Difficulty: Intermediate | Prerequisites: Ch. 14 (competitive markets) and Ch. 15 (monopoly). You need to understand how MR = MC works under both market structures before tackling the ones in between.
Most real-world markets sit between the extremes of perfect competition and pure monopoly. Chapter 16 covers monopolistic competition, where many firms sell differentiated products and free entry drives long-run profit to zero, yet each firm retains some pricing power. Chapter 17 covers oligopoly, where a small number of firms interact strategically, creating a tension between cooperation (act like a monopoly) and self-interest (cheat to grab market share). Game theory, particularly the prisoners' dilemma, is the main analytical tool for oligopoly.
Monopolistically competitive firms behave like monopolists in the short run (downward demand, MR = MC) but like competitive firms in the long run (free entry drives profit to zero). Oligopolists are interdependent: each firm's profit depends on what the others do. The Nash equilibrium typically lands between the monopoly outcome and the competitive outcome, with output higher and prices lower than monopoly, but not as low as perfect competition.
Oligopoly
A market structure in which only a few sellers offer similar or identical products. Each firm's actions affect the others' profits.
In simple terms, a handful of big players who all watch each other closely.
Monopolistic competition
A market structure in which many firms sell products that are similar but not identical (differentiated). Free entry and exit drive long-run profit to zero.
Think of it as a market where every seller has a tiny monopoly on their own version of the product, like restaurants or hair salons.
Product differentiation
Each firm produces a product at least slightly different from its competitors, giving it a downward-sloping demand curve.
Concentration ratio
The percentage of total market output supplied by the four largest firms. A higher ratio indicates more market dominance by a few sellers.
Excess capacity
In monopolistic competition, firms produce on the downward-sloping portion of their ATC curve, below the efficient scale. The gap between their actual output and the efficient scale is excess capacity.
Game theory
The study of how people behave in strategic situations, where each person's best action depends on what the others do.
Prisoners' dilemma
A particular game that illustrates why cooperation is difficult to maintain even when it is mutually beneficial. Each player has an individual incentive to defect, leading to an outcome worse for both.
Nash equilibrium
A situation in which each player chooses the best strategy available given the strategies chosen by all other players. No one can improve their outcome by changing strategy alone.
Dominant strategy
A strategy that is best for a player regardless of what the other players do.
Collusion
An agreement among firms in a market about quantities to produce or prices to charge.
Cartel
A group of firms acting in unison, typically to restrict output and raise prices as if they were a single monopolist.
Many sellers, each small relative to the market
Product differentiation: each firm's product is slightly different, giving it a downward-sloping demand curve
Free entry and exit: no barriers, so long-run economic profit is zero
Identical to a monopolist: find Q where MR = MC, then read P off the demand curve
If P > ATC, the firm earns profit. If P < ATC, it makes losses.
Profits attract entry, which shifts each incumbent firm's demand curve left (customers have more choices), reducing demand and profit
Losses cause exit, which shifts remaining firms' demand curves right, increasing demand and reducing losses
Equilibrium: the demand curve is tangent to the ATC curve. Profit = zero, because P = ATC at the chosen quantity.
Two key properties of long-run equilibrium:
P > MC (as in monopoly, because the downward-sloping demand curve makes MR < P)
P = ATC (as in competition, because free entry drives profit to zero)
Excess capacity: The monopolistically competitive firm produces below the efficient scale (on the downward-sloping portion of ATC). A perfectly competitive firm produces at the minimum of ATC.
Markup over marginal cost: The monopolistically competitive firm charges P > MC. A perfectly competitive firm charges P = MC.
Monopolistically competitive firms have an incentive to advertise because product differentiation gives them something to promote
Perfectly competitive firms (identical products) and monopolists (no competitors) have little incentive to advertise
Advertising can signal quality: a firm willing to spend heavily on advertising signals that it has a product worth buying repeatedly
Brand names serve a similar function, giving firms an incentive to maintain quality
Oligopolists are best off cooperating (act like a joint monopolist: restrict output, charge high prices, split the profit)
But each individual firm has an incentive to cheat (produce more than its agreed share to capture a larger slice of the market)
This tension makes the monopoly outcome unstable without enforcement
Two well owners, zero marginal cost, downward-sloping demand for water
Monopoly outcome: produce 60 gallons total (30 each), charge the monopoly price
But each has an incentive to produce 40 gallons instead of 30, because even though total profit falls, their individual profit rises
Nash equilibrium: both produce 40 gallons, total output = 80, price and profit lower than monopoly
Greater than monopoly output but less than competitive output
Price is lower than monopoly price but higher than the competitive price (which equals MC)
As the number of sellers grows, the oligopoly outcome approaches the competitive outcome
Each firm weighs two things when considering whether to produce one more unit:
Output effect: selling one more unit at the current price raises revenue
Price effect: one more unit on the market pushes the price down on all units, reducing revenue
As the oligopoly grows larger, the price effect shrinks (each firm's share is smaller), and the market behaves more competitively
Two prisoners, each better off confessing regardless of what the other does (confessing is a dominant strategy)
Both confess, landing on an outcome worse for both than if they had cooperated (both silent)
This is the same logic that drives oligopolists to cheat on cartel agreements
Never play a strictly dominated strategy
Rational play by rational players can lead to Pareto-inferior outcomes (the road to bad outcomes is paved with rational intentions)
Figure out your own and others' payoffs to find your best choice
If you do not have a dominated strategy, put yourself in the other player's shoes to predict their move
Repeated games make cooperation more sustainable: firms can threaten punishment (tit-for-tat strategy)
Tit-for-tat: start by cooperating, then do whatever the other player did last time. Friendly, punishes defection, forgives if the other returns to cooperation.
