THE DEMOGRAPHIC TRANSITION
Keith Montgomery
Department
of Geography and Geology
The "Demographic Transition" is a model that describes population change over time. It is based on an interpretation begun in 1929 by the American demographer Warren Thompson, of the observed changes, or transitions, in birth and death rates in industrialized societies over the past two hundred years or so.
By "model" we mean that it is an idealized, composite picture of population change in these countries. The model is a generalization that applies to these countries as a group and may not accurately describe all individual cases. Whether or not it applies to less developed societies today remains to be seen.
Before proceeding you should review some demographic terminology or be sure to follow the links given below as the terms arise.
The model is illustrated below:
As shown, there are four stages of transition. They will be described first in terms of a typical fully developed country today, such as The United States or Canada, the countries of Europe, or similar societies elsewhere (e.g. Japan, Australia etc.).
STAGE ONE is associated with pre Modern times, and is characterized by a balance between birth rates and death rates. This situation was true of all human populations up until the late 18th.C. when the balance was broken in western Europe.
Note that, in this stage, birth and death rates are both very high (30-50
per thousand). Their approximate balance results in only very slow
population
growth. Over much of pre-history, at least since the "Agricultural
Revolution" 10,000 years ago, population growth was extremely slow. Growth
rates would have been less than 0.05%, resulting in long doubling
times of the order of 1-5,000 yrs.
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Given its characteristics, Stage One is sometimes referred to as the "High Stationary Stage" of population growth ("high" birth and death rates; "stationary" rates and "stationary" total population numbers).
Death rates were very high at all times in this stage for a number of reasons, including:
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| Survivorship curve: Survivorship curves keep track of the fate of any given birth cohort. They show the percent still living at a given age. Nowadays in the developed world few children die before reproduction. In Great Britain in 1999 only 1% of all children born alive died by the age of five (compared to 10% in India, and 35% in Niger). However, 300 years ago it was quite a different matter, as the graph above illustrates. In the City of York (England) in the 17th. Century, only 15% made it to the threshold of reproduction (15 yrs.). Only 10% remained alive by the age of twenty. With so few females living to reproduction, only a high fertility rate could maintain the population. Note that changes with economic development, as shown by Niger and India. Note also the impact of bias against females in India on their survival -- otherwise, India's curve in 1999 is very similar to Great Britain's for the late 19th. C. (not shown). |
The high rate of birth (even higher if one were to adjust it for women of childbearing age) could be due any or all of the factors that are associated with high fertility even today in many less developed countries. With a high death rate among children, there would be little incentive in rural societies to control fertility except in the most unbearable of circumstances.
Stage One, then, characterizes all world regions up until the 17th.C. Some demographers sum up its character as a "Malthusian stalemate".
STAGE TWO sees a rise in population caused by a decline in the death rate while the birth rate remains high, or perhaps even rises slightly. The decline in the death rate in Europe began in the late 18th.C. in northwestern Europe and spread over the next 100 years to the south end east. Data from Sweden clearly show this stage (and two other stages following it):
The decline in the death rate is due initially to two factors:
From the relationship between scurvy and measles in England and Wales (scurvy is caused by a dietary deficiency in vitamin C), one could surmise that general improvements in human well-being, an increase in public health awareness, and a decline in poverty was most at work in the decline of infectious diseases.
A consequence of the decline in mortality in Stage Two is an increasingly
rapid rise in population growth (a "population explosion") as the gap between
deaths and births grows wider. Note that this growth is not due to an increase
in fertility (or birth rates) but to a decline in deaths. This change in
population growth in north western Europe begins the population rise that
has characterized the last two centuries, climaxing in the second half
of the 20th.C. as less developed countries entered Stage Two (next two
plots):
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Another characteristic of Stage Two of the demographic transition is
a change in the age structure of the population. In Stage One the majority
of death is concentrated in the first 5-10
years of life. Therefore, more than anything else, the decline in death
rates in Stage Two entails the increasing survival of children. Hence,
the
age structure of the population becomes
increasingly youthful. This trend is intensified as this increasing number
of children enter into reproduction while maintaining the high fertility
rate of their parents. The age structure of such a population is illustrated
below by using an example from the Third World today:
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Birth Rate: 36 per thousand
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In general the decline in birth rates in developed countries began towards the end of the 19th.C. in northern Europe and followed the decline in death rates by several decades (see example of Sweden, in Stage Two above).
There are several factors contributing to this eventual decline, although some of them remain speculative:
In the following figure, note that once infant mortality had fallen to around 70 (which occurred around 1910 in Sweden -- see figure above), then the fertility rate declines rapidly.
In a similar way, there is a close correspondence between fertility and infant mortality across the world today:
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In Mexico one can see the decline in growth by means of its increasing impact on the age structure. The youngest base of the population is no longer expanding.
At some point towards the end of Stage Three the fertility rate falls
to
replacement levels. However population
growth continues on account of population
momentum. This can be seen in the Mexico example, and it is responsible
for the continued growth in the population of Sweden in the 1980s. An animation
of population momentum in Indonesia can be viewed HERE.
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Birth Rate: 13 per thousand
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STAGE FOUR is characterized by stability. In this stage the population
age structure has become older:
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Birth Rate: 12 per thousand
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In some cases the fertility rate falls well below replacement and population
decline sets in rapidly:
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Demographic Indicators
Birth Rate: 9 per thousand
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THE TRANSITION IN LESS WELL DEVELOPED COUNTRIES
Mexico and Sweden illustrate the salient differences and similarities between less and more developed countries.
As do Sweden and Mauritius:
These differences include:
But the greatest similarity concerns the fertility behavior of both populations (at different times) with respect to infant mortality. here shown for Brazil, Chile and Sweden:
The demographic transition model summarizes change in population growth
over time. Another form of transition exists in the world today
and is associated with the differences in growth rates across countries
of differing wealth. This is implied by the alternative labels on the traditional
transition model (pre Modern, Urbanizing/Industrializing, etc.).
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By using these concepts we can then explain the differences we see in population growth rates across the world today:
THE IMPACT OF HIV/AIDS IN AFRICA
Sources are:
The Demographic Impact
of HIV/AIDS (UN Publication)
The
impact of HIV/AIDS in Zimbabwe
AIDS
in Africa
Rising
Death Rates Slow Population Growth Rates
SOURCES:
HERE are my sources.
REALLY USEFUL LINKS
US Census Bureau International Data Base Population Pyramids.
US Census Bureau International Data Base Summary Demographic Data.
US Census Bureau World Population Profile 1998: HTML brief summary; PDF full version (167 pages).
Population Perspectives (various, contrasting papers on the "population issue", including Malthus' Essay.)
Global and U.S. National Population Trends
UN Population Division, World Population Trends
Population Policy: Consensus and Challenges (review of policy changes up to, and through the Cairo Conference: preview of Cairo Conference available HERE.)
GEO 350 (Resource Conservation) links on population
Population Policies for sub-Saharan Africa (World Bank)
News stories: Smaller families to bring big change
in Mexico
For one-child policy, China rethinks the iron hand
Special Report on the 1999 UN Conference on Population and Development
