Lesson 12: Monopolistic Competition & Oligopoly

Warm-Up

Your town has nineteen coffee shops. Each is a tiny bit different — location, vibe, oat-milk selection — so each can charge a little more or less than rivals without losing everyone. That's monopolistic competition. Meanwhile, two rideshare apps watch each other like poker players: if one cuts prices, the other must respond within hours. That's oligopoly, where strategy — not just cost curves — decides outcomes, and where the AP exam deploys its favorite tool: the payoff matrix.

Core Concept

Monopolistic competition: the hybrid

Conditions: many sellers, differentiated products (the monopolistic part), low barriers to entry (the competitive part), heavy non-price competition (advertising, branding, quality).

Differentiation → each firm faces its own downward-sloping but highly elastic demand curve. So the graph machinery is monopoly's (D with MR below it, two-step pricing), but the long-run logic is perfect competition's (entry kills profit).

Short run: identical to monopoly analysis — Q at MR = MC, P from demand, profit/loss from P vs. ATC.

Long run: profits attract entrants selling close substitutes → each incumbent's demand curve shifts left (and flattens) → profits shrink → entry stops when economic profit = 0. Graphically, demand ends up tangent to ATC exactly at the MR = MC output.

[GRAPH: Monopolistic competition, long run. Downward D tangent to U-shaped ATC at output q, with MR = MC directly below the tangency point. Price = ATC at q (zero economic profit). Minimum of ATC lies to the RIGHT of q*, gap labeled "excess capacity". P > MC gap visible.]

Long-run scorecard: - Zero economic profit (like perfect competition) - Not allocatively efficient: P > MC — some valuable units go unproduced - Not productively efficient: production left of minimum ATC — the firm has excess capacity (it could produce more at lower average cost, but demand for its particular variety is too small) - The inefficiencies are the "price" society pays for variety

Oligopoly: few firms, mutual interdependence

Conditions: a few large firms dominate (high concentration), significant barriers to entry, products standardized (steel) or differentiated (phones, airlines). Defining feature: mutual interdependence — each firm's best move depends on rivals' moves. That's why we model it with game theory instead of a single cost-curve graph.

Game theory toolkit

A payoff matrix shows each player's outcome for every strategy combination:

Two airlines set fares High or Low. Payoffs (Skyway, Jetgo) in millions:

Reading protocol (do this mechanically): 1. Skyway's best responses: If Jetgo plays High → Skyway compares 10 (High) vs. 14 (Low) → Low. If Jetgo plays Low → 2 vs. 5 → Low. Low is best no matter what → Low is Skyway's dominant strategy. 2. Jetgo (symmetric): High → 10 vs. 14 → Low; Low → 2 vs. 5 → Low → dominant strategy Low. 3. Nash equilibrium: a cell where neither player can gain by unilaterally switching → (Low, Low) = (5, 5).

Definitions: - Dominant strategy: best choice regardless of the rival's choice. (A player may have none.) - Nash equilibrium: strategy pair where each player is doing their best given the other's choice. Check every cell: would either player deviate? A game can have one, several, or (in pure strategies) no Nash equilibrium.

This game is a prisoner's dilemma: both prefer (10, 10), but individual incentives drag them to (5, 5). Self-interest defeats cooperation.

Collusion and why it cracks

Collusion (illegal in the U.S.) or a cartel (e.g., OPEC): firms agree to act like a shared monopoly — restrict output, raise price, split profits. The payoff matrix explains both its appeal and its fragility: at the collusive cell (High, High), each firm can gain by secretly cheating (cut price, grab share: 10 → 14). Since both face that temptation, cartels tend to unravel toward the Nash equilibrium — especially with many members, secret price cuts, or no repeated-game punishment.

Repeated interaction softens the dilemma: future retaliation makes cheating costlier, which is why real oligopolies often sustain tacit cooperation (price leadership) without any illegal agreement.

Worked Examples

Example 1 (easy): Long-run destination

Food trucks in Austin earn big profits this year. Barriers are trivial. Predict the long run for a typical incumbent, in words and on a graph.

Solution: Entry of similar trucks → incumbent's demand shifts left/flattens → profits fall → long-run equilibrium where D is tangent to ATC at the MR = MC output: zero economic profit, P > MC, output below min-ATC (excess capacity).

Interpretation: Say "tangent" on the FRQ — it's the graphical signature graders look for.

Example 2 (medium): No dominant strategy for one player

Payoffs (Row, Column):

Find dominant strategies and Nash equilibria.

Solution: - Row: vs. Left → 6 > 4 → Up; vs. Right → 2 < 3 → Down. No dominant strategy for Row. - Column: vs. Up → 4 > 3 → Left; vs. Down → 2 < 5 → Right. No dominant strategy for Column. - Nash check each cell: (Up, Left): Row 6 (switch → 4, stays), Column 4 (switch → 3, stays) → Nash. (Down, Right): Row 3 (switch → 2, stays), Column 5 (switch → 2, stays) → Nash. (Up, Right): Row would switch (2→3). (Down, Left): Column would switch (2→4). - Two Nash equilibria: (Up, Left) and (Down, Right).

Interpretation: Dominant strategies are sufficient but not necessary for Nash. Always run the cell-by-cell deviation check.

Example 3 (AP-style): Cartel cheating

Two oil producers agree to restrict output. Payoffs (millions): both restrict (50, 50); both flood (30, 30); one floods while the other restricts (65 to the cheater, 20 to the restrainer). (i) Does each have a dominant strategy? (ii) Nash equilibrium? (iii) Why does the agreement fail?

Solution: - (i) Restrainer's view: if rival restricts → 65 (flood) > 50 (restrict); if rival floods → 30 > 20 → Flood is dominant for both. - (ii) (Flood, Flood) = (30, 30). - (iii) Each producer gains by cheating regardless of the other's move; without enforcement, individual incentive overrides the joint interest — the classic prisoner's dilemma of cartels.

Interpretation: "Cartel + payoff matrix" is a recurring FRQ 3. The answer is always some version of this three-step.

Common Mistakes

1. Drawing monopolistic competition's long run with profit. Long run = tangency, P = ATC, zero economic profit. Profit rectangles belong to the short run only.
2. Claiming long-run monopolistic competitors are efficient. Zero profit ≠ efficiency: P > MC (allocative ✗) and output < min ATC (productive ✗, excess capacity).
3. Reading the wrong payoff. Convention: the row player's payoff is listed first. Confirm which number belongs to whom before anything else.
4. Equating Nash equilibrium with the best joint outcome. The dilemma's whole point: (5,5) is Nash while (10,10) is better for both. Nash = no unilateral improvement.
5. Assuming every player has a dominant strategy. Check both players separately; Example 2 has none, yet two Nash equilibria exist.
6. Forgetting oligopoly's defining trait. It's interdependence — not firm count per se. If the question stresses firms reacting to each other, it's oligopoly.

Practice Problems

Use this matrix for questions 5–7. Two firms choose Advertise or Don't. Payoffs (Alpha, Beta) in millions:

10. (FRQ-style) Bella's Bakery operates in the monopolistically competitive market for artisan cupcakes and is currently earning positive economic profit. (a) Draw a correctly labeled graph for Bella's showing D, MR, MC, ATC, profit-maximizing quantity q₁, price P₁, and the profit area. (b) Explain what happens to Bella's demand curve as new bakeries enter, and why entry occurs. (c) Draw or describe Bella's long-run equilibrium. Identify the relationship between D and ATC, and the firm's economic profit. (d) In long-run equilibrium, is Bella's productively efficient? Allocatively efficient? Explain both.