Population and cities

For most of human history, the global population grew very slowly because high birth rates were matched by high death rates. It is estimated that there were around one billion people alive in about 1800.

Then growth accelerated. The world reached two billion around 1927, four billion in 1974 and, according to the United Nations, eight billion in November 2022. The main cause was not a rise in births but a fall in deaths, as better food supplies, clean water, sanitation, vaccines and medicine allowed far more children to survive.

• Stage one: high birth rates and high death rates keep the population stable and small.
• Stage two: death rates fall while birth rates remain high, so population grows rapidly.
• Stage three: birth rates fall as child survival improves, education spreads and economies urbanise.
• Stage four: low birth and death rates bring population growth to a halt.
• Some countries enter a further stage, where births fall below deaths and the population shrinks.

Researchers find that fertility falls most reliably when girls and women receive more education, when child mortality declines so parents no longer need many births to ensure some children survive, when access to contraception improves, and when people move to cities, where children are more costly to raise and less likely to work.

A total fertility rate of about 2.1 children per woman is called replacement level, enough to keep a population stable in the long run. The global rate has fallen from around five in the 1950s to about 2.3 today, and many countries, including most of Europe, East Asia and the Americas, are below replacement level.

When birth rates fall and people live longer, the average age rises. Japan, Italy and South Korea have some of the oldest populations in the world, and South Korea has recorded fertility rates below one child per woman.

Ageing brings challenges: fewer workers supporting more retirees, pressure on pensions and health care, and labour shortages. Countries respond with policies such as raising retirement ages, encouraging immigration, supporting families with childcare and parental leave, and investing in automation.

In 1950 less than a third of humanity lived in towns and cities. Around 2007, for the first time, more than half did, and the United Nations projects that about two thirds of people will be urban by 2050. Most of that growth is happening in Asia and Africa.

People move to cities for jobs, education and services, and cities are generally more productive than rural areas because they concentrate workers, businesses and ideas. Megacities of more than 10 million people include Tokyo, Delhi, Shanghai, São Paulo, Lagos and Mexico City.

When cities grow faster than housing and infrastructure, many newcomers end up in informal settlements without secure tenure, clean water or sanitation. Around a billion people are estimated to live in such conditions. Traffic congestion, air pollution and pressure on water supplies are other common problems.

Well-planned cities can also be efficient and sustainable. Dense neighbourhoods with good public transport use less energy per person than sprawling suburbs, and cities from Singapore to Copenhagen are often cited for their efforts in housing, transport and green space.