Population momentum

Population momentum is the proportion between the size of a stable (unchanging) population to the total size of the initial population that experiences a drastic shift in fertility to replacement rate (2.1 children per woman). Demographers refer to population momentum as the size of the resulting stationary-equivalent population relative to the current size of the population.

Momentum occurs because older cohorts differ in absolute size from those cohorts currently bearing children. This impacts the immediate birth and death rates in the population that determine the natural rate of growth. For the a population to have an absolute zero amount of natural growth, the US National Library of Medicine National Institutes of Health suggests that three things must have to occur.

1.) Fertility rates would need to level off to the replacement rate (the net reproduction rate should be 1). If the fertility rate remains higher than the replacement rate, then the population would continue to grow.

2.) Next, the mortality rate stops dealing meaning that it remains constant.

3.) Lastly, the age structure has to adjust to the new rates of fertility and mortality. This last step takes the longest to complete.

Population momentum has implications for population policy for a number of reasons.

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

2. With respect to lowest-low fertility countries 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. The way a population momentum can become negative is if the fertility rate is under the replacement level for a long period of time.

3. Population momentum shows that replacement level fertility is a long-term concept rather than an indication of current population growth rates. Depending on the extant age structure, a fertility rate of two children per woman may correspond to short-term growth or decline.

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Wikipedia