Until 1950, the effects of changes in fertility and mortality on the age structure of a population were not well understood by demographers.
The development of demographic models in the early 1950s, however, greatly improved the understanding of this relationship, and it is now possible to determine the relative role of fertility and mortality in determining the age structure of any population.
It has been earlier seen how the age structure of the population in the West underwent substantial changes and how it experienced a decrease in the proportion of children under the age of 15 as well as an increase in the population of 65 and over, with the result that the values of the median age also increased.
These populations could, thus, become “old” with the passage of time. Demographers interpreted the increase in the proportion of the old-age population as a consequence of the joint effect of the declining rates of fertility and mortality.
While the declining fertility was considered to be responsible for the shrinking of the base of the population pyramid, the declining mortality was thought to contribute to the widening of the pyramid.
During the 1950s, however, researches of various demographers such as V.G. Valarous (1950), Frank Lorimer (1951), Alfred Sauvey (1954), Ansley Coale (1956), and the United Nations (1958) demonstrated that the aging of the populations in several Western countries was almost entirely due to a decline in the fertility rate, and that it was virtually negligible because of the decline in the death rates.
It was further elaborated that the increase in the proportion of the old people and the increase in the median age of the population in Western countries was a product of the history of falling birth rates which these countries had experienced.
It must, however, be noted that this explanation refers to past trends, more specifically to trends up to about 1940. In the future, however, the decline in the mortality would be an important factor in any further aging of the population.
Whether a population of any country is “young” or “old” is mainly determined by the fertility of the women in that country. When fertility is high, birth rates are also high, and the number of children borne by the women is large and the -population is “young.”
On the other hand, when fertility is low, birth rates are low, the number of children born is low and the population is “old.” When birth rates are high, the population has a larger proportion of children relative to the adults of parental age.
The sustained high level of birth rates results in a large proportion of children and a small proportion of “old” population and a low median age. When low level of birth rates persists, the result is a small proportion of children and a large proportion of “old” people.
Declining birth rates cause an aging of the base as well as the apex of the age pyramid and an increase in the proportion of adults, with a consequent high median age and an “old population.”
It may, therefore, be concluded that the economically developed countries experienced an aging of the population because of their declining birth rates, while the age structure of the developing countries has remained virtually unchanged because their birth rates have remained more or less at high levels, though their death rates have been declining since the 1950s.