As it has been said in the section’s introduction, demography not only studies the populations as volume-stocks and their change within a period of time, but also pretends to study its’ structure, since the population growth is frequently due to the population structure, which is also conditioned by its growth.
The concept of structure, in demography, is quite simple and clear: it is merely the population distribution accordingly to a classification criterion, exposed in percentage and based on individual socio-demographic features, as sex, age, marital status, etc. Therefore, this data conversion to percentage allows making comparison between the structures of two populations, although their volume may be different.
The population pyramid is the graphical illustration that shows the sex and age distribution of a population. It a special diagram, because it breaks the universal rules for showing data in a Cartesian diagram, that is, the rule of the horizontal x-axis (the independent variable).
In this case, the independent variable is the age, and the dependent one is the sum of people that have each age. However, they are represented in a reversed position; thus, the age is on the vertical axis.
In order to calculate the percentages, each sex and age value is divided by the total amount of population. It is necessary to be aware, so as to avoid the common error of diving by the sex amount. In this manner, both sexes’ parts of the pyramid have the same area and show the differences, which usually consist of higher survival rates for women. Another universal convention is putting the male values on the left, and the female ones on the right.
The following graph is a 2010 population age-sex pyramid. The pyramid itself has a pyramid structure, where each younger age rate is followed by a shorter older rate. This fact means that there is a growing population, with a moderate birth rate.
The above generalisation cannot be applied on the younger generations (0 – 25 years old). This is a confirmation of the progressive birth rate decrease in the last 25 years, as the pyramid base doesn’t expand any more, it is getting stable.
Therefore, we can say that it a transition pyramid, from an expansive model —with a recent high growth rate— to a more regressive one, with a growth rhythm decrease that can cause a world population stagnation. This decrease is due to a general fecundity decline, which is the result of the new motherhood patterns (see the next section) or the new birth rate reduction policies, as the Chinese ones.
If we analyse the top of the pyramid, we can see that there is a quite small percentage of old population, i.e. the general tendency seems to be the progressive population aging. The top also clearly shows the difference between males and females, since within the total amount of population, both sex rates are relatively equal (50,37% of males and 49,63% of females), but within the rates from 65 years, the balance is broken in favour of women (44,21% against el 55,79%). And, if we pay attention to the highest rates, over 85 years, we can see than the imbalance is even bigger (32,50% against 67,50%), since woman have higher life expectancy.
But, as always, this global scale view hides big evidences that only can be seen at national scale. A good example of this fact would be showing two strongly different population pyramids, as the Japan’s and the Niger’s one. These two countries have totally contrasting forecast: a prominent regression in Japan’s case; and a great growth in Niger’s one.
First of all, it is necessary to emphasise that the x-axis has been changed in order to correctly represent the percentage of each age group in both pyramids: whereas no age/sex group from the Japan’s pyramid exceeds the 4% of the total amount of the population, the 0-4 years group embodies the 10% of the population in the Niger’s pyramid.
Concluding, the differences between these two countries are extremely obvious. Firstly, Japan’s case is a regressive one, since there is a constant birth rate decrease (accordingly, the pyramid’s base is quite narrow) and a big amount of aged generations coming, due to the high life expectancy —the highest rated in the world— and, therefore, the progressive aging of the population. On the other hand, Niger’s pyramid is evidently an expansive one, as it shows an extremely high birth rate and a very high death rate too. So, every age group is narrower than the previous younger one and consequently, there is no elderly population.
As it has been said at the beginning of this section and showed through the population pyramids, the structure and the volume, and its variation in time, that is, the growth, are evidently two interrelated phenomena that depend on each other. Which is the mechanism that makes a population have such high birth and death rates and which is the relationship between it and the population growth rhythm? What is the explanation for these big differences between Japan and Niger? The answer can be found in the Demographic transition theory.