The United Nations estimated that world population passed eight billion in November 2022, roughly double the figure of 1974. Yet the most important population story of the past two centuries is not growth alone but a change in its rhythm, one that has now reached almost every country.

Where people live

People are spread very unevenly. Most of humanity lives in a few great clusters, in East Asia, South Asia and Europe, with smaller concentrations in West Africa, the Nile valley and the northeastern United States. Simple density figures hide this. Arithmetic density is the number of people per unit of total land area, while physiological density is the number of people per unit of land that can be farmed. Egypt has a moderate arithmetic density, but almost all Egyptians live on the narrow strip of the Nile valley and delta, so its physiological density is among the highest in the world.

Measuring births and deaths

Demographers use a few standard measures. The crude birth rate and crude death rate count births and deaths per 1,000 people each year. The total fertility rate is the average number of children a woman would have over her lifetime if current rates held. Replacement fertility is about 2.1 children per woman in countries with low child mortality: two to replace the parents, plus a little extra because some children die young and slightly more boys than girls are born. Where child deaths are common, the replacement level is higher.

A population pyramid shows the number of males and females in each age group as horizontal bars. A wide base that narrows quickly signals high fertility and a young population. A column or an upside down shape signals low fertility and an aging one.

Malthus and his critics

In 1798 the English clergyman Thomas Robert Malthus published An Essay on the Principle of Population. He wrote: "Population, when unchecked, increases in a geometrical ratio. Subsistence increases only in an arithmetical ratio." Malthus argued that population tends to grow faster than the supply of food, so that misery, famine and disease would keep numbers in check. His defenders note that hunger and resource limits are real and that very rapid growth can strain poor societies. His critics answer that he missed two things: farming became far more productive, and families chose to have fewer children as they grew richer and safer.

The demographic transition

The second of those changes is the subject of the demographic transition model, sketched by the American demographer Warren Thompson in 1929 and developed by Frank Notestein in 1945. The model describes a shift from high birth and death rates to low birth and death rates. In stage one, both rates are high and population barely grows. In stage two, death rates fall first while birth rates stay high, thanks to better food, clean water, sanitation and vaccines, so population surges. In stage three, birth rates fall too, as more children survive, cities grow, schooling spreads, women gain education and paid work, and contraception becomes available. In stage four, both rates are low and growth slows to near zero. Some writers add a stage five, in which fertility falls well below replacement and population begins to shrink.

Even after birth rates fall, a population can keep growing for decades. Population momentum is growth that continues because a large share of people are of childbearing age, the legacy of earlier high fertility.

The model has critics. It was drawn from the European experience, and it describes rather than explains: it does not say exactly why fertility falls, or when. Many countries passed through it far faster than Europe did, and a few have stalled midway. Governments have also tried to force the pace, sometimes brutally. China's one-child policy, enforced from 1980 to 2015, involved forced abortions and sterilizations, a reminder that population policy reaches into the most private human choices. The lesson most widely accepted since the 1994 United Nations population conference in Cairo is that fertility falls most reliably when people, and women especially, gain education, health care and real choices.

Worked example: two pyramids

Compare Niger and South Korea. Niger has one of the highest fertility rates in the world, around six children per woman in recent estimates, and its pyramid has a very broad base: about half its people are under fifteen. It sits in stage two, moving into stage three. If fertility falls while that large young generation enters working age, Niger could gain a demographic dividend, a period when the share of workers is unusually high compared with the share of dependents.

South Korea had about six children per woman in 1960. By 2023 its rate had fallen to about 0.72, among the lowest ever recorded for any country. Its pyramid now narrows at the bottom. It is past stage four, and its problem is the reverse of Niger's: a shrinking workforce supporting a growing number of older people. The dependency ratio, which compares children and older people with those of working age, rises at both ends of the transition for opposite reasons.