Population momentum

Population momentum, also called demographic inertia, is the idea that the number of babies born can keep rising even after the average number of children each woman has goes down. This happens because when more girls are born, they will become women who can have babies about twenty to forty years later. So, the size of the population keeps growing for a long time, even if each woman is having fewer children.

A good example is the Echo Boom. When the baby boomers—people born after World War II—grew up and had their own children, there was a big rise in the number of babies born. Another example is China, where the population kept growing even during the one-child policy from 1979 to 2021, because there were already many young women who could have children.

Population momentum matters because it shows that a population will keep growing even if the number of children each woman has drops. This growth continues until the number of children per woman reaches the replacement rate, which is about two children per woman, and the number of women who can have babies stays the same. At that point, population momentum ends, and the population size stays balanced.

Example

Imagine a family with three groups of people: the grandparents, the parents, and the children. Suppose each group of parents has four children on average. This means each group is twice the size of the group before them. If we start with 100 grandparents, there would be 200 parents and 400 children, making a total of 700 people.

Now, let’s say the number of children each family has drops to two. The grandparents pass away, and the parents become the new grandparents. The number of children stays the same as the parents. After some time, the numbers balance out and stop changing.

Even though families started having fewer children, the population still grew because there were many young people ready to start their own families. This is called population momentum.

When China introduced rules limiting families to one child, the population kept growing at first. This happened because many young people were already growing up. It wasn’t until much later, in 2023, that China’s population began to shrink for the first time.

Further steps to zero population growth

Population momentum affects the number of births and deaths, which decide if a population grows or shrinks. To stop a population from growing completely, three important things need to happen.

First, the number of children families have should stay at the replacement rate, where each generation replaces itself. If families have more children than this, the population will keep growing.

Second, the mortality rate — how often people pass away — should stop going down and stay the same.

Finally, the age groups in the population need to balance out with these new rates. This part takes the longest to happen.

Implications

Population momentum means that changes in how many babies families have can take a long time to show up in the total number of people. For example, countries where families have many children might keep growing even if they start having fewer babies. On the other hand, places where families already have very few children might keep losing people even if they try to have a little more.

This shows that having just enough babies to replace each parent (about two babies per family) doesn’t always mean the population will stay the same right away. It depends on how many young people there are now. Sometimes, even with this level of babies, the number of people might still go up or down for a while.

Calculation

To figure out population momentum for a group of people, we imagine what would happen if the number of babies born each year suddenly dropped to a level where the population stays the same size. Over time, the population would stop growing.

Population momentum is found by comparing the size of the population after it has stopped growing to the size before any changes were made. This helps us understand how past birth rates continue to affect the current population size, even after those rates have changed.

Causes

Population momentum happens because of changes in how countries grow over time. When fewer people die young and people live longer, more young people grow up. If families still have many children, the population keeps getting bigger. Even if families start having fewer children right away, the population will still grow for a while because there are already many young people ready to have children.