This chapter presents an overview of important demographic statistics and introduces the demographic transition framework as the most important model of population change in the world today.
1. The Demographic Equation
Demographers consider several important variables when describing population change. These variables are used to compare between different countries and regions consistently. The basic demographic equation is:
It should be fairly intuitive that people moving into a country increase its population, and people moving out decreases the population. So too for births and deaths. Migration can be an important factor in a country's population change, but this chapter focuses on population change due to births and deaths. We call this natural increase. The equation for a country's natural increase is much simpler:
To calculate a country's natural increase for a year, we would simply count the number of births in that country for the year and subtract the number of deaths. But we want to be able to compare growth rates between countries of different sizes. Would comparing the total number of births from a small country like Honduras (population around 8 million) to the population of the United States (population 316 million) tell us anything about the relative rate of change in either country's population? Not really. But making that comparison would be useful and interesting.
The answer is to normalize the data so we can compare statistics between countries of very different populations. To normalize births and deaths, we construct a rate with a standard denominator (the bottom part of the fraction). For births and deaths we use 1,000 people as the standard. To calculate these rates the following formulas are used:
Crude birth rate and crude death rate are commonly abbreviated CBR and CDR. The term "crude" refers to the fact that they are measures of births and deaths without taking into account the age structure of the population--everyone is counted on the denominator for births and deaths: babies, women of child bearing age, grandmothers, men, very old people...everyone. Other, less "crude" measures consider factors like number of children born per woman of child-bearing age (we'll get to that statistic a bit later).
To use our already mentioned example of Honduras, let's say there were 204,115 babies born in Honduras during 2013. The total population in July 2013 (including some of those babies, which is why we use an estimate in the middle of the year and not at the end) was 8,448,465. Now, 8 million 448 thousand 465 is the same as 8,448.465 thousand. Our crude birth rate calculation becomes:
The number 24.16 has been normalized to be as though Honduras is a country of exactly 1,000 people. We could do the same for China and calculate the birth rate there, and the number would be the number of babies born as if China was a country of exactly 1,000 people. Isn't that handy? We can compare birth rates and death rates around the world without having to do mental gymnastics about "oh wait, one country is very small and the other is very large, so..." Nope: just compare because the statistics have been normalized!
The normalized statistics of crude birth rate range from under 10 per 1,000 to close to 50 per 1,000. 10-20 is low and around 40 is high. The crude birth rate is a good indicator of whether a population is growing rapidly or not-with low birth rates, it's hard for a population to grow very fast.
Death rates range from the low single digits to around 20 per 1,000. Death rates are a little more complicated to talk about as "low" or "high" because the death rate depends in part on the age structure of the population and in part on the most common causes of death. An old population with few children has a higher death rate, just as a population facing serious health threats does. A very young population where there are lots of children from everyone having large families can have a relatively low death rate because young people don't die very often once they make it through the first five years of life. But an older, not super elderly population with excellent access to health care can also have a low death rate.
We have one more statistic to discuss in the basic demographic equation. Remember that the birth rate minus the death rate is the rate of natural increase, and we can show that the population is shrinking when the rate of natural increase is negative. Here's the demographic equation written with the measures we've just learned:
The rate of natural increase, or RNI for short, is the increase or decrease in a country's population per year. If that seems too easy, it is: the RNI is usually expressed as a percent, not a per 1,000. But the math is fairly simple to calculate the RNI if you know the CBR and the CDR. Simply subtract the CDR from the CBR and then divide the result by 10. The math is harder to explain than the instructions are to follow, but dividing by 10 converts the per 1,000 number into a percent: (CBR-CDR)/10 = RNI.
Back to Honduras to learn more about this RNI. Honduras had a crude death rate of 5.09/1,000 population in 2013. That gives us the following equation: (24.16 - 5.09)/10 = RNI.
Let's make it simpler and drop the precision to the hundredths of a birth or death and just write:
(24-5)/10 = RNI
that reduces to
19/10 = RNI
or 1.9% = RNI
Honduras has a low crude death rate compared to its crude birth rate, meaning that its natural increase rate is relatively high.
