Chapter 10 – Monopolistic Competition & Oligopoly
Most real-world markets are neither perfectly competitive nor pure monopolies. They fall in between — imperfectly competitive. This chapter explores two such structures: monopolistic competition (many firms selling differentiated products) and oligopoly (a few large firms dominating an industry).
Table of Contents
Glossary of Key Terms
| Term | Definition | |——|———–| | Monopolistic competition | Many firms competing to sell similar but differentiated products | | Differentiated product | A product consumers perceive as distinct from competitors’ products | | Product differentiation | Making consumers think your product is different (physical, location, intangible, perceived) | | Oligopoly | A market dominated by a few large firms | | Duopoly | An oligopoly with only two firms | | Collusion | Firms acting together to reduce output and keep prices high | | Cartel | A formal agreement among firms to collude (produce monopoly output, charge monopoly price) | | Game theory | Branch of math analyzing strategic decisions where payoffs depend on others’ choices | | Prisoner’s dilemma | A game where both sides gain more from cooperation, but self-interest drives them apart | | Dominant strategy | The strategy that is best regardless of what the other player does | | Kinked demand curve | Perceived demand when competitors match price cuts but not price increases | | Imperfectly competitive | Markets between the extremes of perfect competition and monopoly |
1. Monopolistic Competition
1.1 What Is Monopolistic Competition?
Monopolistic competition involves many firms competing against each other, but selling products that are distinctive in some way. Each firm has a “mini-monopoly” on its particular style, flavor, or brand — but must compete with many substitutes.
Examples: restaurants (600,000+ in the U.S.), clothing stores, breakfast cereals, golf balls, laundry detergent.
1.2 Four Sources of Product Differentiation
| Source | Description | Example |
|---|---|---|
| Physical aspects | Design, features, ingredients | Unbreakable bottle, non-stick surface, extra spicy |
| Location | Where the product is sold | Gas station at a busy intersection |
| Intangible aspects | Service, guarantees, reputation | Free delivery, money-back guarantee |
| Perception | Advertising creating brand loyalty | Consumers can’t taste the difference blindfolded, but prefer certain brands |
1.3 The Perceived Demand Curve
| Market Structure | Demand Curve | Price Power |
|---|---|---|
| Perfect competition | Horizontal (perfectly elastic) | None — price taker |
| Monopolistic competition | Downward-sloping, relatively elastic | Some — price maker |
| Monopoly | Downward-sloping (market demand) | Substantial — constrained only by demand |
A monopolistic competitor’s demand curve is more elastic than a monopolist’s because substitutes exist. Raise your price, and some customers switch to competitors. But it’s not perfectly elastic — your product is differentiated, so not all customers leave.
1.4 Choosing Price and Quantity
The process is identical to a monopolist:
- Find the quantity where MR = MC
- Go up to the demand curve to find the price at that quantity
- Calculate profit: $(P - AC) \times Q$
Authentic Chinese Pizza
| Q | Price | TR | MR | TC | MC | AC |
|---|---|---|---|---|---|---|
| 10 | $23 | $230 | $23 | $340 | $34 | $34 |
| 20 | $20 | $400 | $17 | $400 | $6 | $20 |
| 30 | $18 | $540 | $14 | $480 | $8 | $16 |
| 40 | $16 | $640 | $10 | $580 | $10 | $14.50 |
| 50 | $14 | $700 | $6 | $700 | $12 | $14 |
| 60 | $12 | $720 | $2 | $840 | $14 | $14 |
| 70 | $10 | $700 | −$2 | $1,020 | $18 | $14.57 |
MR = MC = $10 at Q = 40. Price = $16. Profit = ($16 − $14.50) × 40 = $60.
2. Entry, Exit, and Long-Run Equilibrium
2.1 Profits Attract Entry
Unlike monopoly, monopolistic competition has low barriers to entry. When firms earn economic profits:
- New firms enter with similar (differentiated) products
- The original firm’s demand shifts left (fewer customers at each price)
- MR shifts left → profit-maximizing quantity falls
- Entry continues until P = AC → zero economic profit
Conversely, when firms have losses → firms exit → demand shifts right → losses shrink to zero.
2.2 Long-Run Zero Profit
In long-run equilibrium, monopolistic competition reaches zero economic profit — just like perfect competition. But the way it gets there is different:
- In perfect competition: P = min ATC (tangent at the bottom of AC curve)
- In monopolistic competition: P = AC on the downward-sloping portion of the AC curve (tangent above the minimum)
This means monopolistic competitors produce at a higher average cost than perfectly competitive firms.
Diagram — Long-Run Equilibrium Tangency (Zero Profit):
Key insight: The demand curve is tangent to ATC on its downward-sloping portion — not at the minimum. This creates excess capacity: the firm could produce more at lower ATC, but demand is insufficient.
