Declining Birth Rates and Global Population Change

Two books in the second half of the 20th century, The Population Bomb in 1968 and The Population Explosion in 1990, raised the alarm about overpopulation and rising birth rates. While the global population is now more than 8 billion, recent debate has switched to concerns about declining population and, more specifically, declining birth rates. (See, for example, this article in The Atlantic.)

We noted in a previous blog post that while world population has increased by 5 billion over the past 60 years, the increase has been uneven across poor and rich countries. This blog post first documents the increase in population and decline in birth rates across countries with different standards of living, then computes a counterfactual without the decline in birth rates.

Population Growth and Birth Rates in Poor and Rich Countries

We begin by ranking countries in the increasing order of their real gross domestic product (GDP) per capita in 1960. Then, we combine them into five groups, or quintiles. The number of countries in each quintile in our sample is almost the same. (We excluded China from our sample because its Great Leap Forward, which lasted from 1958 to 1962, and one-child policy, which was in force during 35 years of our sample period, might have affected our calculations on population and birth rates.)

The first figure illustrates the population in the richest countries (top quintile) and poorest countries (bottom quintile) from 1960 to 2023. It is evident from the figure that the population growth in the bottom quintile is remarkably higher than that in the top quintile.

The next figure illustrates the evolution of the crude birth rate (CBR) in the bottom and top quintiles. (Crude birth rate in a year is the number of births in that year divided by the population in that year, often expressed as a percentage, i.e., births per 100 people.)

In general, there is a negative relationship between CBR and GDP per capita: Poor countries have higher birth rates than rich countries. However, the birth rates have been declining in both quintiles for the past 60 years, and they seem to be converging. In 1960, the difference in the birth rates between the bottom and top quintiles was 2.4 percentage points, but this difference had declined to 0.9 percentage points by 2023.

A Counterfactual: Population Change without a Decline in Birth Rates

A natural question is what the population would be if the birth rates had not declined, as shown in the previous figure. To answer this question, we computed a counterfactual by fixing each quintile’s CBR at its 1960 value. More precisely, note that the evolution of population in the data is determined by

Population in year t+1 = Population in year t multiplied by
(1 + CBR in year t – crude death rate in year t + rate of net migration in year t).

With this formula, we can start in 1960 and determine the population in each year until 2023.

In the counterfactual, we hold the CBR in every year at its 1960 value but let the other variables—crude death rate and rate of net migration—evolve as they are in the data. In other words, we determine a counterfactual evolution of population by

Population in year t+1 = Population in year t multiplied by
(1 + CBR in 1960 – crude death rate in year t + rate of net migration in year t).

Note that CBR in 1960 is higher in both quintiles compared with CBR in subsequent years.

The counterfactual population in 2023 in the bottom quintile is 5.5 billion instead of the observed population of 2.6 billion. In other words, if the birth rate had not declined in the poorest-quintile countries, there would have been nearly 3 billion more people than there actually were in these countries in 2023. In the top quintile, the corresponding numbers in 2023 are 1.2 billion and 798 million, respectively; i.e., without the decline in the birth rate, there would have been about 400 million more people in the richest-quintile countries in 2023. Accordingly, the share of world population in 2023 in the bottom quintile would have been 52% instead of the actual share of 48%, and that in the top quintile would have been 11% instead of 15%.

Conclusion

In sum, population growth is higher in the poor countries compared with that in the rich countries. However, both groups of countries have experienced a persistent decline in birth rates for the past 60 years. Absent the decline in birth rates, there would have been a significantly larger population on the planet in 2023, and this additional population would be in poor countries. While higher birth rates would boost population levels in the rich countries, whether public policies can induce such birth rates is open for debate. So far, the results of public policies aimed at encouraging more births have been discouraging. (See, for example, this Harvard Magazine article.)