The Demographic Transition Model
' Does the DTM still provide a 21st century framework for looking at demographical change in countries which are experiencing development? To what extent is the tool really useful or should we make it obsolete?
The "Demographic Transition" is a model that describes population change over time. It is based on an interpretation begun in 1929 of the observed changes, or transitions, in birth and death rates in industrialized societies over the past two centuries.
Figure 1
The term "model" means 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 but may not accurately describe all individual cases. Whether or not it applies, or should be applied to less developed societies today remains to be disputed.
The DTM ( demographic Transition model ) (F.1) was first observed in the two centuries preceding 1950 in what are today's developed countries. Prior to the transition, these developed countries experienced high death rates matched by high birth rates, resulting in a relatively stable population size over time. But then improving living standards and public health measures caused death rates to drop, followed by a gradual drop in birth rates, which by the 1970s once again matched death rates. Between the onset-of-mortality decline and the drop in birth rates, population surged in developed countries, actually quadrupling. But the original 4 stages are over, and most developed countries are now projected to experience population shrinkage in the future (stage 5 see f.1). This historical evidence has proved so far that countries that have experienced industrial change have gone through the stages of the transition model; these countries are mainly in Europe and North America.
Figure 2
After observing these changes in countries like Britain and Germany Demographers predicted that today's NIC's (newly industrialised countries) would undergo a similar transition. Indeed, in the period following World War II, mortality decline accelerated in these countries. As the demographic transition model would predict, that led to a surge in population growth (See F.2) Also as expected, the death rate decline was later followed by a compensatory drop in birth rates. However instead of taking two centuries for the process to complete itself as it did in the developed countries, it will happen in less than one century.
There are many weaknesses of the DTM being used as a tool for predictions in demographic change. The model assumes that in time all countries pass through the same four/ five stages. It now seems unlikely, however, that many LEDCs, especially in Africa, will ever become industrialised.
The model assumes that the fall in the death rate in Stage 2 was the consequence of industrialisation. Initially, the death rate in many British cities rose, due to the insanitary conditions which resulted from rapid urban growth, and it only began to fall after advances were made in medicine. The delayed fall in the death rate in many developing countries has been due mainly to their inability to afford medical facilities. In many countries, the fall in the birth rate in Stage 3 has been less rapid than the model suggests due to religious and/or political opposition to Birth Control, this is evident in countries like Brazil, whereas the fall was much more rapid, and came earlier, in China following the government-introduced 'one child' policy (F3).
The timescale of the model, especially in several South-east Asian countries such as Hong Kong and Malaysia, is being squashed as they develop at a much faster rate than did the early industrialised countries, therefore making the time scale, and consequently the utility of the DTM obsolete.
Figure 3
Countries that grew as a consequence of emigration from Europe (USA, Canada, and Australia) did not pass through the early stages of the model which would also add to the idea that the DTM cannot be used as a general tool for all countries.
Still another factor can skew the numbers in a demographic transition or render it meaningless, which is lethal disease. In some countries today, AIDS rages out of control, with more than 40 million people afflicted globally. In 2001 alone, an additional five million people were diagnosed with AIDS. In future other factors may enter the picture such as groundwater depletion and global water shortage. In Bangladesh today, due to arsenic poisoning of the ground water in thousands of rural tube wells, millions of villagers are falling sick and dying as this silent killer reaches epidemic proportions.
In conclusion, the only way demographers could use the DTM would be in population projections or as a descriptive model. Population projections represent simply the playing out into the future of a set of assumptions about future fertility, mortality, and migration rates. It cannot be stated too strongly that such projections are not predictions, though they are misinterpreted as such frequently. A projection is a "what-if" exercise based on explicit assumptions that may or may not themselves be correct. If the assumptions represent believable future trends, then the projection's outputs may be plausible and useful. If the assumptions are unbelievable, then so is the projection.
As the course of demographic trends is hard to anticipate very far into the future, demographers should calculate a set of alternative projections that, taken together, are expected to define a range of plausible futures, rather than to predict or forecast any single future from the model. Because demographic trends sometimes change in unexpected ways, it is important that all demographic projections be updated on a regular basis to incorporate new trends and newly developed data, and therefore should not rely on one model.
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