When the age structure of population is classified by sex in the form of a histogram, it is called age-sex pyramid. The base of the pyramid shows very low age starting from zero, it increases as we move upwards. The top of the pyramid shows the maximum age above 85 years. Population pyramids are of 5-year intervals.
On the left of the pyramid is shown the age of male population and on the right of the female population, thereby indicating the sexes. Pyramids are of two types: One, based on absolute numbers of population and the second, on the proportion of population. The former shows the size and composition of population of a country. The second type is used to compare the age-sex composition of two countries, on a single scale of which one is small and the other is big, such as India and Sri Lanka.
Figures 2.1 and 2.2 depict two different population pyramids.
Figure 2.1 reflects the condition of age-sex pyramid of developing countries like India while the second figure reflects the condition of age-sex pyramid of developed countries like the United States of America. Fig. 2.1 shows a very wide base, while it is of a rectangular shape in Fig. 2.2. This shows the distinct variation in the age structure of population of both developing and developed countries. In developing countries, there is a marked decline in the death rate because the population is becoming younger, while in developed countries, both birth and death rates stablise at a low level.
During the first stage of demographic transition, the death rate is high due to high infant mortality rate and the existence of low economic development. But with economic development, the decrease in mortality rate becomes possible which ultimately increases the proportion of the younger population. The population pyramid becomes conic when we move upwards if other things remain constant. This happens due to decline in death rate.
Views of Thomson and Lewis Regarding Population Pyramid:
According to Thomson and Lewis, there are five types of population pyramids. All these different types of pyramids indicate the relation of population growth with economic development. In other words, the population pyramid reflects the condition of birth and death rates in different stages explained in the demographic transition theory.
First, we find in Fig. 2.3 a very wide base and slow slopes which indicate the demographic condition of those countries where both the birth rate and the death rate are high. This pyramid also shows large numbers of youth and dependents in total population. This is the condition of the first stage of demographic transition when both the birth rates and death rates were high in the majority of countries of the world in the 17th and 18th centuries.
Second, in Fig 2.4, the base is wider than that of Fig. 2.3, and its slopes are becoming narrow more rapidly. The same is the case with the second stage of demographic transition with high birth rate and low death rate. The developing countries like India, Pakistan, Bangladesh and others are passing through this second stage where population increases at a rapid rate and the expectation of life as well as the ratio of dependent population also increases.
Third, the histogram of the population of developed countries approximates the rectangular shape which is shown in Fig. 2.5. This indicates low birth and low death rates. The shape of the Fig. 2.5 pyramid is very similar to that of the pyramid of United States of America and Eastern Europe before the Second World War. This pyramid reflects the condition of the third stage of demographic transition, where birth and death rates are low, the ratio of dependent population is also low and the expectation of life is very high.
Fourth, in Fig. 2.6 the bell-shaped pyramid shows low death rate, as the pyramid in Fig. 2.5 but the birth rate increases at a slower rate in comparison to the birth rate in the pyramid of Fig. 2.5. The fourth stage of demographic transition shows that the net increase in population is negligible but the ratio of youth and dependents is high in total population.
Fifth, in Fig. 2.7, the pyramid reflects that the birth rate decreases at a faster rate than the death rate and if this situation continues then the growth rate of population will become negative. At present Germany is the country in which the population growth is -0.1.
We may conclude with Thomson and Lewis: “A population is always changing, whereas a pyramid is a static picture. The proportions of people in the various age and sex categories change because of the continuous action, i.e., mortality, fertility and migration. The population pyramid freezes this motion at a particular moment in time.The pyramid can be viewed as a picture of the biological history of population—the result viewed as a picture of biological history of population—the result of 100 years of births, deaths and migration.”