When fertility is lower than the replacement level of 2.1 children per woman and there is a longer life expectancy, this changes the age structure over time.
One concern from this is that the age-dependency ratio will be affected, as the working-age population will have more dependents in older age to support.
Projections calculating migration replacement are primarily demographics and theoretical exercises and not forecasts or recommendations.
However, this demographic information can help prompt governments to facilitate replacement migration by making policy changes.
For example, Marois (2008) calculates the gross number of immigrants needed to prevent total population decline in Quebec.
Instead of using replacement migration to combat declining and aging populations, government policy and social changes could be implemented.
[9] Increased migration could decrease the old age dependency ratio, which is expected to grow considerably in the next decades.
[9] However, the immigration need to effectively counter the greying of many industrialised economies is unrealistically high.
[13] A 2016 paper on the impact of migration on the projected population trends of the Scandinavian countries reached similar conclusions.