What Has Been Driving World Population Growth?

In the late 1960s and early 1970s, there was major concern about the growing population. The concern came from a book, The Population Bomb by Paul Ehrlich, that argued that the world’s population was growing too fast due to rising birth rates and that the earth would not be able to support the population. However, over the last six decades, the world’s population has more than doubled, despite the world’s birth rate falling from about 3.2% in 1960 to about 1.7% in 2022.

Population Growth Varies across Countries

The increase in population has not been the same across countries from 1960 to 2022. Lower-income countries, those which had a gross national income (GNI) per capita of less than $13,846 in 2022, have been experiencing higher population growth, especially compared with the high-income countries, those which had a GNI per capita of at least $13,846 in 2022. We take these groups from the World Bank. The World Bank classifies countries with anything less than $13,846 GNI per capita as middle- and low-income countries, but for simplicity, we are calling them lower-income countries.

The population gap between high-income and lower-income countries has been increasing steadily since 1960. In that year, high-income countries had about 26% of the world’s population, but by 2022, their population share had declined to 16%. Lower-income countries had the opposite trend—their share of the world’s population was 74% in 1960, then increased to 84% in 2022. This means that out of the world population of about 8 billion in 2022, lower-income countries had a population of about 6.7 billion. 

As shown in the equation below, population growth depends on the crude birth rate (CBR) and crude death rate (CDR).We measure CBR and CDR as the number of births per person and the number of deaths per person, respectively, represented as a percentage of the population.

Population Growth = CBR − CDR

There are two ways for population to grow—an increase in the CBR and/or a decrease in the CDR. In other words, if the CDR stays constant, there has to be an increase in the CBR for there to be a rise in population, and if the CBR stays constant, the CDR has to decrease.

Since the lower-income countries had an increase in population share, they had higher population growth than the high-income countries.Population growth could be affected by immigration. Since most people will be emigrating out of lower-income countries to high-income countries, immigration cannot be an important factor for the population growth in lower-income countries. In fact, over the past 60 years, the population growth rate each year of high-income countries has stayed relatively low, with the population growth rate in lower-income countries about double that of high-income countries. This means that there are differences across the CBR and CDR for both groups of countries that cause lower-income countries to have higher population growth.

Birth Rates of Both Country Groups Have Been Declining

First, consider how the CBR affects both country groups. In the first figure, the CBR is plotted from 1960 to 2021. The CBR has been declining steadily for both country groups since the 1960s.

Crude Birth Rates

SOURCES: World Bank’s World Development Indicators.

NOTES: Countries are considered high-income if they had a GNI per capita of $13,846 or more in 2022; they are considered lower income if their GNI per capita was less than $13,846 in 2022. We take these groups from The World Bank; the World Bank classifies countries with anything less than $13,846 GNI per capita as middle- and low-income countries, but for simplicity, we are calling them lower-income countries.

It appears that the CBR of lower-income countries is converging to the CBR of high-income countries. Considering that the CBR in both country groups is becoming similar—both are decreasing over time—the CBR cannot be the main reason why population has been increasing for lower-income countries relative to high-income countries.

Crude Death Rates Have Different Patterns

The other component that influences population growth is the CDR. Shown in the figure below, the CDR of high-income countries has remained relatively flat from 1960 to 2021, at approximately 1%. The relative lack of change in the CDR combined with the decrease in the CBR for high-income countries is why there is no huge increase in the population of high-income countries.

Crude Death Rates

SOURCES: World Bank’s World Development Indicators.

NOTES: Countries are considered high-income if they had a GNI per capita of $13,846 or more in 2022; they are considered lower income if their GNI per capita was less than $13,846 in 2022. We take these groups from The World Bank; the World Bank classifies countries with anything less than $13,846 GNI per capita as middle- and low-income countries, but for simplicity, we are calling them lower-income countries.

With the lower-income countries, the CDR has a different pattern—it originally started higher than that of the high-income countries. Over the course of six decades, it fell below the CDR of high-income countries. The CDR of this group of countries fell from about 2% to 0.9%. Comparing the CDR and CBR of the lower-income countries, we can see that the CDR fell more than the CBR in relative terms.

The decline in mortality is an important reason why the population of lower-income countries has been increasing relative to high-income countries. To stress the importance of mortality decline in population growth of lower-income countries, consider a counterfactual exercise: Since population growth is the CBR minus the CDR, if we kept the CDR constant at the 1960 level, 2%, and allowed the CBR to change as it did in the data, we can see what the population of lower-income countries might have been if the CDR did not decline. Through this exercise, we see that the population of lower-income countries would be about 3.8 billion, which is 2.9 billion lower than it is in 2022.

Population growth does mean there are more people on the earth. However, the main driver of population growth has been the decline in mortality, which means that people are living longer.

Notes

1. We take these groups from the World Bank. The World Bank classifies countries with anything less than $13,846 GNI per capita as middle- and low-income countries, but for simplicity, we are calling them lower-income countries.
2. We measure CBR and CDR as the number of births per person and the number of deaths per person, respectively, represented as a percentage of the population.
3. Population growth could be affected by immigration. Since most people will be emigrating out of lower-income countries to high-income countries, immigration cannot be an important factor for the population growth in lower-income countries.