Population momentum

Population momentum is an effect which causes population growth. This phenomenon refers to the percentage of the population that are in their child bearing years who have not yet had children, and thus are scheduled to eventually have children which add to the population through reproduction. The higher the percentage of people aged, for example 18 and under, the larger the population growth will be because there is such a large percentage of the population capable of having children. This means the population will continue to grow, even if the fertility rate reaches replacement level. The reason is that population momentum would have an effect is that high fertility levels in the past caused a largely young population which still has to reach child-bearing years.

For example, consider country A, which has 50% of its population under 18, and country B, where only has 10% of its population is under 18. If both countries have a present population of 1 million, and a fertility rate of 2 children per woman, the effect of population momentum can be illustrated.

Country A: 500,000 people in child bearing years = 250,000 couples, who each have 2 children - 500,000 new babies.

Country B: 100,000 people in child bearing years = 50,000 couples who each have 2 children - 100,000 new babies.

It can be seen that whilst both countries have the same starting population in terms of numbers and the same fertility rate, because such a high percentage of country A's people are in their child bearing years, their population growth is 5 times that of Country B.

Countries with a Population momentum
The following list are countries that maintain growth despite a fertility rate of under 2.1:


 * Spain (immigration change the trend, despite a fertility rate 1.3, and even 0.7 in some provinces)
 * Canada
 * Brazil
 * Turkey
 * Algeria
 * Tunisia
 * China
 * South Korea
 * Taiwan
 * Vietnam
 * Thailand
 * Iran