Population Momentum Explained

Population momentum refers to the tendency of a population to continue growing after the fertility rate has fallen to replacement level.

This occurs because of the population's age structure. If a population previously had high fertility, it will have a large cohort of young people. As this large cohort enters reproductive age, the total number of births can remain high, leading to growth even if each woman is having fewer children (i.e., at or below replacement level).

Replacement Level Fertility

Replacement level fertility is the average number of children each woman needs to have to replace the population. In most developed countries, this is about 2.1 children per woman. Reaching this level means that, eventually, the population size should stabilize, assuming no significant migration.

Understanding Continued Growth

Population momentum explains why stabilization doesn't happen immediately:

• A population with many young people (due to past high fertility) will continue to grow as these young people reproduce.
• This growth phase continues even if the birth rate per woman drops to replacement level.
• Once the large young cohort passes through its reproductive years, and the number of births begins to fall relative to the number of deaths, the population will eventually stabilize or even decline.

Therefore, the population continues to grow after replacement level fertility is achieved due to the existing age distribution.