In an oligopoly market structure, a few large firms dominate the market, and each firm recognizes that every time it takes an action it will provoke a response among the other firms. These actions, in turn, will affect the original firm. Each firm, therefore, recognizes that it is interdependent with the other firms in the industry. This interdependence is unique to the oligopoly market structure; in perfect and monopolistic competition, we assume that each firm is small enough that the rest of the market will ignore its actions.
Increasing Returns to Scale
The existence of oligopoly requires that a few firms are able to gain significant market power, preventing other, smaller competitors from entering the market. One source of this power is increasing returns to scale. Increasing returns to scale is a term that describes an industry in which the rate of increase in output is higher than the rate of increase in inputs. In other words, doubling the number of inputs will more than double the amount of output. Increasing returns to scale implies that larger firms will face lower average costs than smaller firms because they are able to take advantage of added efficiency at higher levels of production.
Types of Returns to Scale
Most industries exhibit different types of returns to scale in different ranges of output. Typically in competitive markets, there could be increasing returns at relatively low output levels, decreasing returns at relatively high output levels, and constant returns at one output level between those ranges. Monopolies and oligopolies, however, often form when an industry has increasing returns to scale at relatively high output levels. When a few large firms already exist in this type of market, any new competitor will be smaller and therefore have higher average costs of production. This will make it difficult to compete with the already-established firms. Therefore, the oligopoly firms have a built-in defense against new competition.
Take the example of the cell phone industry in the United States. As of the fourth quarter of 2008, Verizon, AT&T, Sprint, and T-Mobile together controlled 89% of the U.S. cell phone market. The cell phone industry has increasing returns to scale: the cost of providing cellular access to 100,000 people is more than half the cost of providing cellular access to 200,000 people. Any new entrant into the cell phone market will either need to pay one of the larger companies for access to its already-existing network, or try to build a network from scratch. Both options result in higher costs, higher prices, and difficulty in competing with the major networks .