The exponents of this theory comment that population growth as described by Malthus is not always bad. Increase in population helped the economic growth of Western European countries rather than hindering it.
This theory of population tries to explain that the size of population which is the best for a country of a particular time.
The theory conceives the right number of population relative to the amount of actually developed natural resources, availability of capital and technology.
Optimum population is that level of population at which output per capita for real income per head is the highest. If the population happens to be less than optimum level, it is under-populated and if this is more than optimum level, it is over populated.
Let us assume that the stock of natural resources, capital and technology is given and remains fixed at a given time. Now also assume that the population at initial stages is found less but begins to increase. With the increase in population, the labour supply shall also increase. With small population and large resources, the production appears to be less because resources are not fully and optimally utilized.
So there will be three advantages if population increase (i) an increase in population will permit a full utilization of natural resources and capital equipment (ii) advantage of large scale production arise and (iii) increase in the level of production ensure higher per capita income and increased standard of living.
Contrary to it, with fixed resources, capital and technology, but increase in population shall result into (i) low standard of living, (ii) open and disguised unemployment and (iii) food problem shall be caused due to over population.
So when resources are not fully utilized due to less man power because manpower resources are short then it is a sign of underdevelopment. In this case if population increases to match the resources then it is a symptom of economic development rather than retardation.
Dalton has given a formula through which it can be judged to what extent actual population of a country deviates from the optimum population. The degree of deviation is called maladjustment. The following formula seeks to measure the extent of maladjustment.