The average and marginal cost curves just deduced are the keys to the solution of the second-level problem, the determination of the most profitable level of output to produce in a given plant. The only additional datum needed is the price of the product, say p ₀.

The most profitable amount of output may be found by using these data. If the marginal cost of any given output (y) is less than the price, sales revenues will increase more than costs if output is increased by one unit (or even a few more); and profits will rise. Contrariwise, if the marginal cost is greater than the price, profits will be increased by cutting back output by at least one unit. It then follows that the output that maximizes profits is the one for which MC(y) = p ₀. This is the second basic finding: in response to any price the profit-maximizing firm will produce and offer the quantity for which the marginal cost equals that price.

Such a conclusion is shown in Figure 3. In response to the price, p ₀, shown, the firm will offer the quantity y* given by the value of y for which the ordinate of the MC curve equals the price. If a denotes the corresponding average variable cost, net revenue per unit will be equal to p ₀ - a, and the total excess of revenues over variable costs will be y*(p ₀ - a), which is represented graphically by the shaded rectangle in the figure.

The conclusion that marginal cost tends to equal price is important in that it shows how the quantity of output produced by a firm is influenced by the market price. If the market price is lower than the lowest point on the average variable cost curve, the firm will “cut its losses” by not producing anything. At any higher market price, the firm will produce the quantity for which marginal cost equals that price. Thus the quantity that the firm will produce in response to any price can be found in Figure 3 by reading the marginal cost curve, and for this reason the marginal cost curve is said to be the short-run supply curve for the firm.

The short-run supply curve for a product—that is, the total amount that all the firms producing it will produce in response to any market price—follows immediately, and is seen to be the sum of the short-run supply curves (or marginal cost curves, except when the price is below the bottoms of the average variable cost curves for some firms) of all the firms in the industry. This curve is of fundamental importance for economic analysis, for together with the demand curve for the product it determines the market price of the commodity and the amount that will be produced and purchased.

One pitfall must, however, be noted. In the demonstration of the supply curves for the firms, and hence of the industry, it was assumed that factor prices were fixed. Though this is fair enough for a single firm, the fact is that if all firms together attempt to increase their outputs in response to an increase in the price of the product, they are likely to bid up the prices of some or all of the factors of production that they use. In that event the product supply curve as calculated will overstate the increase in output that will be elicited by an increase in price. A more sophisticated type of supply curve, incorporating induced changes in factor prices, is therefore necessary. Such curves are discussed in the standard literature of this subject.

It is now possible to derive the relationship between product prices and factor prices, which is the basis of the theory of income distribution. To this end, the marginal product of a factor is defined as the amount that output would be increased if one more unit of the factor were employed, all other circumstances remaining the same. Algebraically, it may be expressed as the difference between the product of a given amount of the factor and the product when that factor is increased by an additional unit. Thus if MP ₁(x ₁) denotes the marginal product of factor 1 when x ₁ units are employed, then MP ₁(x ₁) = f(x ₁ + 1, x ₂, . . . ,x _n; k) - f(x ₁, x ₂ . . . ,x _n; k). The marginal products are closely related to the marginal rates of substitution previously defined. If an additional unit of factor 1 will increase output by f ₁ units, for example, then one more unit of output can be obtained by employing 1/f ₁ more units of factor 1. Similarly, if the marginal product of factor 2 is f ₂, then output will fall by one unit if the use of factor 2 is reduced by 1/f ₂ units. Thus output will remain unchanged, to a good approximation, if 1/f ₁ units of factor 1 are used to replace 1/f ₂ units of factor 2. The marginal rate of substitution is therefore f ₂/f ₁, or the ratio of the marginal products of the two factors. It has already been shown that the marginal rate of substitution also equals the ratio of the prices of the factors, and it therefore follows that the prices (or wages) of the factors are proportional to their marginal products.

This is one of the most significant theoretical findings in economics. To restate it briefly: factors of production are paid in proportion to their marginal products. This is not a question of social equity but merely a consequence of the efforts of businessmen to produce as cheaply as possible.

Further, the marginal products of the factors are closely related to marginal costs and, therefore, to product prices. For if one more unit of factor 1 is employed, output will be increased by MP ₁(x ₁) units and variable cost by p ₁; so the marginal cost of additional units produced will be p ₁/MP ₁(x ₁). Similarly, if additional output is obtained by employing an additional unit of factor 2, the marginal cost will be p ₂/MP ₂(x ₂). But, as shown above, these two numbers are the same; whichever factor i is used to increase output, the marginal cost will be p_i/MP_i(x_i) and, furthermore, the firm will choose its output level so that the marginal cost will be equal to the price, p ₀.

Therefore it has been established that p ₁ = p ₀ MP ₁(x ₁), p ₂ = p ₀ MP ₂(x ₂), . . . , or the price of each factor is the price of the product multiplied by its marginal product, which is the value of its marginal product. This, also, is a fundamental theorem of income distribution and one of the most significant theorems in economics. Its logic can be perceived directly. If the equality is violated for any factor, the businessman can increase his profits either by hiring units of the factor or by laying them off until the equality is satisfied, and presumably the businessman will do so.

The theory of production decisions in the short run, as just outlined, leads to two conclusions (of fundamental importance throughout the field of economics) about the responses of business firms to the market prices of the commodities they produce and the factors of production they buy or hire: (1) the firm will produce the quantity of its product for which the marginal cost is equal to the market price and (2) it will purchase or hire factors of production in such quantities that the price of the commodity produced multiplied by the marginal product of the factor will be equal to the cost of a unit of the factor. The first explains the supply curves of the commodities produced in an economy. Though the conclusions were deduced within the context of a firm that uses two factors of production, they are clearly applicable in general.