Intermediate Microeconomic Theory | Perloff 5th Ed., Chapter 14 (Sections 14.1–14.2)
Tags: oligopoly, monopolistic competition, cartel, collusion, market structure, Nash equilibrium, barriers to entry, antitrust, price fixing, OPEC
Difficulty: Intermediate | Prerequisites: Chapter 11 (Monopoly), Chapter 13 (Game Theory and Nash Equilibrium)
This is the first half of a chapter that sits between monopoly and perfect competition, covering the messy middle ground where most real markets live. You need to understand monopoly pricing (MR = MC) and the basics of Nash equilibrium before diving in. These sections set up the landscape of market structures (monopoly, oligopoly, monopolistic competition, perfect competition) and then focus on what happens when firms try to cooperate through cartels.
Market outcomes (price, quantity, profit) depend on how many firms there are, whether entry is blocked, and whether firms coordinate or compete independently. Cartels try to act like a monopoly to earn monopoly profits, but individual members always have an incentive to cheat, and antitrust laws in most countries make explicit collusion illegal.
Oligopoly
A market structure with a small number of firms and substantial barriers to entry. Each firm's decisions affect its rivals, so strategic interaction matters. Think of it as the zone between monopoly and competition where firms have to watch each other.
Monopolistic competition
A market structure in which firms have market power (can set price above marginal cost) but free entry drives long-run economic profit to zero. In simple terms, firms have some pricing power because of differentiation or few rivals, but new entrants keep eating into profits until nobody is making money above their costs.
Cartel
A group of firms that collude, coordinating their pricing or output decisions. In simple terms, a cartel is firms teaming up to act like a single monopoly so they can all charge higher prices.
Collusion
When firms coordinate their actions (setting prices or quantities together) rather than competing independently. Can be explicit (direct agreements) or tacit (signalling through actions without direct communication).
Tacit collusion
Firms coordinating their behaviour without explicit communication, for example by one firm raising its price and waiting to see if rivals follow. Not necessarily illegal, unlike explicit collusion.
Nash equilibrium (in oligopoly context)
A set of actions (quantities or prices) such that, holding the actions of all other firms constant, no firm can obtain a higher profit by choosing a different action. This is the foundational equilibrium concept for all three oligopoly models in this chapter.
Market power
The ability to profitably set price above marginal cost. All market structures except perfect competition have some degree of market power.
Barriers to entry
Factors that prevent or discourage new firms from entering a market, such as government licences, patents, high fixed costs, or control of essential resources. Barriers keep the number of firms small in oligopolies and monopolies.
Price fixing
A per se violation of antitrust law in which firms explicitly agree to set prices jointly. No mitigating justifications are accepted.
Lerner Index
(p - MC) / p. A measure of market power. Ranges from 0 (perfect competition, price equals marginal cost) toward 1 (high market power). The fewer firms in a market, the larger the Lerner Index tends to be.
The key properties that distinguish market structures are the number of firms, entry conditions, long-run profit, ability to set price, whether strategy depends on rivals, and whether products are differentiated.
Monopoly: One firm, no entry, positive long-run profit possible, price setter, no rivals to consider, single product.
Oligopoly: Few firms, limited entry, positive long-run profit possible, price setter, must consider rivals' behaviour, products may be differentiated.
Monopolistic competition: Few or many firms, free entry, zero long-run economic profit, price setter, must consider rivals' behaviour, products may be differentiated.
Perfect competition: Many firms, free entry, zero long-run economic profit, price taker, no need to consider individual rivals, undifferentiated products.
Insurmountable barriers to entry (government licences, patents, control of resources) keep the number of firms small. In monopolistic and perfectly competitive markets, entry occurs until no new firm can profitably enter, so long-run economic profit is zero.
All noncompetitive market structures involve downward-sloping demand curves, which give firms the ability to set price above marginal cost. This creates a market failure (too little consumption relative to the efficient level). The fewer the firms, the higher the price tends to be.
Oligopolistic and monopolistically competitive firms must pay attention to rivals' behaviour. A monopoly has no rivals. A perfectly competitive firm ignores individual rivals because the market price tells it everything it needs to know.
When a firm acts independently, it considers only how its own output affects its own profit. It does not care that increasing its output lowers other firms' profits. A cartel internalises these effects: it accounts for how any member's output change affects all members. By restricting total output to the monopoly level, the cartel raises price and earns the highest possible collective profit.
Each member produces q_m = Q_m / n (monopoly output divided equally). At the cartel price, each member's marginal cost is below the market price, which creates the temptation to cheat.
Cartels fail for three main reasons:
Legal prohibition. In most developed countries, cartels are illegal. The Sherman Antitrust Act (1890), the Clayton Act (1914), and the Federal Trade Commission Act (1914) in the US, plus similar laws in Canada, Australia, New Zealand, and the EU, make price fixing a per se violation.
Insufficient market control. A cartel that does not control enough of the market cannot raise prices meaningfully. The copper producers' cartel failed repeatedly because it controlled less than a third of world production.
Cheating incentives. At the cartel output, each firm's marginal cost is below the market price. A firm can increase its profit by secretly producing more than its quota, selling at the high cartel price. If enough firms cheat, the price falls and the cartel collapses.
