A simple method for estimating inter-generational replacement based on fertility and migration – European examples

Concern over unprecedentedly low levels of childbearing in Europe has become increasingly marked among both scientists and policy-makers. In conjunction with these concerns over fertility, there has also been considerable debate on the role migration can play in compensating for fertility below the replacement level.
The issue of how to measure inter-generational replacement has been addressed by several scholars in recent years. In this paper we make use of a very simple method to assess how far migration alters the extent of replacement for a birth cohort as it ages.
We term the measure used here the overall replacement ratio (ORR). It is calculated by taking the size of a female birth cohort divided by the average size of the cohorts of mothers in the year of birth. For example, we can compare the size of the 1975 cohort over time to the number of women in the main childbearing ages in 1975. Using annual estimates of the size of the 1975 cohort enables us to track the impact of migration on its implied level of replacement. Where immigration is significant, the ratio climbs over time, often reaching the replacement level by the age of 30 in many countries where fertility is well below the replacement level. The paper presents estimates of the ORR for a range of European countries representing different replacement regimes. In contrast with a frequently expressed notion of a limited impact of migration on population trends in Europe, we demonstrate that for many countries net migration has become a key factor in their population trends during the last decades.