One of the most common misconceptions about population growth is that a population stops growing once replacement level fertility is reached (a roughly 2-child average). In fact, due to population momentum, a country’s population can continue growing for another 70 years once replacement level fertility is reached.
What is Population Momentum?
Population growth has a lot to do with the age structure of the population. The higher the percentage of young people (especially those under age 15), the more the population will continue to rise as this large cohort (segment of the population) enters their reproductive years (15-49). Once this young group moves beyond childbearing age, however, the momentum will decrease, and population can begin to stabilize so that births and deaths balance (assuming fertility rates remain at or below replacement levels).
Example of Population Momentum
Malaysia is a great example. At just over 30 million people, this Southeast Asian nation reached replacement level fertility around the year 2000 (down from a 6-child average in the 1950s). Yet, with 25 percent of its population under age 15, Malaysia is expected to grow until 2070 (according to UN projections), at which time the country’s under-15 cohort will have dropped to 15 percent of the population.
For world population, the UN projections shows global fertility rate dropping to replacement level sometime in the middle of this century. Due to population momentum, the size of our global population is expected to grow beyond 2100.