How is a population's age structure related
to its stage of economic development? The
population age structure indicates the relative
proportions of older, middle-aged, and younger
people within a population; it is often illustrated
by a population pyramid. Populations can be
classified as predominately young, middle-aged,
or old, depending on the age structure that results
from fertility and mortality rates. A population
ages through a process known as the
demographic transition, in which economic development
causes a fall in both fertility and
mortality rates. Developed countries such as the
United States, Sweden, and Germany have relatively
old populations, whereas developing nations
in Africa and parts of Asia have relatively
young populations. An important aspect of the
population age structure is the total dependency
ratio, the combined ratio of children and old
people to people of working age.
What is the relative life expectancy of
men versus women, and how is it related
to the sex ratio? Life expectancy is the average
number of years people in a given population
can expect to live or, more precisely, the
mean age at death. In most nations, women
live longer than men—six to eight years longer
in developed countries, three to five years longer
in developing countries. The sex ratio is the
number of males to every 100 females. At birth
males outnumber females about 106 to 100,
but because women live longer than men, the
sex ratio declines with age until elderly women
greatly outnumber elderly men.
In the United States, how have fertility,
mortality, and migration rates changed
over the past century, and what has been
the effect on the nation's population? In the
United States after World War II, rising fertility
rates created a baby boom. Fertility began to decline
in the 1960s with the introduction of the
birth control pill and increased levels of education
and employment among women. It reached
a historic low in the early 1970s, creating what
demographers call a baby bust. The twentieth
century also brought dramatic declines in mortality
rates. Medical advances against infant and
childhood diseases began the trend; later, improved
treatments for heart disease lowered
mortality rates among older people. The result
has been population aging and an evening out of
the population pyramid.
Two waves of migration to the United States
occurred during the twentieth century: the first,
around the beginning of the century, from Italy
and central Europe, and the second toward the
end of the century, from Latin America and
Asia. Because most of these immigrants were
young, they changed the nation's age structure,
increasing the proportion of younger people to
older Americans. Overall, the nation's total dependency
ratio has been falling since World War
II. The composition of the dependent portion of
the population is also changing: Once the majority
of dependents were children, but soon, when
immigrants and baby boomers retire, the majority
of dependents will be elderly.
How has life expectancy in the United
States changed over the past century, and
does it differ from one group to the next?
Over the past 100 years, life expectancy in the
United States increased from about 47 years to
over 75. Life expectancy varies significantly
with both race and gender. On average, whites
live longer than African Americans, and
women live longer than men. Because women's
life expectancy has increased faster than men's,
the sex ratio has declined, from 101.1 to 100 in
1910 to only 65 to 100 in 1990.
