The growth of population in a region depends upon fertility, mortality and migration.
Fertility or the birth rate is measured in terms of total number of live births per thousand population per year.
Generally, the fertility rate is affected by various social, economic and demographic factors.
Mortality or the death rate is measured in terms of total number of deaths per thousand population per year.
The difference between these two rates (i.e. fertility and mortality) is called the natural growth rate.
The term migration refers to the movement of people from one area to the other or from one country to another.
The rate of migration affects the growth of population of a region by increasing or decreasing the number of people living there.
The growth rate of population may be positive or negative.
A positive growth rate of population means an increase in the number of people living in a region; whereas negative growth rate means declining population.
A positive growth rate occurs when the number of births and in migration exceeds the number of deaths and out migration; the negative growth rate means just opposite to positive growth rate.
If we look at this 100 years population growth then, it can be broadly grouped under the following four categories.
 Period of stagnant growth rate (before 1921)
Before 1921 the increase in population was sporadic, irregular and slow.
This was mainly due to high birth and death rate.
Therefore, the natural growth was insignificant.
In 1911-21 the absolute increase declines marginally due to famines, epidemics etc.
After 1921 the population has been increasing.
Therefore, 1921 is known as demographic divide in the population study of India.
 Period of steady growth rate (1921-1951)
Since 1921 to 1951 there was a steady increase in population.
This is because of steady delcline in death rates.
The decline was mainly due to improvement in sanitation and medical facilities.
Other factors which helped were development in road facilities which helped in meeting the exigencies of food shortage and substantial improvement in agricultural economy.
Therefore, the population growth during this period was known as mortality induced growth.
 Period of rapid growth rate (1951-1981)
This is a very crucial phase as far as population growth of India is concerned.
The population was almost doubled during these three decades.
During this period there was a rapid decline in death rate whereas the decline in birth rate was marginal.
Birth rate was reduced from 41.7 to 37.2 whereas death rate was reduced from 22.8 to 15.0 during this period.
Therefore the difference between birth rate and death rate was very high and as a result natural growth rate remains very high.
This was due to acceleration in developmental activities further improvement in medical facilities, improvement in living conditions of the people etc.
This period of growth is termed as fertility induced growth.
 Period of declining growth rate (after 1981)
In the last two decades i.e. 1981-91 and 1991-2001, the rate of growth started declining gradually.
It signals the beginning of a new era in the demographic history of India.
During this period birth declined significantly, from 37.2 in 1971-81 to 24.8 in 1991-2001; whereas the decline in death rate continued in a slower rate.
The death rate has declined from 15.0 to 8.9 during this period.
This declining trend is a positive one and may be attributed to effective government role in promoting family welfare programmes and people’s awareness.