Revised 24 May 2019
Population growth is an input-output process just like capital investment and depreciation in our economic system. Population growth in any one year is the result of the crude birth rate being higher than the crude death rate. The growth rate (r), the crude birth rate less the crude death rate (b-d), describes the increase of births over deaths per 1,000 people per year within a population. A growth rate of 10 births over deaths per 1,000 people per year is the same as a 1.0% annual increase in the natural population with no migration. Populations can also grow due to positive net migration – the numbers of immigrants entering the country in any one year exceed the number of emigrants who leave the country.
Pre-agricultural society is an example of low growth rate populations. The average high crude death rate would have been only fractionally less than the average high crude birth. On average, only slightly more than two children per women would have survived to a mature reproduction age.
Crude birth and death rates in a human population can and will fluctuate from year to year. A necessary condition of Zero Population Growth (ZPG) is that the average crude birth rate from generation to generation equals the average crude death rate. If the average crude death rate were to remain stable at 5 deaths per 1,000 population per year, then ZPG would occur when the average crude birth rate is also 5 births per 1,000 population per year. In a population of 1 million people, this would be 5,000 deaths replaced by 5,000 births over one year. A ZPG population can be maintained by low average crude birth and death rates and also by high average birth and death rates which was very nearly the case from 10,000 BC to 1000 AD when the growth rate of 0.04% was very low with a subsequent population doubling time of about 1,750 years.
When a population is growing rapidly, it is more illuminating to focus on changes in the growth rate rather than the population level as shown in Figure 5.
Figure 5: World Population & Growth Rate 1960 to 2017 (Data: United Nations 2019)
The world population grew from 3.0 billion people in 1960 to 7.5 billion people in 2017. From 1960 until 2017 the growth rate had been almost linear rather than exponential. The reason why is provided by examining the change in the growth rate. The growth rate increased rapidly prior to 1960 to a high of 2.11% in 1971 when almost 78 million people in that year were added to the world population. The world population growth rate then declined monotonically to 1.16% in 2017 with the exception of a period between 1975 to 1990 when the growth rate fluctuated up and down.
Figure 6a and Figure 6b show the population growth and growth rates respectively from 1960 to 2017 for countries grouped by income.
Figure 6a: Population Growth by Income
(Data: United Nations 2019)
Figure 6b: Population Growth Rate by Income
(Data: United Nations 2019)
The size of the populations in the middle-income countries in 1960 and subsequent annual growth rates have dominated growth in the world population. Although the size of the populations in all three income groups have increased monotonically from 1960 to 1970, growth rates in the high-income and middle-income countries have slowed down substantially over the same time frame from 1.28% and 2.37% in 1963 to 0.56% and 1.11% in 2017 respectively. In contrast, the growth rate of low-income countries was 2.26% in 1963, increased to a peak of 2.96% in 1993 and then fluctuated down to 2.58% in 2017.
There are substantial differences in the growth rates of countries within income groups and especially in the low-income group of countries. For example, the growth rate of Uganda in 1963 was 3.42% which slowed down to 3.31% in 2017.