Rev. Thomas Robert Malthus was an English clergyman, economist and demographer. In the 18th century, he published his famous treatise essay on the principles of population. This paper survived and has become the basis of Malthusian population theory. He had come to conclude that "the rate of growth of the world's population was outstripping the capacity of land to provide the food necessary to subsistence and that only recurring famine, pestilence, or wars would tend to keep down the number of inhabitants" (Rook, A. 1958).

This theory states that population increases faster than food supply and if unchecked, it may lead to human misery, It presupposes that man's biological capacity to reproduce himself exceeds his capacity to increase the food supply.

The theory further assumes that population grows at a geometric rate or progression Le.[2, 4, 8, 16, 32] and if unchecked can double itself every 25 years.

Food supply on the other hand increases at arithmetic rate or progression i.e. [2, 4, 6, 8, 10 etc.] due to the operation of the law of diminishing returns based on the idea that the supply of land is fixed and that, the rest is used first and the worst is cultivated last.

At each stage of population growth, Malthus argued that, the amount of food produced is often less than ever before and that at a certain point in time, population growth exceeds food supply. In such a situation, the resources would be unable to support a given size of the population. The additional population is likely to starve to death leading to Malthusian population trap.

The population trap is the point when the population growth rate is equal to that of the food production and it is inevitable that the population growth would stop after that point. 

Illustration of the population trap.

Point T is the point of population trap where food supply is just sufficient to sustain the population growth rate. Beyond point "T" i.e. The shaded area is a region of misery and starvation and death when population size exceeds food supply.

To avoid entering the period of misery, Malthus suggested two checks or measures that can control the population growth rate.