Glossary term
Replacement migration
The level of international immigration a country would need to offset population decline or workforce shrinkage caused by below-replacement fertility and population ageing. Coined in the UN Population Division's landmark 2001 study examining whether migration could compensate for demographic decline in eight low-fertility countries.
The UN's 2001 study and its findings
The concept entered mainstream demographic debate through the UN Population Division's 2001 report Replacement Migration: Is It a Solution to Declining and Ageing Populations? The study examined eight countries and regions — France, Germany, Italy, Japan, South Korea, Russia, the UK, the US, and the EU as a whole — and asked how much net migration would be needed to maintain three different demographic targets: a constant total population, a constant working-age population, and a constant old-age support ratio (workers per retiree).
The findings were striking. For most countries, maintaining population size required moderate increases in migration above historical levels. But maintaining working-age population required much larger increases — 3–4× historical levels for Western Europe. And maintaining the support ratio required migration flows so large they were described as "not feasible" — in some scenarios exceeding the total population of sending countries.
How net migration already offsets decline
In practice, many high-income countries already rely heavily on immigration to avoid population decline, even if not at full "replacement" levels. Germany's population would be declining without its net migration inflow of roughly 400,000–700,000 per year. Italy and Spain have seen their fertility fall so low (TFRs near 1.2–1.3) that even current migration levels do not fully offset natural decrease in some regions. Japan, which until recently accepted very little immigration, is projected to lose one-third of its population by 2100 under low-migration scenarios from the UN WPP 2024.
The United States is an unusual case: its TFR (around 1.6–1.7) is below replacement, but annual net migration of roughly 1 million produces positive population growth overall. Canada and Australia have explicitly calibrated their immigration programmes to manage demographic and labour force targets — arguably the closest real-world implementation of replacement migration logic.
Limits and political dimensions
Replacement migration is a useful analytical concept but has significant limits as a policy framework. First, migrants age too — an immigrant population that arrived in working age becomes an elderly population that also requires support, shifting the demand curve rather than eliminating it. Second, the cultural, political, and social dimensions of high migration levels create constraints that pure demographic arithmetic ignores. Third, countries differ dramatically in their capacity to absorb and integrate migrants economically.
The OECD International Migration Outlook consistently notes that immigration increases the total labour supply but has mixed effects on wages, housing costs, and public services that must be managed through complementary policies. Most economists treat immigration as one lever among several — alongside pension reform, later retirement, increased automation, and higher female labour force participation — rather than as a standalone solution.
For the related concept of long-run population stabilisation, see net migration and the demographic transition.
Frequently asked questions
What is replacement migration?
Replacement migration refers to the international immigration a country would need to counteract population decline or workforce shrinkage caused by below-replacement fertility and population ageing. The term was popularised by the UN Population Division's 2001 report examining how much migration would be needed for eight low-fertility countries to maintain their total population, working-age population, and old-age support ratios.
How much migration would Europe need to maintain its working-age population?
The 2001 UN replacement migration study estimated that to maintain Western Europe's working-age population through 2050, net migration would need to be 3–4 times higher than the levels prevailing at the time. For specific countries with very low fertility — like Italy or Spain with TFRs below 1.3 — the numbers were even more extreme.
Does immigration fully solve the ageing problem?
Not on its own. To maintain constant support ratios (workers per retiree) rather than just population size, the required migration levels become very large in the UN's modelling. Most economists argue that immigration is a valuable but partial response, best combined with later retirement ages, higher productivity, pension reform, and increased labour force participation among existing residents. The OECD consistently emphasises this multi-lever approach.
Which countries rely most on immigration for population growth?
Countries where population growth comes entirely from net migration include Germany, Italy, Spain, Portugal, Greece, and Japan. Canada and Australia have explicitly structured their immigration systems to manage population and labour force levels. The United States receives about 1 million net migrants per year, which compensates for its below-replacement fertility.