For most of the last century, the fear was too many people. That fear has now flipped.

In March 2024, The Lancet published a study forecasting that by 2050, 155 of the world’s 204 countries and territories will have birth rates too low to keep their populations from shrinking. By 2100, that rises to 198 of 204, or 97 per cent. The work comes from the Institute for Health Metrics and Evaluation at the University of Washington.

Its message is one the last century would have struggled to believe: for most of the planet, the problem is now too few births, not too many.

The fear that defined a century

The big population worry of the twentieth century ran the other way. Its most famous version came from Stanford biologist Paul Ehrlich, whose 1968 bestseller The Population Bomb opened with a blunt prediction: “The battle to feed all of humanity is over. In the 1970s and 1980s hundreds of millions of people will starve to death in spite of any crash programs embarked upon now.”

It did not happen. The mass famines Ehrlich described never arrived, mostly because he had not counted on the Green Revolution, which sharply raised food production. The world grew from about 3.6 billion people in 1968 to roughly 8.2 billion today, and the collapse he predicted never came.

The fear was not unreasonable when he voiced it. In 1950, women had around five children each on average, and populations were climbing fast. What the doom-mongers missed was the direction of travel. Birth rates had already begun a long slide that would eventually redraw the map.

What the Lancet study actually found

The new numbers point the opposite way. The average number of children per woman fell from about five in 1950 to 2.2 in 2021. That is close to the 2.1 needed to keep a population steady from one generation to the next. As of 2021, 110 of 204 countries and territories were already below that line. The 2050 and 2100 forecasts extend a trend already well underway.

This is one modelling exercise, and its authors are careful to say so. Forecasts this far out depend on data quality and on the assumption that past trends hold, which they may not. Read them as a strong clue about where the world is heading, not a fixed timetable. As senior author Stein Emil Vollset put it: “We are facing staggering social change through the 21st century. The world will be simultaneously tackling a ‘baby boom’ in some countries and a ‘baby bust’ in others.”

The geography of the reversal

The change is not spread evenly. The study describes a “demographically divided world,” in which a small group of low-income countries keeps driving growth while most of the planet shrinks. The share of the world’s babies born in sub-Saharan Africa is set to rise from 29 per cent in 2021 to 54 per cent by 2100. Meanwhile, many countries are projected to see deaths outnumber births by 2050, so their populations would fall without people moving in.

Co-lead author Austin Schumacher framed the other side of the divide as a warning, not a certainty: “A large challenge for countries in sub-Saharan Africa with the highest fertility is to manage risks associated with burgeoning population growth or risk potential humanitarian catastrophe.” The paper treats that risk as depending on access to education, contraception, and investment in health and climate resilience, rather than as something fixed.

Why the old fear outlived the evidence

Part of the reason is that fear of too many people lodges in the mind in a way that decline does not. A famine is a vivid, immediate image. A birth rate drifting slowly below replacement is a line on a chart, easy to ignore until the workforce and the school rolls start to thin. The shift had been building for half a century before most countries crossed below the line, and by then the older story had hardened into common sense.

There is also a delay between what a forecast shows and how policy responds. A birth rate below replacement does not shrink a population straight away. Numbers from earlier generations keep the total rising for years. So a country can be well past the tipping point in its birth rate while its headcount is still growing, which makes the reversal easy to dismiss right up until it isn’t.

What a divided world leaves unresolved

The framing has flipped, but the harder question is what to do with two opposite problems at once. Ageing, shrinking societies face strain on pensions, care systems, and the supply of workers. Fast-growing, younger ones face strain on schools, jobs, food, and water. These are not the same challenge in reverse, and a fix for one region can hurt the other. The clearest example is migration, which could ease a worker shortage in a shrinking country and a youth surplus in a growing one at the same time, but only if both sides treat it as something to manage rather than resist.