Drivers of Population Growth: Natural Increase vs. Net Migration

At the global level, changes in population growth are a result of changes in birth rate and death rate (the so-called natural increase). At the national level, changes in population are due to not only the changes in the natural increase but also the changes in the inflows and outflows of people, or net migration. This blog post documents the changes in population across various national-income groups and decomposes the changes based on the natural increase and net migration.

We begin by ranking countries in the increasing order of their real gross domestic product per capita in 1960. Then, we combine them into three groups: the bottom one-third, the middle one-third, and the top one-third. There are 106 countries in our sample and the number of countries in each group (or tertile) is almost the same: 36 countries in the poorest group, i.e., the bottom tertile; 35 countries in the middle tertile; and 35 countries in the top tertile.

The first figure illustrates the population in the three tertiles in 1960 and in 2023. The bottom tertile grew from 1.53 billion in 1960 to 4.56 billion in 2023, while the top tertile grew from 0.72 billion in 1960 to 1.26 billion in 2023.

The second figure illustrates the change in population in each tertile and the two components of the change: natural increase and net migration.

By construction, for each tertile,

That is,

Note that the height of the blue column must equal the height of the red column plus the height of the green column. As is evident from the figure, the components look different across the three tertiles. Almost all the change in population in the bottom tertile (more than 3 billion) is due to natural increase. In the top tertile, however, both natural increase and net migration contribute substantially to the change in population (540 million). Put differently, absent net migration, the change in population from 1960 to 2023 in the top tertile would have been 392 million instead of 540 million, whereas the change in the bottom tertile would have been 3.16 billion instead of 3.03 billion.

Population in our 106-country sample has increased by more than 4 billion over the past six decades. This increase has been uneven across poor and rich countries. Furthermore, natural increase accounts for most of the change in population in poor countries, while both natural increase and net migration are important in accounting for the change in population in rich countries.