What population projections can and cannot tell us

Population projections, such as the United Nations World Population Prospects, are among the most widely used forecasts in the world. They shape plans for schools, pensions, housing and health systems.

Why they are relatively reliable: the people who will be adults in twenty years have already been born, so a large part of the future is already fixed. Demographic transition, the shift from high birth and death rates to low ones as societies develop, has been observed in many countries and gives a pattern to build on.

Where uncertainty lies: fertility is the biggest unknown, because it responds to economics, education, culture and policy, and projections of it have been revised many times. Migration is also hard to predict, and shocks such as pandemics or wars can change the picture.

Limits: projections are not predictions. The UN publishes ranges, not just a single line, and the range widens with time.

A careful reader uses the central line for planning, looks at the range for risk, and remembers that the first decades are far better known than the last.