The Theory of Demographic Transition: Overview

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  • 0:03 Demography
  • 0:35 Demographic Terms
  • 3:52 The Theory of…
  • 10:04 Lesson Summary
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Lesson Transcript
Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

Populations change over time. The growth or decline of a population can have an effect on the quality of life for people within that population. In this lesson, you'll learn about the theory of demographic transition, which is a model used to study and predict population changes.


Demography is the statistical study of human populations. Demographers look at information in order to determine specific characteristics of a population.

In this lesson, we will use the fictitious population of Dharma Island that has recently formed in the ocean to look at the theory of demographic transition, which is basically a population change model that demographers can use to review how a population has changed in the past as well as how to make predictions about the future of the population.

Demographic Terms

Before we can look at the theory of demographic transition using our population on Dharma, we first need to go over a few terms that will be used. Fertility rate refers to the number of births in a population. This is normally given as a number out of another number. For example, the fertility rate on Dharma was around 31 births per 1,000 women age 15 - 44. You may also see it as the number of children per woman such as 1.6 children per woman. This is an average of children who were born to women generally of reproductive age (around 15 - 44 years old).

Opposite of fertility rate is mortality rate, which is the number of deaths in a population. Again, this is normally given as a number out of another number. On Dharma, the mortality rate was 8 out of 1,000 people.

Other terms that are used when talking about demography include age composition and life expectancy. Life expectancy refers to the age at which someone is predicted to die. Again, let's look at information about Dharma. The life expectancy of women was 81 years, and the life expectancy of men was 76 years.

The age composition of a population can best be described as the age structure of the population. This refers to the number of people in certain age groups. A population may be described as having a large number of young people or maybe a large number of older individuals. Demographers often use population pyramids in order to illustrate information about age composition and life expectancy. Let's take a look at a few pyramids now to understand the basics.

This population pyramid shows a population whose life expectancy is not very high
Chart Showing Age Composition

In this diagram, we have the number of males on the left in blue and the number of females on the right in pink. The overall shape of the population looks like an upside down ice cream cone. We can see from this that the age composition of the population is much larger at younger ages than older ages. The number of females and males is about the same. The life expectancy is not very high as there are few older people in the population.

This next diagram again has the males on the left in blue and the females on the right in pink. This looks more like a triangle, but again, we can see that the age composition of the population is heavily based in the younger population. The main difference between this pyramid and the first is that the life expectancy is longer. We can see this by viewing the top of the pyramid. In this case, there are more people alive in the older ages than on the first diagram.

This next population pyramid has a more even shape, indicating that the age composition is pretty well balanced between the younger and older groups within the population. The life expectancy is greater for this population than the previous population, which we can see by the large number of individuals in the older age groups. We can also tell that the fertility rate isn't as high because the number of young people is more even with the rest of the population.

In this last population pyramid, we can identify one main difference from the last graph. In this population, the birth rate is declining. It may be difficult to see, but if we look closely at the bottom of the diagram, we can see that there is a small dip indicating that the number of young people is lower than the rest of the population.

The Theory of Demographic Transition

Now that we have the basic terms down, let's look at the theory of demographic transition using our population on Dharma. Demographers have identified four main stages of transition, but some have also identified a fifth stage. Let's start with stage 1.

Stage 1

In stage 1 of demographic transition, populations have high fertility rates and high mortality rates
Stage One of Demographic Transition

The island of Dharma has recently become inhabited. The population is just starting out and does not yet have the best sanitation and water quality. Some of the water is contaminated, allowing diseases to spread. There isn't much health care available on this remote island, which does not help the stop of diseases that are being caused by the lack of sanitation. There aren't yet supermarkets from which the people can buy food and supplies, so they need to work the land. Larger families are better for this because the children can go to work and help support the family. Dharma is currently in stage 1, or the high stationary stage of demographic transition. Stage 1 has high fertility and high mortality rates.

Because both rates are high, the overall size of the population doesn't change much. As with the population on Dharma, mortality rates are high due to poor sanitation, poor water quality, high rates of disease and high rates of famine. The fertility rates are high due to the lack of family planning and the cultural need for children. Populations in stage 1 tend to be rural and agricultural, so families need kids in order to work the land and to survive. Relating this information back to the pyramids, in the first pyramid the number of births was very high and based on the small number of older individuals, we can assume that the death rate was also very high. This pyramid is an example of a population in stage 1 of demographic transition.

Stage 2

Because of the high death rates on Dharma, the population decides that it is necessary to improve the water quality, sanitation and health care system. While it is more than a small feat to do this, the people of Dharma know that it is needed. As these improvements are made, the population moves into stage 2, or the early expanding stage of demographic transition. Populations in this stage have a rapid drop in mortality rates and high fertility rates.

During this stage, the overall population starts to increase. As we see with the people of Dharma, the decrease in deaths in stage 2 is due to improved sanitation, water quality, health care and food production. The number of births is still high because populations at this stage still require large families for survival and there is still limited family planning. This stage was seen in the second population pyramid, which had a high fertility rate but a lower death rate than the first pyramid.

Stage 3

Stage 3 brings a decrease in fertility rates and a stabilization of mortality rates
Stage Three of Demographic Transition

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