The theory of long-run profit-maximizing behaviour rests on the short-run theory that has just been presented but is considerably more complex because of two features: (1) long-run cost curves, to be defined below, are more varied in shape than the corresponding short-run cost curves, and (2) the long-run behaviour of an industry cannot be deduced simply from the long-run behaviour of the firms in it because the roster of firms is subject to change. It is of the essence of long-run adjustments that they take place by the addition or dismantling of fixed productive capacity by both established firms and new or recently created firms.

At any one time an established firm with an existing plant will make its short-run decisions by comparing the ruling price of its commodity with cost curves corresponding to that plant. If the price is so high that the firm is operating on the rising leg of its short-run cost curve, its marginal costs will be high—higher than its average costs—and it will be enjoying operating profits, as shown in Figure 3. The firm will then consider whether it could increase its profits by enlarging its plant. The effect of plant enlargement is to reduce the variable cost of producing high levels of output by reducing the strain on limited production facilities, at the expense of increasing the level of fixed costs.

In response to any level of output that it expects to continue for some time, the firm will desire and eventually acquire the fixed plant for which the short-run costs of that level of output are as low as possible. This leads to the concept of the long-run cost curve: the long-run costs of any level of output are the short-run costs of producing that output in the plant that makes those short-run costs as low as possible. These result from balancing the fixed costs entailed by any plant against the short-run costs of producing in that plant. The long-run costs of producing y are denoted by LRC(y). The average long-run cost of y is the long-run cost per unit of y [algebraically LAC(y) = LRC(y)/y]. The marginal long-run cost is the increase in long-run cost resulting from an increase of one unit in the level of output. It represents a combination of short-run and long-run adjustments to a slight increase in the rate of output. It can be shown that the long-run marginal cost equals the marginal cost as previously defined when the cost-minimizing fixed plant is used.

Cost curves appropriate for long-run analysis are more varied in shape than short-run cost curves and fall into three broad classes. In constant-cost industries, average cost is about the same at all levels of output except the very lowest. Constant costs prevail in manufacturing industries in which capacity is expanded by replicating facilities without changing the technique of production, as a cotton mill expands by increasing the number of spindles. In decreasing-cost industries, average cost declines as the rate of output grows, at least until the plant is large enough to supply an appreciable fraction of its market. Decreasing costs are characteristic of manufacturing in which heavy, automated machinery is economical for large volumes of output. Automobile and steel manufacturing are leading examples. Decreasing costs are inconsistent with competitive conditions, since they permit a few large firms to drive all smaller competitors out of business. Finally, in increasing-cost industries average costs rise with the volume of output generally because the firm cannot obtain additional fixed capacity that is as efficient as the plant it already has. The most important examples are agriculture and extractive industries.

The theory of production has been subject to much criticism. One objection is that the concept of the production function is not derived from observation or practice. Even the most sophisticated firms do not know the direct functional relationship between their basic raw inputs and their ultimate outputs. This objection can be got around by applying the recently developed techniques of linear programming, which employ observable data without recourse to the production function and lead to practically the same conclusions.

On another level the theory has been charged with excessive simplification. It assumes that there are no changes in the rest of the economy while individual firms and industries are making the adjustments described in the theory; it neglects changes in the technique of production; and it pays no attention to the risks and uncertainties that becloud all business decisions. These criticisms are especially damaging to the theory of long-run profit maximization. On still another level, critics of the theory maintain that businessmen are not always concerned with maximizing profits or minimizing costs.

Though all of the criticisms have merit, the simplified theory of production does nevertheless indicate some basic forces and tendencies operating in the economy. The theorems should be understood as conditions that the economy tends toward, rather than conditions that are always and instantaneously achieved. It is rare for them to be attained exactly, but it is just as rare for substantial violations of the theorems to endure.

Only the simplest aspects of the theory were described above. Without much difficulty it could be extended to cover firms that produce more than one product, as almost all firms do. With more difficulty it could be applied to firms whose decisions affect the prices at which they sell and buy (monopoly, monopolistic competition, monopsony). The behaviour of other firms that recognize the possibility that their competitors may retaliate (oligopoly) is still a theory of production subject to controversy and research.

Authoritative intermediate-level discussions may be found in William J. Baumol, Economic Theory and Operations Analysis, 4th ed. (1977). Vernon L. Smith, Investment and Production (1961), especially emphasizes the relationship between long-run costs and investment. Probably the best technical presentation is Paul A. Samuelson, Foundations of Economic Analysis, enlarged ed. (1983). George J. Stigler, Production and Distribution Theories (1941, reissued 1994), provides a discussion of the evolution of the theory. J. Viner, “Cost Curves and Supply Curves,” Zeitschrift für Nationalökonomie, 3:23–46 (1931), remains a classic text.