Principles of Microeconomics / Unit 9 MIC 0833 review, 2 to 3 minutes
The Four Structures on One Table
The idea
The four market structures are best held as one table with seven rows, and reading down the rows shows that each structure is a variation on one rule. Number of firms: many, many, few, one. Product: identical, differentiated, either, unique. Entry: free, easy, hard, blocked. The demand curve the firm faces: flat at the market price; downward sloping and elastic; downward sloping and dependent on rivals; the whole market's. Marginal revenue against price: equal in perfect competition, below price in the other three, because any firm that must cut its price to sell more loses revenue on earlier units. The rule for quantity: marginal revenue equals marginal cost in every structure without exception, which for a price taker reads price equals marginal cost. Long-run profit: zero under free or easy entry, so perfect competition and monopolistic competition both end at zero economic profit, while oligopoly and monopoly can keep profit behind barriers. Efficiency: perfect competition passes both tests; monopolistic competition fails both by a little, through excess capacity and a price above marginal cost; oligopoly and monopoly fail both by more. The table has a diagonal. As one moves from many firms to one, the firm's demand curve steepens, its price rises above marginal cost, entry weakens as a discipline, and profit becomes durable. What never changes is the decision rule; what changes is the demand curve it is applied to and whether anyone can arrive to compete the result away.
In practice
Worked example, one product across four structures. Wheat: a thousand farms, identical grain, entry by planting; the farm takes twenty-five dollars and grows to marginal cost; profit is competed to zero. Pizza: forty shops, each a little different, entry by opening; the shop sells where marginal revenue meets marginal cost, sets sixteen, and entry pulls it to the tangency at fourteen and zero profit. Cement in a region: three plants behind a large efficient scale; each prices watching the others, and profit persists unless a price war breaks out. Water: one utility behind a network; it would price at the monopoly point, and unit ten explains why it is not allowed to.
Where people slip
Three cautions. Zero long-run profit does not make monopolistic competition efficient; entry removes the profit and leaves the excess capacity. Marginal revenue equals marginal cost is not a special monopoly rule; every firm in the table uses it, and the price taker's version only looks different. And the table classifies the model, not the world; most real markets are a blend, and the question to ask is which row a market most resembles on entry and on the demand curve the firm faces.
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