Unlike other population theories, the theory of demography transition is based on the actual demographic experience of Western countries, which have moved from a condition of high mortality and high fertility with consequent slow growth of population to conditions of low mortality and low fertility, once again leading to a slow growth of population.
Earlier demographers such as Landry’s (in 1909) and Warren Thompson (in 1929) had attempted to construct a typology to describe the transition from conditions of high mortality and high fertility to conditions of low mortality and low fertility.
In 1947, C.P. Blacker attempted to identify the following five phases of the demographic transition:
1. The high stationary stage characterised by high birth rates and high death rates;
2. The early expanding stage, with falling birth rates but rapidly decreasing mortality;
3. The late expanding stage, with falling birth rates but rapidly decreasing mortality;
4. The low stationary stage, with low birth rates balanced by equally low mortality; and
5. The declining stage, with low mortality and deaths exceeding births.
None of these demographers can, however, is called the father of the theory of demographic transition, for none made an attempt to explain these changes either in fertility or mortality.
In 1945, it was Frank W. Notestein who presented the theory of demographic transition in an almost mature form, with explanations for the changes in fertility. In that sense, he may be credited with expounding the theory of demographic transition.
Note stein pointed out that the rapid growth of population during the past three centuries was mainly due to the decline in the death rate, resulting from the process of modernisation, which involved rising standards of living, rising incomes, and advances in sanitation and in medical knowledge.
Fertility also registered a decline, though this response to moderation was not as spectacular.
Throughout the modern West, birth rates reached very low levels by the middle of the 1930s. This decline was achieved because of the widespread acceptance of contraception under the influence ideal of the small family, so common in any urban of the new industrialised society.
Note stein characterised three types of populations, according to their stage of demographic evolution.
1. Population in the stage of “incipient decline,” where fertility below the replacement level or those approaching this stage (e g-> populations of Europe, the United States, Australia and New Zealand).
2. Populations in the stage of “transitional growth,” where “birth and death rates are still high and growth are rapid, but the decline of the birth rate is well established.” (e.g., populations of the Soviet Union, Japan and some countries in Latin America).
3. Populations in the stage of “high growth potential” where “mortality is high and variable and is the chief determinant of growth, while fertility is high and thus far has shown no evidence of a downward trend.
In these populations, rapid growth is to be expected just as soon as technical developments make possible a decline in mortality (e.g., populations in most countries ‘of Asia, Africa and Latin America).
The process of demographic transition in the course of economic development, as experienced by today’s industrialised countries, may be briefly explained as follows: “All nations in the modern era, which have moved from a traditional, agrarian-based economic system to a largely industrial, urbanised base, have also moved from a condition of high mortality and fertility to low mortality and fertility.”
Ansley J. Coale and Edger M. Hoover have studied the changes m the birth and death rates typically associated with economic development.’ Their explanation is as follows: The agrarian peasant economy is characterised by high death and birth rates.
The death rates usually fluctuate in response to the variations in harvests and the incidence of epidemics. They are high because of poor diet, Primitive conditions of the sanitation and lack of preventive and curative medical and public health programmes.
The birth rates in economies are high and are a functional response to high mortality. The ideals of prolific fertility are, therefore, ingrained in customs and beliefs of such societies.
When this happens, death rates register striking reductions a consequence of better and regular Sunni of food as well as improved medical knowledge and care.
At somewhat later stage, birth rates also begin to fall. The acceptance of the ideal of a small family size common about initially in urban groups at the higher end of the socio-economic scale, and then spreads to small cities, lower-income groups and eventually to rural areas.
The decline in the birth rate usually occurs after a substantial time lag, as compared to the decline in tithe death rate. This delayed response of the birth rate to economic change comes about because any decline in fertility results only when changes occur in longstanding attitudes and customs prevalent in society.
The birth and the death rates pursue a somewhat parallel downward course though, of course, the decline in the birth rate lags behind finally, as further reductions in the dearth rate become increasingly difficult to achieve, the birth rate again approaches the level of the death rate, and population grows only at a very slow rate.
During this stage, death rates are relatively low and un fluctuating, while birth rates may fluctuate from year too year for they are mainly dependent on voluntary decisions of individual couples.