Excess Capacity Calculation:
If a monopolistic competitor has $ATC = \frac{200}{Q} + 5 + 0.2Q$ and produces at Q = 20 (long-run tangency):
$ATC(20) = 10 + 5 + 4 = $19$
Minimum ATC: Set $\frac{dATC}{dQ} = 0$: $-\frac{200}{Q^2} + 0.2 = 0 \implies Q^* = \sqrt{1000} \approx 31.6$
$ATC(31.6) = 6.32 + 5 + 6.32 = $17.64$
Excess capacity = 31.6 − 20 = 11.6 units. The firm produces 37% below its most efficient scale.
3. Efficiency and the Benefits of Variety
3.1 Not Productively Efficient
Monopolistic competitors do not produce at the minimum of their average cost curve — they produce on the downward-sloping portion. More output at lower AC would be possible, but the firm’s limited demand prevents it.
3.2 Not Allocatively Efficient
Since MR = MC but P > MR (downward-sloping demand), we get P > MC. Society values additional units more than they cost to produce, but the firm doesn’t produce them.
3.3 The Benefit: Variety
The trade-off for inefficiency is variety and innovation:
- Consumers get many styles, flavors, and qualities to choose from
- Firms constantly innovate to attract customers
- Blue jeans, white shirts, and plain spaghetti for everyone would be efficient but boring
The cost of this variety: higher prices per unit than under perfect competition. Whether society produces the “optimal amount of variety” is an unresolved debate.
3.4 The Role of Advertising
- U.S. spending on advertising: ~$180 billion/year (2014)
- Advertising can make demand more inelastic (steeper) or shift it right
- Successful advertising → higher prices or more sales → higher profits
- But competing ads may simply offset each other — canceling out the effect
4. Oligopoly
4.1 What Is an Oligopoly?
Oligopoly arises when a small number of large firms have all or most of the sales in an industry. Key features:
- Mutual interdependence: each firm’s decisions depend on what others do
- High barriers to entry: economies of scale, brand loyalty, large capital requirements
- Firms can either compete (act like perfect competitors) or collude (act like a monopoly)
Examples: Boeing & Airbus (aircraft), Coca-Cola & Pepsi (soft drinks), major auto manufacturers, cable television.
4.1a Measuring Market Concentration: The HHI
The Herfindahl-Hirschman Index (HHI) measures market concentration:
\[HHI = \sum_{i=1}^{n} s_i^2\]where $s_i$ is each firm’s market share (as a percentage).
| HHI Range | Classification | Example | |—|—|—| | < 1,500 | Unconcentrated | Restaurants | | 1,500 – 2,500 | Moderately concentrated | Airlines | | > 2,500 | Highly concentrated | Wireless carriers | | 10,000 | Pure monopoly (one firm = 100%) | Local water utility |
Worked Example — U.S. Smartphone OS Market (2023):
| Firm | Market Share | $s_i^2$ |
|---|---|---|
| Apple (iOS) | 57% | 3,249 |
| Google (Android) | 42% | 1,764 |
| Others | 1% | 1 |
| HHI | 5,014 |
HHI = 5,014 → highly concentrated duopoly. The DOJ would scrutinize any merger in this market.
Compare: If there were 10 equal firms (10% each): HHI = $10 \times 10^2 = 1,000$ (unconcentrated).
4.2 Why Do Oligopolies Exist?
Three main reasons:
- Economies of scale: Market demand supports only 2–3 firms at minimum efficient scale
- Patents: Government grants patents to a few firms for similar products
- Brand loyalty + advertising: Creating a recognizable brand requires enormous investment (competing with Coke or Pepsi requires massive marketing budgets)
4.3 Collusion vs. Competition
The oligopolist’s temptation: If firms cooperate and restrict output, they earn monopoly-level profits. But each firm has an incentive to secretly increase output and grab market share.
Collusion = firms acting together to reduce output and raise prices (illegal in the U.S. and EU)
Cartel = formal collusion agreement (OPEC is the most famous example)
Adam Smith (1776): “People of the same trade seldom meet together, even for merriment and diversion, but the conversation ends in a conspiracy against the public, or in some contrivance to raise prices.”
The Lysine Cartel
In the 1990s, major lysine producers (including Archer Daniels Midland) met secretly to fix prices and divide markets. FBI wiretaps captured ADM’s president stating: “Our competitors are our friends. Our customers are the enemy.” Result: lysine prices doubled. After FBI investigation, ADM paid a $100 million fine and executives received prison sentences.
4.4 The Kinked Demand Curve
A mechanism for enforcing tacit cooperation without a formal agreement:
- If a firm cuts price → competitors immediately match → very little increase in sales (steep/inelastic lower portion)
- If a firm raises price → competitors don’t follow → big loss of customers (flat/elastic upper portion)
Result: a kink at the current price/quantity. Neither price increases nor decreases are attractive, so the status quo holds.
Diagram — Kinked Demand Curve:
The MR gap: Because demand has a kink, the MR curve has a vertical discontinuity at Q*. MC can shift anywhere within this gap without changing the profit-maximizing price or quantity — explaining why oligopoly prices tend to be sticky.
