Chapter 20 — Monopolistic Competition and Oligopoly

The Messy Middle

Chapters 18 and 19 covered the two polar extremes of market structure: perfect competition (many firms, no market power) and pure monopoly (one firm, maximum market power). Most real markets are neither. They live in between — in what we'll call the "messy middle."

The messy middle has two forms:

Monopolistic competition: many firms, each selling a slightly different product, with free entry and exit. Think: restaurants, clothing brands, apps, small service businesses. Each firm has some market power (because its product is unique) but faces strong competition (because many alternatives exist).

Oligopoly: a few large firms that dominate the market. Think: airlines, wireless carriers, auto manufacturers, soft drinks, search engines (sort of). Each firm has significant market power, and the key analytical challenge is strategic interaction — each firm's best decision depends on what the other firms do.

This chapter covers both, with the Millbrook restaurant scene as the running example for monopolistic competition and the airline/OPEC story for oligopoly.

20.1 Monopolistic competition

The four features

Monopolistic competition has four properties:

1. Many firms. Enough that no single firm dominates the market. A city might have 200 restaurants, 50 clothing stores, or 10,000 apps in a category.

2. Differentiated products. Each firm's product is slightly different from its competitors'. Luigi's pizza is not identical to Domino's, which is not identical to the campus cafeteria's. The differences can be real (different recipes, different quality) or perceived (branding, ambiance, location).

3. Free entry and exit. New firms can enter (open a restaurant, launch an app) and existing firms can exit (close down, pivot) without large barriers. This is similar to perfect competition and unlike monopoly.

4. Some market power. Because products are differentiated, each firm faces a downward-sloping demand curve — not a horizontal one as in perfect competition. Luigi's can raise prices slightly without losing all customers (some people specifically want Luigi's pizza, not just any pizza). But the demand curve is fairly elastic because there are many substitutes.

Short-run equilibrium

In the short run, a monopolistically competitive firm looks a lot like a monopolist: it faces a downward-sloping demand curve, MR < P, and it maximizes profit where MR = MC. If demand is strong, the firm can earn positive economic profit.

Long-run equilibrium: zero economic profit (again)

Here's the key: because entry is free, positive economic profit attracts new firms. New restaurants open on the same block. New apps launch in the same category. Each entrant takes some customers from existing firms, shifting each firm's demand curve to the left. Entry continues until economic profit is driven to zero — just as in perfect competition.

In long-run monopolistic-competition equilibrium: - P > MC (the firm has some market power; it charges above marginal cost) - P = ATC (zero economic profit; the demand curve is tangent to the ATC curve) - Excess capacity: the firm produces at a point to the left of minimum ATC — it could produce more at lower average cost but doesn't because it would have to cut prices. This "excess capacity" is the price of variety: society gets many differentiated products (which consumers value) at the cost of each firm not producing at its most efficient scale.

The Millbrook restaurant scene

Downtown Millbrook has about 60 restaurants within walking distance of the MSU campus. They range from fast-food chains (McDonald's, Subway) to independent sit-down restaurants (the Riverside Bistro where Maya works, a Vietnamese pho shop, a BBQ joint, two pizza places, a taco truck, a coffee shop with sandwiches, a vegan café, and dozens more).

Each restaurant sells a differentiated product: different food, different atmosphere, different prices, different locations. No two are identical. This is monopolistic competition in its natural habitat.

Short-run dynamics: when a new restaurant opens (say, a ramen shop that's the first of its kind in Millbrook), it initially attracts a lot of curiosity. Lines are long. Prices are high relative to cost. The owner earns positive economic profit.

Long-run dynamics: within 12–18 months, a second ramen option opens (either another restaurant or an existing restaurant that adds ramen to its menu). Some customers shift. The original shop's demand falls. Profit shrinks. If the market can't sustain two ramen shops profitably, one of them closes. If it can, both survive at zero economic profit.

What consumers get: variety. Millbrook's 60 restaurants offer a much wider range of dining options than a perfectly competitive market (where every restaurant would serve identical food) or a monopoly (where one restaurant would serve everyone). The cost of this variety is that no single restaurant operates at its most efficient scale — but most consumers consider the trade-off worthwhile.

20.2 Oligopoly

The defining feature: strategic interaction

An oligopoly is a market dominated by a few large firms. The critical analytical feature is strategic interaction: each firm's optimal decision depends on what it expects the other firms to do.

In perfect competition, firms don't think about each other (each is too small to matter). In monopoly, there's only one firm to think about. In oligopoly, firms are constantly watching each other: if I lower my price, will my competitor match? If I launch a new product, will they copy it? If I invest in advertising, will they respond with their own ad campaign?

