THE DEMOGRAPHIC TRANSITION
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.
terminology or be sure to follow the links given below as the
The model is illustrated below: History
of the glacial theory Development
of the glacial theory
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.
HIGHLIGHTS IN WORLD POPULATION GROWTH
|1 billion in 1804
||3 billion in 1960 (33 years later)
||5 billion in 1987 (13 years later)
|2 billion in 1927 (123 years later)
||4 billion in 1974 (14 years later)
||6 billion in 1999 (12 years later)
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
Death rates were very high at all times in this stage for a number of
Spikes in the rate of death were caused by outbreaks of infectious
diseases such as influenza, scarlet fever, or plague. However, on a daily basis,
it was primarily the lack of clean drinking water and efficient sewage disposal,
and poor food hygiene that created an environment in which only a minority
of children survived
childhood. Water and food borne diseases such as cholera, typhoid, typhus,
dysentery, and diarrhea were common killers, as were TB, measles, diphtheria,
and whooping cough. Today in the developed world, at least, these are now
minority causes of death.
- Lack of knowledge of disease prevention and cure;
- occasional food shortages.
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
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:
- First, improvements in food supply brought about by higher yields as
agricultural practices were improved in the Agricultural Revolution of the
18th.C. These improvements included crop rotation, selective breeding, and
seed drill technology. In England, the greater wealth this brought
about enabled people to marry earlier, thus raising the birth rate slightly
at the same time. Another food related factor was the introduction of the
potato and maize (corn) from the Americas. These new crops increased the
quantity of foodstuffs in the European diet, especially in northern Europe.
- Second, there were significant improvements in public health that reduced
mortality, particularly in childhood. These are not so much medical
breakthroughs (which did not come until the mid 20th.C.) as they are
improvements in water supply, sewage, food handling, and general personal
hygiene following on from growing scientific knowledge of the causes of
disease. This is illustrated below for the case of measles and TB in the
USA over the past 100 years. However, bear in mind that killer infectious
diseases such as TB are airborne and not water borne, so public engineering
works such as sewer and water supply cannot take all the credit. In fact,
perhaps the most important factor here was increased female literacy allied
with public health education programs in the late 19th. and early 20th.
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):
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
Birth Rate: 36 per thousand
Total fertility rate: 4.8 births
Natural increase: 2.9% per year 1990-2000
Age structure: 43% under 15 yrs.age
STAGE THREE moves the population towards stability through a
decline in the birth rate. This shift belies Malthus's belief that
changes in the death rates were the primary cause of population change.
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 rural areas continued decline in childhood death means that at some
point parents realize they need not require so many children to be born to
ensure a comfortable old age. As childhood death continues to fall parents
can become increasingly confident that even fewer children will suffice.
- Increasing urbanization changes the traditional values placed upon
fertility and the value of children in rural society. Urban living also
raises the cost of dependent children to a nuclear family (education acts
and child labor acts increased dependency through the late 1800s). People
begin to assess more rationally just how many children they desire or need.
Once traditional patterns of thinking are broken the decline is likely
- Increasing female literacy and employment lower the uncritical acceptance
of childbearing and motherhood as measures of the status of women. Valuation
of women beyond childbearing and motherhood becomes important. In addition,
as women enter the work force their life extends beyond the family and the
connections they make with other women serve to break their isolation and
change their attitudes towards the burdens of childbearing. Within the
family they become increasingly influential in childbearing decisions.
- Improvements in contraceptive technology help in the second half of the
20th.C. However, contraceptives were not widely available in the 19th.C. and
likely contributed little to the decline. Fertility decline is caused by a
change in values than by simply the availability of contraceptives and
knowledge of how to use them. Today in the world there exists a close
correspondence between fertility and contraceptive use, but this likely
means that those families that have chosen to limit family size find
contraceptives the easiest and most effective way to do so.
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:
The age structure of a population entering Stage Three is illustrated below by
using an example from the Third World today:
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
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.
Birth Rate: 13 per thousand
Total fertility rate: 1.9 births
Natural increase: 0.3% per year 1990-2000
Age structure: 19% under 15 yrs.age
STAGE FOUR is characterized by stability. In this stage the population
age structure has become older:
Birth Rate: 12 per thousand
Total fertility rate: 1.8 births
Natural increase: 0.1% per year 1990-2000
Age structure: 18% under 15 yrs.age
In some cases the fertility rate falls well below replacement and population
decline sets in rapidly:
Birth Rate: 9 per thousand
Total fertility rate: 1.2 births
Natural increase: -0.1% per year 1990-2000
Age structure: 14% under 15 yrs.age
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:
- A later (20th.C.) transition in LDCs.
- A faster decline in death rates (50 yrs. vs. 150 yrs.). Death
control has been imported from MDCs and applied rapidly. In most LDCs
childhood mortality remains high, but 1/3 to 1/2 what it was 50 years ago.
However the most rapid improvements have occurred in places in which female
literacy has increased the most. Therefore, it is not simply the application
of modern drugs that is responsible but, rather, behavioral changes that
have improved survival (e.g. changes related to hygiene). These types of
behavioral change are readily adopted because, in so far as they improve
survival, they act to support traditional values that favor life over death
in almost all societies.
- A relatively longer lag between the decline in death rates and the
decline in birth rates (death rates are lower before decline in birth
rate starts). Fertility change requires a more conscious effort than
mortality change and requires social and behavioral changes that conflict
more with traditional values. This has been slower coming in LDCs because
economic change has been delayed in many cases. The same economic pressures
that existed in urban areas 100 years ago in MDCs have been slower to
develop in LDCs because many, particularly in Africa, remain very rural.
Hence, attitudes and values have been slower to change.
- Higher maximum rates of growth in LDCs: over 3.5% growth per year at the
height of Stage 2 in Mauritius and Mexico, compared to 1.3% in the same
stage in Sweden. Also, therefore, age structures are far younger in LDCs.
These data yield doubling times of 20 years versus 55 years.
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:
ANOTHER FORM OF THE TRANSITION
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.).
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
Demographic Impact of HIV/AIDS (UN Publication)
impact of HIV/AIDS in Zimbabwe
Death Rates Slow Population Growth Rates
are my sources.
REALLY USEFUL LINKS
US Census Bureau International Data Base Population
US Census Bureau International Data Base Summary
US Census Bureau World Population Profile 1998: HTML
brief summary; PDF
full version (167 pages).
Perspectives (various, contrasting papers on the "population
issue", including Malthus' Essay.)
and U.S. National Population Trends
UN Population Division, World
Policy: Consensus and Challenges (review of policy changes up to, and
through the Cairo Conference: preview of Cairo Conference available HERE.)
United Nations International
Conference on Population and Development (ICPD), 5-13 September 1994, Cairo,
GEO 350 (Resource
Conservation) links on population
policy in India
Policies for sub-Saharan Africa (World Bank)
The Population Council
News stories: Smaller
families to bring big change in Mexico
one-child policy, China rethinks the iron hand
Report on the 1999 UN Conference on Population and Development
of the glacial theory Development
of the glacial theory