Patterns of Population Growth
The human population cannot keep growing exponentially forever, because Earth and its resources are limited. The question is, when and how will our population growth slow? Two centuries ago, English economist Thomas Malthus observed that human populations were growing rapidly. Malthus predicted that such growth would not continue indefinitely. Instead, according to Malthus, war, famine, and disease would limit human population growth.
Today, scientists have identified a variety of other social and economic factors that can affect human populations. The scientific study of human populations is called demography (duh-MAH-gruh-fee). Demography examines the characteristics of human populations and attempts to explain how those populations will change over time. Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly.
The Demographic Transition Over the past century, population growth in the United States, Japan, and much of Europe has slowed dramatically. Demographers have developed a hypothesis to explain this shift. According to this hypothesis, these countries have completed the demographic transition, a dramatic change in birth and death rates.
Throughout most of history, human societies have had high death rates and equally high birthrates. With advances in nutrition, sanitation, and medicine, more children survive to adulthood and more adults live to old age. These changes lower the death rate and begin the demographic transition.
The figure below shows that when the death rate first begins to fall, birthrates remain high. During this phase of the demographic transition, births greatly exceed deaths, and population increases rapidly. This was the situation in the United States from 1790 to 1910. Many parts of South America, Africa, and Asia are still in this phase.
Demographic Transition Birthrates, death rates, and the age structure of a population help predict the rate of population growth. Birthrates and death rates fall during the demographic transition. In Stage I, both the birthrate and death rate are high. During Stage II, the death rate drops while the birthrate remains high. Finally, in Stage III, the birthrate also decreases.
As societies modernize, increase their level of education, and raise their standard of living, families have fewer children. As the birthrate falls, population growth slows. The demographic transition is complete when the birthrate falls to meet the death rate, and population growth stops.
So far, the demographic transition has been completed in only a few countries. Despite the trend in the United States, Europe, and Japan, the worldwide human population is still growing exponentially. Most people live in countries that have not yet completed the demographic transition. Much of the population growth today is contributed by only 10 countries, with India and China in the lead, where birthrates remain high.
Age Structure Population growth depends, in part, on how many people of different ages make up a given population. Demographers can predict future growth using models called age-structure diagrams, or population profiles. Age-structure diagrams show the population of a country broken down by gender and age group. Each bar in the age-structure diagram represents individuals within a 5-year group. Percentages of males are to the left of the center line and females to the right in each group.
Consider the figure at right, which compares the age structure of the U.S. population with that of Rwanda, a country in east-central Africa. In the United States, there are nearly equal numbers of people in each age group. This age structure predicts a slow but steady growth rate for the near future. In Rwanda, on the other hand, there are many more young children than teenagers, and many more teenagers than adults. This age structure predicts a population that will double in about 30 years.
Age Distribution