Page 97 - IJPS-10-4

International Journal of
Population Studies Mortality shapes population age structures

Figure 2 presents the evolution of two synthetic than in Figure 1, but the general tendency is the same: there
indicators of age structure: the average age of the Chinese is an underlying “normal” value for each chosen indicator
population, A (panels on the left), and the OADI (panels (the standard one, with an asterisk). Shocks may, and
on the right). Both indices were calculated twice: first usually do, hit populations, driving the age structure and
on the observed population (or projected one, for the its indicators away from their usual shape and levels: be it
years 2026 – 2051) and then on the standard (stationary) a war, demographic transition, or economic crisis. In the
population of the same period. Both indices are presented following years, however, the effects of these shocks tend
twice: as a time series (top panels) and as a scatter diagram to be reabsorbed, and little by little, things tend to get back
(bottom panels). In the latter case, the bisector is drawn; to normal. However, the standard has changed because it is
it represents the theoretical case when the two indexes determined by the constantly evolving survival conditions.
(observed and standard) give the same result.
The same conclusions emerged from the analysis of
A few insights can be learned from this example. First, the dissimilarity (ID) index. Firstly, it should be noted
both structural measures convey the same message, and in that it is always low: 23% in the worst year (1976), 16%
the following sections, only the average age (A) is utilized. on average over the past 70 years (1951 – 2021), and
Second, the scatter point representation (bottom panels) likely to be even lower in the future (12%; 2026 – 2051;
saves space, which is useful when the dataset increases Figure 3). This means that the share of Chinese who are
(this type of representation is used in the next sections). classified correctly by age (5-year age groups) using the
With the omission of tags (for reasons of space), the time corresponding stationary population (i.e., assuming that
dimension is lost, but it can be imagined as a rightward current mortality is the only force at play) is about 84% on
movement because as time passes, survival conditions average and never below 77%, even in the worst year. This
improve, and this makes the standard (stationary) means that all other factors combined (previous mortality,
population older. Based on our hypothesis, this is the main fertility, and migration) compete to explain only about
driving force behind population aging, and this is precisely 16% of the Chinese age structure on average.
the third aspect that emerges from the Chinese example:
empirical data evolved more or less in accordance with our Figure 4 displays the observed and standard (stationary)
expectations. In the Chinese case, to be sure, the difference Chinese age structures in selected years. At all times, the
between model predictions and observed data is larger effects of past waves of fertility (high until the 1960s and

Figure 2. Structural indicators of the observed (or projected) population and the standard (stationary) population (*) of China, 1951 – 2051 (United Nations
(2022).
Note: Standard values [*] are calculated for the stationary population. Abbreviations: A: Average age of population (Equation V); OADI: Old-age
dependency index (Equation IV).

Volume 10 Issue 4 (2024) 91 https://doi.org/10.36922/ijps.377