# Why Is A Percentage Scale Used For The X-Axis Rather Than Actual Ages?

Close

What we doFocus AreasExpertiseData Center2021 World Population Data SheetData IndicatorsWork With UsWork With UsAboutAbout

### Has the world’s population distribution changed much over time? When could world population stop growing?

Find out the answers to these questions and more.The sections listed below explore eight elements of population dynamics. Charts and graphs supplement each topic with one full-sized chart (in PDF) suitable for class distribution. Along with each topic are a frequently asked question and glossary terms. Teacher’s guides with discussion questions and web resources are also included in each section. For further investigation see also the most recent World Population Data Sheet.

Grade level: middle to high schoolTime required: one weekSubjects: social studies, geography, and world history

## Change

Three Patterns of Population Change

Source: United Nations, World Population Prospects, The 2006 Revision.

Teachers Guide: Discussion QuestionsWhat percentage of the population of the Democratic Republic of Congo, the United States, and Germany are 0–4 years old?Which of the three countries has the greatest proportion of people ages 65 and older?

How can the age-sex structure of a population help determine the needs of that population?What does it mean to have a “young” or “old” population?How can migration affect the shape of a pyramid?What is “zero population growth”? Which pyramid represents this concept?

Data

The dependency ratio is a measure used to indicate the ratio of people in the “dependent” ages (under 15 and ages 65 and older) per 100 people in the “economically productive” ages (15–64 years of age). The formula for the dependency ratio is:

The age dependency ratio for the United States is shown below at 49.

This means that there were 49 people in the dependent ages for every 100 persons in the working ages.

Calculate the dependency ratios for Kenya, Germany, Brazil, and Japan. Compare the components of each of them.

DiscussionDiscuss the implications of high or low dependency ratios for economic resources and development.

Question and Answer: Why Does It Take So Long to Slow or Stop Population Growth?

Growth through natural increase occurs when the birth rate exceeds the death rate. For example, the U.S. birth rate in 2005 was 14 births per 1,000 people and the death rate was 8, yielding a net increase of six persons for every 1,000 persons in the United States, or approximately 1.7 million additional persons for that year. This rate of natural increase occurred in spite of a very small average family size measured by the total fertility rate—an estimate of the number of births to women during their lifetimes.

The rate of natural increase of a population depends on birth and death rates, which are strongly influenced by the population age structure. Births occur primarily to people in the younger-adult age groups. If there are comparatively more young adults than older adults where mortality is highest, then even at replacement fertility levels (when each woman has about an average of two children) there will be more births than deaths.

Hence, a relatively large number of couples each having one or two children can still produce a large excess of births. This phenomenon is known as population momentum.

In the United States, birth rates are higher than death rates at present, partly due to the relatively young age structure of the U.S. population. Immigrants, who are younger on average than the U.S.-born population, play a significant role in keeping the United States younger than most other developed countries. For example, among U.S. Hispanics, 40 percent of whom are foreign-born, there are approximately 10 births for every death.

The momentum of population growth in less developed countries will only be slowed when the large number of young adults resulting from previous high fertility have passed out of the childbearing years and a succeeding smaller generation reproduces at replacement level fertility. This momentum is very pronounced in China, where women have about two children, but the number of women having children is now much larger than in the previous generation. Thus, even though it has reached replacement level fertility, China’s population continues to grow.

Published