Lessons > Population Momentum > For Your Consideration
As you saw in this lesson, two countries with the same birth and death rates by age group will eventually come to have populations with identical structures (population pyramids). However, it takes them a while to get there. This is because the age structure of a population is key to its population growth. Countries like Nigeria, with large numbers of young people, will go through a "growth spurt" as those young people age and have children. Conversely, a country such as Italy, with a large elderly population past childbearing age, will dwindle as those older people die. Eventually these factors even out and a country with a replacement fertility rate such as that of the USA will have a stable population.
But in the short term, a population structure has a momentum of its own. Even with extreme measures such as China's "one-child" laws, a population's growth rate cannot be changed quickly.