The population of the world has grown from a few person to 7.141 billion people. It has been estimated by United Nations Food and Agriculture Organisation that nearly 870 million people are suffering from chronic undernourishment and around 1.29 billion population were living in absolute poverty in 2008 estimated by United Nations and World Bank one in every five people of the planet is illiterate and 205 million people are unemployed in the world.

The world which fails to support the present population could not support more inhabitants. The science of population sometimes called the demography represents the study of statistics such as births, deaths, income or the incidence of disease, which explain the changing structure of human population.

Theories of Population:-

Thomas Robert Malthus was the first economist proposed a systematic theory of population he articulated his views in his book ‘Essay on the Principle of Population’ (1978). Malthus proposed the principle that human populations grow exponentially (i.e. doubling with each cycle) while food production grows at an arithmetic rate (i.e. by the repeated addition of a uniform increment in each uniform interval of time). Thus, while food output was likely to increase in a series of twenty five tear intervals in the arithmetic progression 1,2,3,4,5,6,7,8,9 and so on population was capable of increasing in geometric progression 1,2,4,8,16,32,64,128,256 and so forth. This scenario of arithmetic food growth with simultaneous geometric human population growth predicted a future when humans would have no resources to survive on. To avoid such a situation, Malthus urged control on population growth.

Demographic Transition Theory: –

Two different interpretations have been given for this theory. One by Frank Notestein and W.S. Thompson which says that every country passed through three stages of population growth.
1. High birth rate and high death rate
2. High birth rate and low death rate (population explosion)
3. Low birth rate and low death rate

The theory says the developed nation approaching a new equilibrium with both birth rates and death rates quite low and little population growth. The other theory is given by C.P. Blacker.

There are five phases in this theory.
(1) Early expanding phase marked by high fertility but declining mortality
(2) Late expanding phase with declining fertility but mortality declining more rapidly
(3) Low stationary phase with cow fertility and equally cow mortality
(4) Declining phase with low mortality, low fertility and an excess of deaths over births.

Optimum theory of population: –

The theory has been propounded by Cannan. According to him there is direct relationship between the size of population and available resources in the country. He divided the world in three categories.

1. Over populated
2. Under Populated
3. Optimum size of population

(1) Over populated :- If due to increase in the population per capita income begins to fall, the country is said to be over populated

(2) Under populated: – If the number of people are less than the resources of the country and they are unable to make full use of these resources the country is said to be under populated.

(3) Optimum size of population: – When the size of population is according to the size of natural resources, in this case per capita output will be maximum and country is said to have an ideal size of population or optimum population.