The model of demographic transition [1.] describes – and attempts to explain – the changes in a societies or countries population structure. Its theoretical foundation was set by American demographer Warren Thompson [2.] in 1929 and was later expanded on as an essential part of demographic and societal studies.
The core element is the study and analysis of changes in birth and death rates resulting in the rapid change of the demographic structure within a population’s framework of any society or country during its industrialisation progress.
In its original form the model was split into four main stages depicting the demographic change over a set period of time in history.
Stage one, observable in populations before the 18th century, consists of characteristically balanced – but relatively high – birth and death rates resulting in very slow population growth.
In stage two advancements in food and water supply as well as personal hygiene and improvements in the public health sector lead to a decrease in death rates, whilst birth rates were reasonably stable – resulting in a steady rise of total population.
Whilst death rates amongst children and the younger generations were considerately higher within stage one, stage two progresses to a more youthful population with the decreasing death rate especially manifesting within the same age group.
For stage two and the following stages a differentiation between more developed (MDC) and less developed countries (LDC) becomes important. Whereas population growth stays steady for MDCs toward the end of stage two and progressing throughout stage three, LDCs see an almost exponential growth due to a slower stabilisation ratio of birth and death rates.
Toward the late 19th century a decrease in birth rates can be noted resulting in a progressively stable population structure in terms of total inhabitants for MDCs. This development is carried out through stage three.
In stage four the demographic structure has reached a stable level. In terms of age structure a definitive shift towards an increasingly ageing population is noted.
Recent observations and projections show that this shift can develop quite dramatically for highly industrialised countries and in the case of falling birth rates might lead to rapid population decline.
Based on the early four and later expanded five stage model a new variable system of displaying the progression was developed in the 1980s. This most recent model is flexible enough to be applied to different transition time spans and varying severity.
Macmillan, 2003. Encyclopedia of Population – Second Edition. Gale: Macmillan Library Reference; pp.939-940 [2.]