2. Why do we care about natural increase rates?
Demographers started worrying about natural increase when they noticed some populations were growing rapidly. Perhaps the most famous and earliest person to worry about population growth was Thomas Robert Malthus, an English scholar who noticed that the population of England appeared to be growing at an exponential rate. His book An Essay on the Principle of Population is one of the first books focusing mainly on population, first published in 1798.
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| Thomas Robert Malthus in an 1832 portrait by John Linnell. Public domain image from Wikimedia Commons. |
Malthus' most pessimistic views did not come to pass for a number of reasons. First, food supplies continued to increase with population thanks to advances in farming, including mechanization and food preservation. Second, population growth slowed in England because of the progress of a phenomenon known as the demographic transition. The third reason is often overlooked. Consistent with Malthus' predictions, many of the new generations of rapidly growing England did not in fact have very hopeful economic prospects at home. But they resolved this issue by moving as part of a massive migration out of Europe. In the case of the British, many moved to present or former colonies of the British Empire, like the United States, Canada, or New Zealand.
In the 1960s with the growing awareness of the finite resources of Earth and the possibility that humans could seriously harm the environment, a new generation started to worry about population growth. Population growth had not been systematically estimated worldwide before World War 2, and in the post-war decades researchers came up with some startling numbers: Earth's population was growing at a rate so fast that it would double about every 30 years! Malthusian concerns about the limits of Earth's resources given apparently exponential population growth were reborn.
Concerns were expressed in another book, this time titled The Population Bomb, by Paul and Anne Erlich. The Erlichs predicted widespread starvation and social upheaval in the 1970s and 1980s because of rapid population growth. Critics argue that they were wrong in their predictions and were being too catastrophic. But the Erlichs and their supporters point to the abundant evidence of environmental crisis, starvation, and social upheaval through those decades as evidence of their claims.
Here's a graphic that shows world population growth from the time of Christ until 2013:
Earth's population increased from an estimated 300 million at the time of Christ to 1 billion sometime in the early 1800s. It took about 100 years to gain the next billion, but then only about 30 years to get to 3 billion (in 1960). 4 billion was reached in 1975, 5 billion in the late 1980s, and 6 billion around 2000. Earth's population passed 7 billion in 2011. Here's a graphic display of that trend over the last sixty years, with thirty-five more years forecast:
Those numbers might seem intimidating, but there is a reason this chapter doesn't take too panicked a tone in considering these population numbers: while population numbers are increasing, the rate of growth is decreasing and is forecast to continue to decrease. As the growth rate decreases, world population is forecast to stabilize.
3. The Demographic Transition
What caused Earth's overall rate of natural increase to be so high, and why is it decreasing? Furthermore, how can demographers confidently predict a further decline in population growth rates?
The answer is that demographers use a model of population growth and change called the demographic transition model. The model is based on observed population changes from Europe since the 1800s, so it may not really be a perfect way to predict what will happen in non-European countries, but it is widely used and accepted.
Some models contain five stages, but we'll use a model that has four stages for simplicity's sake:
We'll review each of the stages in turn.
Stage 1: Low growth
There are no countries in the world in this stage, but prior to the mid-1700s and the Industrial Revolution, every place in the world was in stage 1 of the demographic transition. In this stage population growth is low because the death rate is very high, nearly as high as the birth rate on average. An outbreak of disease or a series of crop failures could easily make the growth rate negative.
The first countries to move out of stage 1 were in Europe. The spread of the Industrial Revolution was associated with advances in sanitation, medicine, and food supply, so people slowly learned to control and cure disease and lived longer. The move to stage 2 is displayed in the model by the drop in the death rate.
Much of the world's poorer countries, what geographers call the developing world, did not leave stage 1 until around the middle of the 20th century. In many cases the move to stage 2 was caused by the spread of medical knowledge and technology lowering the death rate, not the industrialization that pushed Europe into stage 2 earlier.
