After reading this article you will learn about:- 1. Introduction to Logistic Curve Theory 2. Assumptions of Logistic Curve Theory 3. Explanation 4. Criticisms 5. Conclusion.
Introduction to Logistic Curve Theory:
The foundation of logistic curve theory was laid by Quetlet in 1835. He said that the growth of population tends to slow down with the increase in density of population. The idea of logistic curve theory was also given by Verhulst in 1838.
But people did not give recognition to it. However, the credit goes to Raymond Pearl and Lowwell Reed in popularising this logistic curve theory of population growth. They did not accept the Malthusian view that population increases in geometrical progression. According to them, such views were unrealistic.
Assumptions of Logistic Curve Theory:
They based their theory on the following assumptions:
1. Population grows in a finite area.
2. The rate of population growth cannot be infinite.
3. The lower limit of population cannot be zero.
4. The point of maximum rate of growth is the point of inflection of the population growth curve. After that point, the rate of growth becomes progressively slower, till finally the curve becomes nearly horizontal, in close approach to the upper asymptote which belongs to the particular cultural epoch and area involved.
Explanation to Logistic Curve Theory:
Given these assumptions, population rises or falls in the form of a cycle and the changes that occur in population growth are obviously circular that determine the cycle.
It differs at various stages:
(a) In the first stage, when the country is in a state of backwardness due to high birth rate and high death rate, the growth of population will be stationary at the low level.
(b) In the second stage, when the birth rate remains unchanged as in the first stage and the death rate falls, population will increase at a rapid rate.
(c) In the third stage, with the fall in the death rate, the birth rate too will fall and the population will increase at a decreasing rate.
(d) In the fourth stage, both the birth rate and the death rate will be low and match each other. Population will not grow. It will be stationary but at a high level.
In the words of Pearl and Reed, “Population wanes and waxes, rises and falls, and increases rapidly and slowly but on the whole it is ever increasing.”
The logistic curve theory is explained diagrammatically in Figure 1.
In the figure, the time period has been shown on horizontal axis and the population growth on vertical axis. AB is the logistic
curve which shows that between the time periods X1-X2 and X3-X4 the increase in population is slow, while between the time periods X2-X3 and X4-X5 population increases at a rapid rate, and between the time period X5-X6 population is stationary.
It can also be explained in terms of Fig. 2 where the population growth has been shown on the horizontal axis and the time period on vertical-axis. From Q6 to Q1 the big ‘S’ is the logistic curve. On this logistic curve, population begins to increase from to Q1 toQ2 i.e., in the beginning population rapidly increases from OX1 to OX2. From Q2 onwards up to Q4it starts declining. At point Q4 population decreases from OX2 to OX3.After point Q4 population again starts growing up to point Q6.It means at point Q6, the overall population increases from OX3 to OX4. In thefigure, points Q2-Q3 and Q4-Q5 show stationary population growth. Thus the logistic curve shows that the population grows in cyclical form.
Pearl’s study was based on two experiments – one on fruit flies and another on hens.
In the first experiment, he gave gelantinized banana pudding in constant quantity to the fruit flies in a bottle. In the beginning, the fertility rate rapidly increased. But with the increased number of fruit flies the density in the bottle increased and with the increase in density, the fertility rate of fruit flies decreased and at last it became negligible.
In the same manner, Pearl also found through the second experiment that with the increase of hens in their pens, density went up, hens gave less eggs as the space became less. Thus, fertility rate came down with more density.
On the basis of these experiments, he concluded that the fertility rate of human population comes down when the density goes up. Besides, he had a belief that there will be high birth rate in poor people or in labour colonies.
He also gave biological explanation of death. He stated that “as the different groups of people differ in structure, habits, mental outlook and thought, they fight as groups for they get mad at each other.”
Pearl and Reed also studied the population growth of a number of cities and countries and came to the conclusion that their logistic curve theory was based on reality.
Criticisms of Logistic Curve Theory:
The logistic curve theory has been criticised on the following grounds:
(1) Ignores Socio-economic Factors:
This theory completely ignores the socio-economic factors and institutional aspects of population problems and the effect of environmental changes on population. It only considers the aspects of biological theory of population growth.
(2) Wrong Analogy:
Peral’s conclusion is that with the constant supply of food in the beginning, the human population increases and gradually it decreases and at last its growth is negligible which is based on the experiments on fruit flies and hens. But in reality, we do not find much analogy between fruit flies or hens and human beings.
According to this theory, the growth of population tends to slowdown with the increase in density of population. At the same time, Pearl and Reed also believe that there will be high birth rate in poor people or in labour colonies. But in labour colonies density is high and the birth rate is also high. So the theory is contradictory.
(4) Population Growth Pattern not Uniform:
It is not necessary that the population increase of all regions or countries will follow the growth pattern mentioned in the logistic curve.
(5) Time Period not clear:
This theory does not clearly show the time when the population will reach the optimum stage or when it will come down or when it will become stationary.
Conclusion to Logistic Curve Theory:
Despite these criticisms, the logistic curve theory is superior to the Malthusian theory of population. It is more realistic than the Malthusian Theory. This is because unlike Malthus, this theory does not believe that population increases in geometrical progression.
Rather, it explains the growth of population in a cyclical manner. In the beginning, population increases rapidly, then at a declining rate and ultimately it increases at a constant rate.