This is the domain of game theory — the mathematical study of strategic interaction.

The prisoner's dilemma

The most famous model in game theory is the prisoner's dilemma. It captures the central tension of oligopoly: each firm has an individual incentive to defect from cooperation, even though all firms would be better off cooperating.

Setup: Two firms (Firm A and Firm B) each choose between two strategies: "cooperate" (keep prices high) or "defect" (cut prices to steal customers).

Payoff matrix:

The logic: - If B cooperates, A's best response is to defect ($15M > $10M). - If B defects, A's best response is to defect ($5M > $2M). - Defection is A's dominant strategy — the best choice regardless of what B does. - By the same logic, defection is B's dominant strategy. - Both defect. Both earn $5M.

But notice: if both had cooperated, both would have earned $10M. The individually rational choice (defect) produces a collectively worse outcome ($5M each) than cooperation ($10M each). This is the dilemma.

Nash equilibrium (named after John Nash, Nobel 1994): a set of strategies where no player can improve their payoff by unilaterally changing their strategy. In the prisoner's dilemma, (defect, defect) is the Nash equilibrium.

Why cartels are unstable

A cartel is an agreement among oligopolists to cooperate — to keep prices high, limit output, or divide the market. The most famous cartel is OPEC (the Organization of the Petroleum Exporting Countries), which periodically agrees to limit oil production to keep prices high.

Cartels face the prisoner's dilemma in exactly the form above. Each member of the cartel has an individual incentive to cheat: if all other members are restricting output and keeping prices high, one member can secretly produce more and sell at the high price — earning $15M instead of $10M. But if everyone cheats, the price collapses and everyone earns $5M.

OPEC's history is a recurring cycle of cooperation and defection: - The cartel agrees to production quotas - For a while, members comply and prices stay high - Eventually, some members (historically: Venezuela, Nigeria, Iraq) exceed their quotas - Other members notice and respond by also exceeding quotas - The price falls - OPEC reconvenes and negotiates new quotas - The cycle repeats

The cartel has been partially successful — oil prices are higher than they would be without OPEC — but the cheating problem is real and persistent. The prisoner's dilemma predicts exactly this pattern.

Real-world oligopolies

Airlines. The U.S. domestic airline industry is dominated by four carriers (American, United, Delta, Southwest) that together control about 80% of the market. They engage in strategic pricing, capacity decisions, and route choices that reflect oligopolistic interaction. When one airline raises prices on a route, others often follow (tacit collusion — not illegal but economically similar to a cartel). When one airline cuts prices, others match (competitive response). The industry oscillates between periods of higher prices (when tacit cooperation holds) and price wars (when someone defects).

Wireless carriers. The U.S. market has three major carriers (AT&T, Verizon, T-Mobile) plus some smaller players. Pricing is highly strategic. When T-Mobile launched its "Un-carrier" strategy (eliminating contracts, offering lower prices), AT&T and Verizon responded with matching offers. Strategic interaction drives the pricing, not just cost-plus-markup.

Soft drinks. Coca-Cola and PepsiCo are a duopoly (two firms). They compete aggressively on advertising, product innovation, and pricing. Neither has been able to drive the other out. The "Cola Wars" are a textbook example of oligopolistic rivalry.

20.3 Comparing the four market structures

20.4 Where this is going

Chapter 21 — Labor Markets — is the last chapter in Part IV and one of the most important in the book. It applies the market-structure framework to the labor market, introduces monopsony (the buying-side equivalent of monopoly), and gives the minimum-wage debate its deepest treatment. By the end of Part IV, you'll have the full set of tools for analyzing any market: from competitive to monopolistic to the labor market where you'll spend most of your working life.

Key terms recap: monopolistic competition — many firms, differentiated products, free entry, some market power oligopoly — few firms, strategic interaction, significant barriers product differentiation — each firm's product is slightly different game theory — the analysis of strategic interaction Nash equilibrium — a set of strategies where no player can improve by changing unilaterally prisoner's dilemma — individual rationality leads to a collectively worse outcome dominant strategy — the best choice regardless of what the other player does cartel — an agreement among firms to cooperate (restrict output, fix prices) OPEC — the most famous real-world cartel

Themes touched: Markets power+imperfect (the messy middle is where most markets live), Tradeoffs (variety vs. efficiency in monopolistic competition; cooperation vs. defection in oligopoly), Behavioral (brand loyalty, advertising effects), Affects daily life (restaurants, airlines, wireless, soft drinks).