Detection methods:
Inspecting each other's accounts
Dividing the market by region or customer (makes cheating visible)
Using industry organisations that track market share by firm
"Low price guarantee" ads that effectively recruit customers to report rivals' price cuts
Enforcement methods:
Most-favoured-customer clauses: if you cut price for one buyer, you must cut it for all previous buyers, making cheating expensive
Threats of violence (in extreme cases)
Government-facilitated enforcement (e.g., baseball's antitrust exemption, the Quebec maple syrup cartel's legal authority to seize production)
Governments sometimes help create cartels (Quebec maple syrup federation, pre-deregulation airline cartels under IATA) and sometimes fight them (DOJ, FTC, European Commission). Government transparency in bidding can inadvertently help cartels monitor cheating.
When antitrust laws prevent collusion, firms may try to merge instead. Whether a merger raises or lowers welfare depends on the trade-off between reduced competition (bad for consumers) and increased efficiency (good for everyone). Governments review mergers and may block those with anticompetitive effects.
OPEC is the most prominent international cartel, organised by countries rather than firms, and thus operates legally.
No-poaching agreements in Silicon Valley (Apple, Google, Intel, Adobe and others) functioned as a buyers' cartel to suppress engineers' wages. Settlements totalled hundreds of millions of dollars.
Quebec maple syrup is cartelised by provincial law. The federation sets quotas, requires all sales through the federation, posts guards on suspected cheaters' farms, and seizes production from non-compliant producers.
"Cartels are always illegal." They are not. International cartels organised by countries (like OPEC) operate legally. Some domestic cartels have government exemptions (e.g., Major League Baseball in the US). Tacit collusion, where firms coordinate through signalling rather than explicit agreements, is not necessarily illegal.
"A cartel benefits all members equally." In practice, members have strong incentives to cheat. The cartel benefits you most if everyone else restricts output while you secretly increase yours.
"Oligopoly firms always collude." Many do not. The next sections cover three models (Cournot, Stackelberg, Bertrand) of non-collusive oligopoly behaviour. Whether firms collude depends on detection ability, penalties, number of firms, and barriers to entry.
"Mergers are always bad for consumers." Some mergers create efficiency gains that outweigh the reduction in competition. US airline mergers in the late 2000s and early 2010s actually reduced fares on routes where the merging airlines had previously competed.
⚠️ Be able to explain why a cartel member has an incentive to cheat using Figure 14.1 logic: at the cartel output, the firm's MC is below the cartel price, so expanding output is profitable if you can treat the price as given.
⚠️ Know the properties table (Table 14.1) and be able to identify which market structure applies to a given real-world example.
⚠️ Understand the difference between explicit collusion (illegal per se) and tacit collusion (potentially legal).
⚠️ Be ready to explain why fewer firms in a market typically means higher prices and more market power (higher Lerner Index).
⚠️ The connection between barriers to entry and cartel stability is a common exam point: fewer firms means easier detection of cheating and easier enforcement.
True or False: In a perfectly competitive market, firms are price setters. (False, they are price takers.)
Fill in the blank: A group of firms that coordinate their pricing or output decisions is called a __________. (Cartel.)
True or False: A cartel maximises its collective profit by producing at the competitive output level. (False, it produces at the monopoly output level.)
True or False: Tacit collusion is always illegal under US antitrust law. (False, only explicit agreements are per se illegal.)
Fill in the blank: The Lerner Index equals __________. ((p - MC) / p.)
Q: Why does each member of a cartel have an incentive to cheat on the agreement?
A: At the cartel output, each member's marginal cost is below the cartel price. If a single firm expands output while others stick to their quotas, it sells the extra units at roughly the cartel price (because one firm's extra output has little effect on the market price). The extra revenue exceeds the extra cost on every unit between q_m and q*, so cheating is profitable.
Q: Why are cartels more likely to succeed when there are fewer firms in the market?
A: With fewer firms, it is easier to detect cheating (each firm's market share is large, so any deviation is noticeable) and easier to impose penalties. Coordination is also simpler with fewer parties. Cartels with large numbers of firms are relatively rare.
Q: Explain the difference between oligopoly and monopolistic competition.
A: Both involve strategic interdependence and downward-sloping demand curves. The key difference is entry conditions: oligopolies have barriers to entry that allow positive long-run profits, while monopolistic competition has free entry that drives long-run economic profit to zero.
Q: A market has an inverse demand curve p = 100 - 2Q and four identical firms, each with MC = 20. If the firms form a profit-maximising cartel with equal output shares, how much does each firm produce?
A: The cartel acts like a monopoly. Set MR = MC: MR = 100 - 4Q = 20, so Q = 20. Each firm produces q = 20/4 = 5 units. The cartel price is p = 100 - 2(20) = 60.
This material connects directly to Chapter 11 (Monopoly), since a cartel that fully colludes produces the monopoly output. It also builds on Chapter 13 (Game Theory), as Nash equilibrium is the foundational concept for all three oligopoly models. The cheating incentive in cartels is essentially a Prisoners' Dilemma: each firm is better off cheating regardless of what others do, but everyone cheating leaves all firms worse off than if everyone cooperated.
oligopoly, duopoly, monopolistic competition, cartel, collusion, tacit collusion, explicit collusion, price fixing, Nash equilibrium, market structure, barriers to entry, Lerner Index, market power, antitrust, Sherman Act, Clayton Act, FTC, OPEC, merger, no-poaching agreement, maple syrup cartel, per se violation, most-favoured-customer clause, residual demand