Breaking Down U.S. Population Growth: Migration and Natural Increase

How has net migration contributed to overall population growth in the U.S. over time? To answer this question, and to place recent developments around immigration flows in longer historical context, the following analysis examines data stretching from 1910 through the height of the COVID-19 pandemic.

A Demographic Accounting of U.S. Population Growth

The framework I use is sometimes referred to as “demographic accounting.” It is the simple notion that a country’s population grows for two reasons only. The first, natural increase, is the difference between the birth rate and the death rate of the country’s population. The second, net migration, is the difference between the rate of migration into the country and the rate of migration out of the country. These flows capture the movement of both the foreign born and native born. Together, the “rate of natural increase” and the “rate of net migration” add up to the rate of population growth:

Rate of natural increase + Rate of net migration = Rate of population growth

It is important to understand that demographic accounting is not a theory that may or may not be true. Instead, it is true by construction and offers a tool to decompose the rate of population growth into its components.

The figure below illustrates the growth rate of the U.S. population from 1910 to 2022 (the red line) and its components: the rate of natural increase (the blue area) and the rate of net migration (the red area).

Population Growth in the U.S. and Its Components, 1910-2022

SOURCES: Centers for Disease Control and Prevention, World Bank, Maddison Project Database 2020 and author’s calculations.

A few elements of the above figure are worth pointing out.

1. The baby boom is quite noticeable in the middle of the 20th century: There is a “bump” in the rate of population growth from approximately 1940 to approximately 1970.
2. The rate of population growth is noticeably lower circa 2020 than it is circa 1920. The population growth rate in 2021 was lower than at any other time in the period examined. The U.S. population is growing, but at a slower pace than it was a century ago.
3. In the first half of the 20th century, net migration contributed little to population growth, even often reducing it (periods when the red line is below the blue line). Since the middle of the 20th century, however, net migration has contributed more to population growth.

Migration as a Factor in U.S. Population Growth over Time

It is interesting to focus on the last observation in the list above. The next figure shows the share of the population growth rate that comes from net migration and the trend line associated with that share. An example can help with interpreting this figure: In 1980, the rate of natural increase was 0.71% while the rate of net migration was 0.25%. So, the rate of population growth that year was 0.71% + 0.25% = 0.96% (see the first figure). The contribution of net migration was then 26%, since 0.25 represents 26% of 0.96. That is, in 1980, just over a quarter of the U.S. population growth rate was due to net migration.

Role of Net Migration in Overall U.S. Population Growth, 1910-2022

SOURCES: Centers for Disease Control and Prevention, World Bank, Maddison Project Database 2020 and author’s calculations.

One remarkable feature of the figure above is the upward trend in the contribution of net migration to the rate of U.S. population growth. As the third observation in the list above pointed out, net migration’s contribution to the population growth rate during much of the first half of the 20th century was near zero or negative. In the second half of the 20th century and subsequent two decades, however, its contribution rose steadily.

The trend line in the figure above permits us to identify three episodes of interest since the 1950s. During the 1970s and again during the 1990s, the contribution of net migration to the rate of U.S. population growth was above trend. These two periods are episodes of high immigration contribution relative to the trend. From about 2000 to about 2020, however, net migration’s contribution to the rate of U.S. population growth was below trend. This period is an episode of low immigration contribution relative to the trend.

Migration and U.S. Population Growth in Recent Years

In conclusion, two points deserve mentioning. First, recent changes in net migration as a contributor to U.S. population growth, viewed against the historical backdrop, appear abnormal. The 2020 surge in net migration as a share of the population growth rate was concomitant with an all-time low in the rate of natural increase. Both events were short-lived, as the figures show.

The table below reproduces the relevant data points to emphasize the magnitude of the effects: The rate of net migration increased almost fivefold from 2019 to 2020, while at the same time, the rate of natural increase fell to a fifth of its 2019 level.

Second, even though net migration has accounted for (on average) almost half of the U.S. population growth rate since about 2000, such a phenomenon is not necessarily new. It is the culmination of a trend that began a century ago.