The law of variable proportion shows that as we increase the quantity of only one input, keeping other inputs fixed, the total product increases at an increasing rate in the beginning, then increases at diminishing rate and after a level of output ultimately falls. The behaviour of Total product according to this law is as under:
(I) TP increases continuously from points O to A.
(iI) It increases at an increasing rate (convex shape) from O to P and at a diminishing rate (concave shape) from P to A.
(iii) TP is maximum at A and remains so upto point B.
(iv) After Point B, Total Product falls.

(i) Average Product increases as long as Marginal Product (MP)> Average Product  (AP). Alternatively, when MP > AP, AP rises.
(ii) Average Product is maximum and constant when AverageProduct (AP) = Marginal Product (MP). Alternatively, when AP = MP, AP is maximum.
(iii) average product falls when Marginal Product < Average product.
(iv) Marginal Product can be zero and negative but Average product is never zero.

(i) Short run:
(a) A short run refers to the period of time in which a firm cannot change some of its factors like plant, machinery, building, etc. due to insufficiency of time but can change any variable factor like labour, raw material, etc.
(b) Thus, in short run, there will be some factors of production that are fixed at predetermined levels, e.g., a farmer may have fixed amount of land.
(ii) Long run:
(a) A long run is a time period during which a firm can change all its factors of production including machines, building, organization, etc.
(b) In other words, it is a period of time during which supplies can adjust itself to change in demand.
Note: (i) Mind, here the terms long run and short run are functional and do not refer to a calendar month or a year.
(ii) This distinction depends merely upon how quickly factor inputs can be change by producers in an industry.

According to the Law of Variable Proportion when only one input is increased while all other inputs are kept constant, Marginal Product and Total Product behave in the following manner:
(i) When Marginal product rises (till Point PI), Total product increases at an increasing rate (convex shape) (till point P).
(ii) When Marginal product falls and remains positive (Till point B₁), total product increases at a diminishing rate (concave shape) (till point A).
(iii) When Marginal Product is zero (at point B₁), Total Product is at its maximum and constant (At point B).
(iv) When Marginal product becomes negative (after point B₁), total product falls (after point B).

The behaviour of Marginal product in the law of variable proportion is as under:
(i) When Marginal product rises (till Point P₁), Total product increases at an increasing rate (convex shape) (till point P).
(ii) When Marginal product falls and remains positive (till point B₁), total product increases at a diminishing rate (concave shape) (till point A).
(iii) When Marginal Product is zero (at point B₁), Total Product is at its maximum and constant (At point B).
(iv) When Marginal product becomes negative (after point B₁), total product falls (after point B).

Causes or Reasons of this Behaviour is as Under:
(i) Phase I
(a) Proper utilization of the fixed factor
1. In the initial stage of production the units of variable input (i.e., labour) is so less that fixed inputs cannot be effectively utilized.
2. Proper utilization of the fixed factor can be attained when more and more units of variable factor (labour units) are applied to the fixed factor (land), the fixed factor will be used intensively and output will increase rapidly.
(b) Specialization and division of labour
1. Initially, there was only one labour working on all the 5 acres of land ploughing, watering, etc.
2. As the number of labour units increases, each worker specialized in a particular activity leads to
specialization of the variable units and this resulted in increased output.
(ii) Phase II
(a) The non-optimal combination of variable factor with the fixed factor
1. When a given quantity of a fixed factor is combined with more and more units of variable factor, the additional units of variable factor will have smaller and smaller quantity of fixed factor to work with them.
2. As many workers share the same fixed factor, the share of each would obviously fall. Therefore, the cooperation of the fixed factor is not available to the same extent. Thus, an increase in the variable factor would add less and less to total output.
(b) Imperfect Substitutes
1. Diminishing return to factor occurs because variable factor and fixed factor are imperfect substitutes to each other.
2. Technically speaking, there is a limit to which variable factor can be applied to fixed factor and that limit depends upon the efficiency of fixed factor. So, variable factor and fixed factor are imperfect substitutes to each other.
(iii) Phase III
(a) Efficiency of Variable Factor Fall
1. In this stage the amount of variable factor becomes excessive relative to the fixed factor. This happens when too many LABOUR are engaged in cultivating on a given piece of land.
2. Instead of helping each other in production they cause overcrowding and chaos and thus hamper each other's work. In such a case, the contribution of additional labour to production is bound to be negative.
3. Thus, the marginal returns become negative and the total returns start diminishing.
(b) Efficiency of Fixed Factor Fall
1. Too much of a variable factors may also lead to the inefficiency of the fixed factor as well. • In case of capital, which is a fixed factor, too much of labour may cause lot of wear and tear of machinery, frequent breakdowns and excessive cost of maintenance. This is bound to affect total production adversely.
2. In such a situation it is advisable to reduce the units of the variable factor than to increase it with a view for getting maximum production.