The theory behind the model
The first thing to note about the Demographic Transition Model was that it was just that - a model, which is a representation of some phenomenon of the real world made in order to facilitate an understanding of its workings. In the case of the Demographic Transition model it was formulated by geographers, (mainly Notestein in 1945) in an attempt to explain a link between population change and socio-economic progress.
The theory was based on the population explosion in Western countries during the 1800s as the transition from high to low mortality and high to low fertility began. The transition from high to low mortality was a result of major public health improvements with a decline in infant mortality rates. Fertility rates fell at a slower rate as these were linked to social and behavioural change. The population grows at a rapid rate in this phase between the drop in mortality and fertility rates. The transition is complete when death and birth rates are comparable and population growth stabilises.
Transition Stages
The Demographic Transition model has 4 and possibly 5 stages of transition, each stage being characterized by a specific relationship between crude birth rate (CBR = number of annual births per one thousand people) and crude death rate (CDR = number of annual deaths per one thousand people).
Stage 1: High stationary: High CBR and high CDR,e.g. before Industrial Revolution of 18th century in the West, population was small and stable.
Stage 2: Early Expanding: Mortality levels (CDR) fall with socioeconomic progress but fertility level remain high so population grows.
Stage 3: Late Expanding: CDR continues to decline but CBR also falls causing population growth to slow.
Stage 4: Low Stationary: CDR and CBR are low with population stabilising.
Possible Stage 5: Population Decline: Fertility rates have fallen below replacement rates and there is a greater elderly than youthful population.
The diagram below shows these transitional stages from the following website:
https://populationeducation.org/what-demographic-transition-model/
Developed countries have been through these transitions while still-developing countries like those in Sub-Saharan Africa are in stage 3.
Typical Demographic Transition Model
Criticisms...and Mark II/III?
Criticisms of the Demographic Transition Theory have focused on it Western-centric bias. It assumes that progression will be similar for all countries whereas varying factors for fertility reduction for example, be they economic or cultural can have a differing impact on the transitions as as fertility does not always decline with economic security.
Subsequent Transition models have been suggested: the Second Demographic Transition takes account of changes in sexual and reproductive behaviour since the 1960s , while the Third suggests immigration and ethnic change will have an effect on population transition