Migration and population change

Population projections are being revised more often and by more, and two assumptions are doing the work. Fertility in high-income countries has fallen faster than any official projection assumed: Korea's total fertility rate reached 0.72 in 2023 and rose only slightly in 2024; Japan, England and Wales, and most of the Nordic countries recorded their lowest rates on record in the last three years, the Nordic fall the most surprising because those countries were the model of family-friendly policy; the UN's 2024 revision brought forward the peak of world population and lowered it. The unresolved question is how much is postponement — births delayed, to be recovered later — and how much a fall in completed family size, and the cohorts that would answer it, women born in the 1990s, have not finished childbearing.

Migration is less predictable and now larger. Net migration to the United Kingdom reached a record in 2022 and 2023 and then fell sharply as visa rules changed; several European countries have seen the same shape; the ONS's 2022-based projections raised the long-run migration assumption substantially over the previous round, and offices elsewhere have moved toward variants or ranges because a single central figure keeps being wrong. Migration responds to policy, conflict and labour demand within a year, where fertility and mortality move over decades.

Both matter to the mortality numbers in ways that are easy to miss. Migrants are healthier than the native-born at arrival and their deaths are often registered elsewhere, which flatters mortality rates in high-migration areas; population denominators between censuses are estimated from migration flows that are themselves revised; and a census that undercounts young men in cities changes every rate for a decade. The evidence that the assumptions are being rewritten is clear; what has not been done systematically is the comparison of each vintage with what followed, which would show which assumptions have historically been wrong by most.

Status

building

Direction

strengthening

Would change our view

A sustained fertility rebound in a country beyond Korea's small uptick, or completed-fertility data showing postponed births being recovered; a national office replacing point assumptions with probabilistic projections as its main output; or an ex-post evaluation across vintages showing that fertility rather than migration has dominated the errors.

Trackers

Gaps

Completed fertility for 1990s cohorts, by construction unavailable yet; emigration, which almost no country measures well; a systematic comparison of projection vintages against outturns; the healthy-migrant effect quantified outside a few countries.