The limiting cases of market structure

An industry is the set of all firms making the same product. The output of an industry is the sum of the outputs of its individual firms. But why indeed do some industries have many firms but others only one? This is a question about market structure. The structure of a market is a description of the behaviour of buyers and sellers in that market. Speaking about market structure, first it is useful to establish two benchmark cases, the opposite extremes between which all other types of market structure must lie. These limiting cases are perfect competition on the one hand and monopoly or monopsony on the other hand. A perfectly competitive market is one in which both buyers and sellers believe that their own buying or selling decisions have no effect on the market price. A monopolist is the only seller or potential seller of the good in that industry. A monopsonist is the only buyer or potential buyer of the good in that industry. The economist's definition of perfect competition is different from the meaning of competition in everyday usage. The economist means that each individual, recognizing that his own quantities supplied or demanded are trivial relative to the market as a whole, acts on the assumption that his actions will have no effect on the market price. Each consumer constructs a budget line on the assumption that market prices are given and unaffected by the quantities he chooses. Changes in market conditions, applying to all firms and consumers, change the equilibrium price and hence individual quantities demanded, but each individual neglects any feedback from his own actions to market price. This concept of competition, which we now extend to firms, differs from everyday usage. Ford and Renault are fighting each other vigorously for the European car market, but an economist would not call them perfectly competitive. Each commands such a large share of the total market that changes in their quantities supplied affect the market price. Each must take account of this in deciding how much to supply. They cannot regard themselves as pricetakers. Only under perfect competition can individuals make decisions that treat the price as independent of their own actions.