Stage 2: Rapid (increasing) growth
In this stage the death rate falls but the birth rate stays high. A crude birth rate of 40 or more is usually considered natural fertility, meaning that essentially nothing is being done to prevent pregnancy. In this stage, women often become pregnant with their first child in their teens and will continue bearing children into their 30s and sometimes 40s. Doing a little basic math it's easy to see how birth rates can be high in these societies--having a child every other year starting from age 15 until 45 years old could result in 15 children to one woman. That's a hypothetical, though, and there is a better measure of the number of children born to a woman. |
| Fourteen Maasai children from different mothers pose for a photo in their boma (settlement) in the Eastern Rift Valley, Tanzania. 2013 photo by Tim Scharks. |
That measure is called the total fertility rate, or TFR. The TFR is a hypothetical measure but based on actual childbirth statistics--it estimates the number of children a statistically average woman would have if the present demographic conditions continued through her lifetime.
Total fertility rate for selected stage 2 countries (2022):
Chad: 6.2 children/woman
Angola: 5.3
Afghanistan: 4.6
Countries with the highest TFRs are among the poorest in the world. This means, unfortunately, that with more children and few resources, there are very few resources per child. One of the greatest tragedies in economically developing countries is the large number of infant deaths, measured as deaths before age 1 per 1,000 live births. This depressing statistic is called the infant mortality rate.
Infant mortality rate for selected stage 2 countries (2022):
Chad: 65/1,000 live births
Angola: 48
Afghanistan: 45
In Chad, more than 1 out of every 20 infants die before their first birthday. But so many children are born, this statistic has relatively little impact on the crude death rate since 19 out of 20 survive and add to the population.
Countries move from stage 2 when social changes decrease the birth rate. For many reasons, women choose to have fewer children and tend to invest more resources in each child.
Demographers have put a lot of time and effort into studying what might cause women to choose smaller families. A number of correlations have been found: lower TFRs and smaller family sizes are associated with economic development and the education of women. When women have established more autonomy and preferences for smaller families, having contraceptive choices available is also important to decreasing TFR and entering stage 3 of the demographic transition.
Stage 3: Slowing growth
As the birth rate falls, the death rate continues to fall so the rate of natural increase may stay high for a while and the population will continue to grow. But the death rate can only go so low, so the falling birth rate "catches up" and the rapid rate of growth seen in stage 2 starts to slow down. In a stage 3 society, population is still growing, but not as fast.In stage 3 the so-far-as-we-know irreversible process of decreasing TFR is underway. We have seen slight rebounds in fertility in some rich countries, but never a major increase in family size.
Smaller families mean more resources dedicated to each child, and countries in stage 3 may enjoy economic growth as a relatively large and relatively well educated labor force matures.
When the birth rate drops to a level where it is very close to the death rate (around a difference of 10 or less), stage 4 begins. In many cases the death rate increases slightly as the population ages because there are not so many children being born.
The transition from stage 1 to 2 is caused by the adoption of technology and knowledge to decrease the death rate. The transition from stage 2 to 3 is caused by a dramatic social change where women choose to have smaller families. But the transition from 3 to 4 is a straightforward progression of trends established in stage 3--there is no change required for this stage to begin.
Stage 4: Low growth
The rich countries of the world are in stage 4, with low rates of natural increase. Some countries in stage 4 are considered to be in "late stage 4", shown on the diagram above where the RNI dips below the line as negative growth. Japan is one such example:Japan (2022)
CBR: 7.0
CDR: 12
RNI: -0.5%
Countries in stage 4 have their own sets of problems related to demographic change. The populations are aging as people live longer; the average number of children per woman is around 2 in these countries, which is just enough to replace the present population. Some countries rely on immigrants from developing countries to provide more labor force and economic growth. But others, like Japan, that are opposed to immigration are having a more difficult time preparing for the future as their population ages and the proportion of young people shrinks.
Last updated January 12, 2023
