak of 10.3 billion in the 2080s and a decline to 10.2 billion by 2100. That is more cautious than the IHME projection, which factors women's education into its model.

Which countries have the highest fertility?

Sub-Saharan Africa, averaging 4.2 to 4.3 children per woman. The highest rates are in Niger (near 6), Somalia (5.9) and DR Congo (5.9). Those are also the countries where girls are married off youngest.

Does this mean we will be forced to have fewer children?

No. The authors reject coercion outright. They talk about access to school for girls and contraception for women. Where a woman decides for herself, fertility falls without any bans.

Why do scientists propose cutting the population to 4 billion?

Because headcount presses on the environment harder than consumption does. In the 2026 study, population growth outweighed growth in per capita consumption for 6 of 8 indicators, including CO2 emissions, energy use and biodiversity loss.

How does women's education affect fertility?

Through three practical mechanisms. An educated woman loses more income during a year of leave, more often knows about contraception, and more often gets her child to a doctor, so children survive and there is no need for spares.

From 8 to 4 Billion: Scientists on Future Population

In 1974 there were 4 billion people on Earth. Today there are 8.3 billion. A 2026 paper in the peer-reviewed journal Sustainable Development argues we should get back to 4 billion by the year 2200. No bans, no catastrophe. The method it describes already works in dozens of countries: schooling for girls, contraception people can actually get, and letting women decide how many children they have.

The team was led by Mark Keegan. They went through data from 1970 to 2020 and asked which pushes harder on eight environmental measures, from CO2 emissions to biodiversity loss: more people, or more consumption per person. Then the arithmetic. If the global fertility rate (the average number of children per woman) falls to 1.75 by 2050 and stays there, world population drifts down to roughly 4 billion by 2200 on its own. Nobody is forced into anything. Older generations pass on, and smaller ones follow them.

What the 2026 study found

The paper is titled "Achieve Sustainability by Easing Population to 4 Billion by 2200," published in Sustainable Development by Wiley. The authors include Mark Keegan, Anastasia Pseiridis, Philip Cafaro, Jane O'Sullivan, William Rees, and several other researchers from universities around the world.

They tracked eight measures: the Living Planet Index (a running count of how wildlife populations are changing), CO2 emissions, energy use, ecological footprint (the total pressure humans put on nature), freshwater use, pesticides (chemicals used against crop pests), mineral extraction, and plastics production. For six of the eight, population growth mattered more than growth in consumption per person.

Even the UN's restrained projection of 11.4 billion by 2100 would, by the authors' math, cut the Living Planet Index in half, double emissions and energy use, and quadruple plastic waste.

Why 4 billion specifically

The number is not arbitrary. Earth has already carried that many of us: population crossed 4 billion in 1974. That is roughly when humanity entered ecological overshoot, the state where we burn through more resources in a year than the planet can put back.

The authors ran three scenarios:

High fertility (UN projection): 11.4 billion by 2100, 21.2 billion by 2200
Low fertility (UN projection): 9 billion by 2100, 3.2 billion by 2200
Rapid progress (IHME): 6.9 billion by 2100, 4.1 billion by 2200

The last one assumes the UN goals on family planning and girls' schooling are met by 2030. It lands at 4.1 billion by 2200, almost exactly what the authors are proposing.

How to get there without coercion

Coercion, eugenics (the idea of controlling who is allowed to have children), and sterilization programs are rejected outright in the paper. Three levers are left, and none of them are new.

Contraception. Right now 259 million women want to avoid pregnancy and are not using a modern method, because none is within reach. Close to half of women in low- and middle-income countries do not make decisions about their own bodies.

School for girls. A girl who finishes secondary school marries later, has her first child later, and ends up with fewer children. It happened in Iran. It happened in South Korea. It is happening now in Ethiopia.

The right to decide. Where a woman chooses when and how often to give birth, fertility slides to replacement level (2.1 children per woman) and below. Dozens of countries have already done this, with no ban from above.

Why educated women have fewer children

The link between education and fertility is one of the sturdiest findings in demography. Nobody is talking women out of motherhood. Three ordinary, practical mechanisms do the work.

Opportunity cost. The more education a woman has, the more a year at home with a baby costs her. A year without pay, without promotion, without seniority. Where a woman with a degree earns two to three times what she would without one, every leave is visibly more expensive.

Knowing about contraception. Educated women more often know modern family planning methods exist and more often have access to them. In Niger, where 81% of women with no education marry before 18, fertility averages close to 6 children. Among women with secondary education it is half that.

Children stop dying. When child mortality falls, families no longer need spares. An educated mother is more likely to get a child to a doctor, vaccinate, and watch nutrition, and the children survive.

The official UN forecast looks different

The UN's World Population Prospects 2024 is more cautious. It sees a peak of 10.3 billion in the mid-2080s, then a slow decline to 10.2 billion by 2100.

The gap with IHME (the Institute for Health Metrics and Evaluation, a research center at the University of Washington) comes down to how the models are built. IHME feeds women's education levels and access to family planning into its calculations. The UN uses more conservative assumptions about how fast rates will fall in Sub-Saharan Africa.

On the main point they agree: fertility is falling nearly everywhere. Since 1990 the global rate has dropped from 3.3 to 2.3 children per woman. More than half of all countries are below replacement level. Italy and Spain are under 1.4, China is at 1.01, South Korea at 0.75.

Where fertility is highest

Sub-Saharan Africa, averaging 4.2 to 4.3 children per woman. Niger is near 6, Somalia 5.9, DR Congo also 5.9.

Even there the numbers move. Ethiopia had 7 children per woman in the 1980s and has 3.8 now. Iran went from 4 to 1.9 in ten years after launching a family planning program. South Korea covered the same ground in 12.

The harder problem is elsewhere. Nine African countries will double in population by 2054: Angola, CAR, DR Congo, Niger, Somalia and others. Those are also the countries where children are married off most often. Worldwide one girl in five marries before 18, in Africa it is closer to one in three (31%).

Why people argue with the study

Not everyone accepts the thesis. Three objections come up, and all three are fair.

It is not only about headcount. An American puts 20 times more CO2 into the atmosphere than someone in Niger. Fewer people in poor countries leaves overconsumption in rich ones exactly where it was.

Economies are aging. Japan, South Korea and Italy are already patching pension systems and labor shortages. The authors answer with numbers: between 1970 and 2020 the working-age share of the global population fell from 77% to 55%, while GDP per capita rose from $4,960 to $10,569. The economy grew during the same years that share was falling.

A 200-year forecast is guesswork. Technology, climate, wars, epidemics, any one of them rewrites the whole picture. The authors call their own numbers cautious projections.

What a reader can do with this

Keegan and his colleagues did not write an instruction manual for governments, and certainly not for your family. The paper makes a scientific case that a school and a reachable doctor change more than one woman's biography: they change the global figures on emissions, water and plastic.

Not much follows from this for a reader personally. How many children you have is yours to decide, and no study disputes that. Girls' education and access to family planning somewhere in Niger or Somalia, on the other hand, is something you can move with money or attention if you want to.