Larger groups find cooperation harder to maintain
Antitrust laws (Sherman Act, Clayton Act) make price-fixing agreements criminal conspiracies and allow triple damages for victims of anti-competitive behaviour
These laws also block mergers that would create excessive market power
Controversial areas: resale price maintenance (manufacturers setting retail prices) and predatory pricing are debated among economists
Students often think monopolistic competition is the same as monopoly. The critical difference is free entry. Monopolistically competitive firms have market power in the short run but earn zero economic profit in the long run, just like competitive firms.
Students sometimes think a firm in monopolistic competition takes the price as given. It does not. It faces a downward-sloping demand curve and chooses its own price, like a monopolist.
Students often confuse Nash equilibrium with the best possible outcome. Nash equilibrium is the best each player can do given what others are doing. It can be Pareto-inferior, as the prisoners' dilemma shows.
Students sometimes think oligopolists always collude. Collusion is unstable because each member has an incentive to cheat. Antitrust laws also make explicit collusion illegal.
The long-run equilibrium graph for monopolistic competition (demand tangent to ATC) is heavily tested. Be able to draw it and explain why tangency means zero profit.
Know the two inefficiencies of monopolistic competition: excess capacity and markup over MC.
Prisoners' dilemma payoff matrices appear frequently. Be able to identify dominant strategies, Nash equilibria, and Pareto-inferior outcomes.
Understand why oligopoly output is between the monopoly and competitive levels, and that it approaches the competitive level as the number of firms grows.
Antitrust policy: know the difference between horizontal and vertical mergers, and why horizontal mergers face more scrutiny.
Real-world application: OPEC is the classic cartel example. Member nations agree to restrict oil output to keep prices high, but each country has an incentive to pump more than its quota. The cartel's effectiveness depends on enforcement, and member countries frequently cheat. Airlines and mobile phone providers are good examples of oligopolies where a few firms set prices strategically.
True or false: A monopolistically competitive firm earns positive economic profit in the long run. (False, free entry drives it to zero)
Fill in the blank: In long-run monopolistic competition equilibrium, the demand curve is ____ to the ATC curve. (tangent)
True or false: In a Nash equilibrium, both players are always as well off as possible. (False, Nash equilibrium can be Pareto-inferior)
Fill in the blank: A dominant strategy is best for a player regardless of ____. (what the other players do)
True or false: As the number of firms in an oligopoly grows, the market outcome approaches perfect competition. (True)
Q: Which condition does NOT describe a firm in a monopolistically competitive market?
a) It sells a differentiated product. b) It takes prices as given by market conditions. c) It can freely enter or exit. d) It faces a downward-sloping demand curve.
A: (b). A monopolistically competitive firm is a price maker, not a price taker. It faces a downward-sloping demand curve because its product is differentiated.
Q: In the long run, a monopolistically competitive firm earns zero economic profit. Why doesn't it just raise its price?
A: Raising the price would move the firm above its demand curve, so it would sell fewer units. With free entry, competitors offering similar products prevent any single firm from sustaining a price above ATC. The demand curve has already shifted left (from entry) until it is just tangent to ATC.
Q: Jack and Jill each own a well. They agree to produce 30 gallons each (the monopoly split). Jack is tempted to produce 40. Using the prisoners' dilemma framework, explain why the agreement is unstable.
A: If Jill sticks to 30, Jack earns more by producing 40 (larger share of the market). If Jill also defects to 40, Jack is still better off at 40 than at 30. Producing 40 is Jack's dominant strategy. Jill faces the same logic. Both defect, and total output rises above the monopoly level. Profit for each is lower than under the agreement, but higher than if they had been the only one to cooperate.
Q: Why does the oligopoly price fall toward marginal cost as the number of sellers increases?
A: With more sellers, each firm's share of the market is smaller, so the price effect of producing one more unit shrinks. Each firm cares less about pushing the price down on its existing sales and more about the revenue from the extra unit. In the limit, each firm acts as a price taker and produces where P = MC.
Q: Compare long-run equilibrium in perfect competition and monopolistic competition. Name two differences.
A: (1) Excess capacity: monopolistically competitive firms produce below efficient scale (below minimum ATC), while perfectly competitive firms produce at minimum ATC. (2) Markup: monopolistically competitive firms charge P > MC, while perfectly competitive firms charge P = MC.
Monopolistic competition borrows its short-run analysis from monopoly (Ch. 15) and its long-run zero-profit result from perfect competition (Ch. 14). Oligopoly connects to game theory, which appears again whenever strategic interaction matters (trade negotiations, environmental agreements, military strategy). The prisoners' dilemma framework also applies to common-resource problems and public goods (Ch. 11). Antitrust policy (Ch. 17) connects to welfare economics: the goal is to push markets closer to the competitive outcome, where total surplus is maximised.
Monopolistic competition, oligopoly, product differentiation, excess capacity, markup over marginal cost, game theory, prisoners' dilemma, Nash equilibrium, dominant strategy, collusion, cartel, duopoly, tit-for-tat, antitrust, Sherman Act, Clayton Act, concentration ratio, imperfect competition, strategic interaction, OPEC, payoff matrix, Pareto inferior, Mankiw Ch. 16, Mankiw Ch. 17