Visualizing Environmental Science

Demographics of Countries 173

of a stable population (neither growing nor shrinking)

demonstrates that the numbers of people at prereproduc-

tive and reproductive ages are approximately the same.

A larger percentage of the population is older—that is,

postreproductive—than in a rapidly increasing popula-

tion. Many countries in Europe have stable populations.

In a shrinking population, the prereproductive age

group is smaller than either the reproductive or postre-

productive age group. Russia, Ukraine, and Germany are

examples of countries with slowly shrinking populations.

Worldwide, 27 percent of the human population is

under age 15. When these people enter their reproduc-

tive years, they have the potential to cause a large increase

in the growth rate. Even if the birth rate doesn’t increase,

the growth rate will increase simply because there are

more people reproducing.

Most of the world population increase since 1950 has

taken place in developing countries, as a result of the

younger age structure and the higher-than- replacement-

level fertility rates of their populations. In 1950, 67 per-

cent of the world’s population was in developing countries

in Africa, Asia (minus Japan), and Latin America. After

1950, the world’s population more than doubled in size,

but most of that growth occurred in developing coun-

tries. As a reflection of this trend, in 2011 the number

widths of these segments are proportional to the popula-
tion sizes: A broader width implies a larger population. The
overall shape of an age structure diagram indicates whether
the population is increasing, stable, or shrinking.
The age structure diagram of a country with a high
growth rate, based on a high fertility rate—for exam-
ple, Ethiopia or Guatemala—is shaped like a pyramid
(Figure 7.13a). The largest percentage of the population
is in the prereproductive age group (0 to 14 years of age),
so the probability of future population growth is great.
A positive population growth momentum exists because
when all these children mature, they will become the par-
ents of the next generation, and this group of parents
will be larger than the previous group. Even if the fertility
rate of such a country has declined to replacement level
(that is, if couples are having smaller families than their
parents did), the population will continue to grow for
some time. Population growth momentum, which can be
positive or negative, explains how a population’s present
age distribution affects its future growth.
In contrast, the more tapered bases of the age struc-
ture diagrams of countries with slowly growing, stable, or
declining populations indicate that a smaller proportion
of the popu lation will become the parents of the next gen-
eration (Figure 7.13b and c). The age structure diagram

Male Female

a. Rapid growth

Ethiopia

b. Slow growth

United States

c. No growth or

decline in growth

Age Italy

100+

75–79

80–84

85–89

90–94

95–99

70–74

65–69

60–64

55–59

50–54

45–49

40–44

35–39

30–34

25–29

20–24

15–19

10–14

5–9

0–4

Birth Year

Before 1906

1926–1930

1921–1925

1916–1920

1911–1915

1906–1910

1931–1935
1936–1940
1941–1945
1946–1950
1951–1955
1956–1960
1961–1965
1966–1970
1971–1975
1976–1980
1981–1985
1986–1990
1991–1995
1996–2000
2001–2005
10
Percentage of population

8 6 4 2 0 2 4 6 810

Male Female

Baby

boom

years

10

Percentage of population

8 6 4 2 0 2 4 6 8 10

Male Female

10

Percentage of population

8 6 4 2 0 2 4 6 8 10

Based on data from Population Reference Bureau.

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Shown are countries with a rapid (Ethiopia), b slow (United States), and c no growth (Italy) or declining population growth.