Chapter 4 Demographic Composition
Under 15, age 15-64, 65+: By looking at these three sectors, scientists can
determine the economic dependency burden in a society. The ratio of youth plus
old-age dependents to the working population gives a measure of a society's
overall dependency on the workers. If a society has a high youth dependency
ratio, its old age dependency ratio will be low.
Age Pyramids: Pictorial representation of age and sex compositions.
Canada's age pryamid: Bulge associated with baby boom. The people over 65 is
increasing while the 15 and under cohort is declining.
General typology of age pyramids:
TYPE A: Rapidly growing populations have wide base and narrow top. Africa,
Asia, Central America.
TYPE B: Age structure remains constant. Represents a stationary population with
a constant rate of growth. No population currently reflects this type.
TYPE C: Countries that have completed the demographic transition. Western
countries. Age distribution is top-heavy and the base is narrow. Declining
Fertility is the major determinant of age composition. Varying mortality while
holding fertility constant has little effect on age distribution. A young population is
produced by high fertility rates. Declining fertility produces an aging population.
However, prolonging life by reducing death rates makes the population
somewhat younger because reduced mortality affects young people as well, so
more people live to child-bearing years, and more births would occur.
Stable population model: Stable populations can either increase or decrease in
size, as long as the rate of change remains constant - what is stable is the shape
of the age distribution. A stationary population is a type of stable population in
which the rate of growth is zero; the size nor the age structure changes. Allows
us to explore "what if" scenarios. If a researcher assumes constant age-specific
birth and death rates in a real population, they can determine how this will later
affect age distribution in the future.
Crude and Intrinsic growth rates:
RNI is the difference between the crude birth and death rates for a given
population. Rate of natural increase based on crude rates does not take into
account current age compositions, so the result is a crude measure of the
rate of growth. A net rate of natural growth can be computed, known as the
instrinsic rate of growth which measures the rate of growth without the
confouding effects of age composition. Industrialized countries would show
negative intrinsic growth rates because of the low fertility combined with low death rates. The reason these populations are showing positive crude
rates is because their age compositions reflect the effects of high fertility
rates in the past.
How does migration affect age-sex structure?
Migration tends to be negligable compared to the effects of birth and mortality
change. However it can make an impact if the numbers emigrating are large and
concentrated in a specigic age/sex category. In Canada, the age pyramid of the