Population momentum

Assume that a population has three generations: First (oldest), Second (child bearing), and Third (children).

Now take the population forward in time to the next generation, line two of the spreadsheet.

The initial population has grown from 700 to 1,200 even though fertility dropped from four to replacement (two) at the end of the third generation.

When China first introduced the one-child policy, population growth continued regardless.

As a result, the Chinese population maintained the same momentum of increase as for the past 20 years.

However, for a population to have an absolute zero amount of natural growth, three things must occur.

With respect to high-fertility countries (for example in the developing world), a positive population momentum, meaning that the population is increasing, states that these countries will continue to grow despite large and rapid declines in fertility.

With respect to lowest-low fertility countries (for example in Europe), a negative population momentum implies that these countries may experience population decline even if they try to increase their rate of fertility to the replacement rate of 2.1.

For example, some Eastern European countries show a population shrinkage even if their birth rates recovered to replacement level.

Population momentum can become negative if the fertility rate is under replacement level for a long period of time.

Depending on the extant age structure, a fertility rate of two children per woman may correspond to short-term growth or decline.

[5] Population momentum is typically caused by a shift in the country's demographic transition.

[1] When mortality rates drop, the young survive childhood and the aging population live longer.

Fertility rates remain high, causing the overall population size to grow.