Population momentum: if the number of children per woman is falling, why is the population still increasing?

Note

This article was published in 2019 and has not been updated with the latest demographic data.

In 1965, the total fertility rate was five, globally. 50 years later this metric has fallen below 2.5. It has halved. The first panel in the chart shows this fundamental change.

The total fertility rate at which a population replaces itself from one generation to the next is called the “replacement fertility rate”. If no children died before they grew up to have children themselves, the replacement fertility rate would be 2. Because some children die, the global replacement fertility rate is currently higher than 2, only slightly lower than the actual global fertility rate. Why, then, has global population growth not come to an end yet?

The number of births per woman in the reproductive age bracket is only one of two variables that matter here. The second one is the number of women in the reproductive age bracket.

If few women were in the reproductive age bracket, the number of births would be low even when the fertility rate is high. However, when an increasing share of women enters the reproductive age bracket, the population can keep growing even if the fertility rate is falling. Demographers refer to this as “population momentum”; it explains why the global number of children will not decline as rapidly as the fertility rate. Hans Rosling explained it better than anyone with the help of toilet rolls.

The second chart in this panel shows that population growth over the last decades has resulted in increasingly large cohorts of women in the reproductive age bracket.

As a result, the number of births will stay high even as the number of births per woman is falling, as the bottom panel in the chart shows. According to the UN projections, the two drivers will cancel each other out, so the number of births will stay close to the current level for many decades to come.

This means that the coming decades will be very different from the last when the annual number of births increased rapidly, as the visualization shows.

Population momentum is one important driver for high population growth. But it also matters that we live much longer today than our ancestors just a few generations ago. Life expectancy is now twice as long in all world regions.

In all of this, it is also important to remember that these are projections, and how the future will actually play out will depend on what we do today.

Population momentum is driven by the increasingly large cohorts of women in the reproductive age bracket. It’s only when both the fertility rate and the number of women level off that population momentum stops. And this is when global population growth will come to an end.

Where this page came from

This page was imported from Our World in Data. “Population momentum: if the number of children per woman is falling, why is the population still increasing?” by Max Roser (June 2, 2019), published by Our World in Data under CC BY 4.0. Changed here: set as a page, its interactive charts shown as pictures. Data from third parties keeps its own licence.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.