4.5 Oligopoly Models (Advanced)
Three classical oligopoly models:
| Model | Assumption | Outcome | |—|—|—| | Cournot | Firms simultaneously choose quantities | Price between monopoly and competitive levels | | Bertrand | Firms simultaneously choose prices | Price driven to MC (competitive outcome!) | | Stackelberg | One firm moves first (leader), other follows | Leader earns more; total output > Cournot |
Cournot Duopoly — Worked Example:
Market demand: $P = 100 - Q$ (where $Q = q_1 + q_2$). Both firms have $MC = 10$.
Firm 1’s problem: Maximize $\pi_1 = (100 - q_1 - q_2)q_1 - 10q_1$
\[\frac{\partial \pi_1}{\partial q_1} = 100 - 2q_1 - q_2 - 10 = 0\]Reaction function: $q_1^* = \frac{90 - q_2}{2} = 45 - 0.5q_2$
By symmetry: $q_2^* = 45 - 0.5q_1$
Solve simultaneously: $q_1 = 45 - 0.5(45 - 0.5q_1) = 22.5 + 0.25q_1$
$0.75q_1 = 22.5 \implies q_1^* = q_2^* = 30$
| Outcome | Monopoly | Cournot | Bertrand (competitive) |
|---|---|---|---|
| Total Q | 45 | 60 | 90 |
| Price | $55 | $40 | $10 |
| Industry profit | $2,025 | $1,800 | $0 |
Cournot is between monopoly and perfect competition — the typical oligopoly outcome.
5. The Prisoner’s Dilemma
5.1 The Classic Setup
Two prisoners are arrested and separated. Each can confess or remain silent:
| B: Silent | B: Confess | |
|---|---|---|
| A: Silent | A: 2 yrs, B: 2 yrs | A: 8 yrs, B: 1 yr |
| A: Confess | A: 1 yr, B: 8 yrs | A: 5 yrs, B: 5 yrs |
- If both cooperate (stay silent): 2 years each (best combined outcome)
- But each has a dominant strategy to confess — regardless of what the other does, confessing is individually better
- Result: both confess → 5 years each (worse than cooperation)
5.2 The Oligopoly Version
Two firms can hold down output (cooperate) or increase output (cheat):
| B: Hold Down | B: Increase | |
|---|---|---|
| A: Hold Down | A: $1,000, B: $1,000 | A: $200, B: $1,500 |
| A: Increase | A: $1,500, B: $200 | A: $400, B: $400 |
- Best combined outcome: both hold down → $1,000 each (monopoly profits)
- Dominant strategy: increase output → each earns only $400
- The dilemma: cooperation maximizes joint profit, but individual incentives push toward competition
5.3 Enforcing Cooperation
Since cartels are illegal and contracts unenforceable, oligopolists use:
- Kinked demand curves (match price cuts, don’t match increases)
- Monitoring each other’s prices and output
- Repeated interactions (the threat of future punishment deters cheating)
- International cartels like OPEC use diplomatic and political pressure
6. Key Takeaways
Monopolistic Competition:
- Many firms sell differentiated products — each has a mini-monopoly on its brand/style
- Firms face a downward-sloping, relatively elastic demand curve
- Profit maximization: produce where MR = MC, charge the price from the demand curve
- In the long run, entry drives economic profits to zero — but P = AC on the downward-sloping portion (not at min ATC)
- Not productively or allocatively efficient, but provides consumer variety and innovation
Oligopoly:
- A few large firms dominate, with high barriers to entry and mutual interdependence
- Firms face a prisoner’s dilemma: cooperate for monopoly profits vs. cheat for individual gain
- Cartels are formal collusion agreements — illegal in the U.S. but exist internationally (OPEC)
- The kinked demand curve helps explain price rigidity in oligopolistic markets
- The dominant strategy often leads to competition, driving profits down — even though cooperation would maximize joint profit
7. Practice Questions
Q1. List the four sources of product differentiation and give an example of each.
Answer:
- Physical aspects — a phone with an unbreakable screen
- Location — a gas station at a highway exit
- Intangible aspects — a restaurant offering free delivery and a satisfaction guarantee
- Perception — brand loyalty to Coca-Cola even though blind taste tests show no difference
Q2. How does a monopolistic competitor’s demand curve compare to (a) a perfectly competitive firm’s and (b) a monopolist’s?
Answer: (a) A perfectly competitive firm faces a horizontal demand curve (perfectly elastic) — it’s a price taker. A monopolistic competitor faces a downward-sloping demand curve — it has some pricing power because its product is differentiated. (b) Both monopolistic competitors and monopolists face downward-sloping demand curves, but the monopolistic competitor’s curve is more elastic because close substitutes exist. Raise the price, and some customers switch to competitors.
Q3. Using the Authentic Chinese Pizza data, explain why MR = MC determines the profit-maximizing output at Q = 40.
Answer: At Q = 30, MR ($14) > MC ($8), so expanding output adds to profit. At Q = 40, MR ($10) = MC ($10) — the last unit adds exactly as much revenue as it costs. At Q = 50, MC ($12) > MR ($6), meaning each additional unit costs more than the revenue it brings. So Q = 40 is the sweet spot.
Q4. Why do monopolistic competitors earn zero economic profit in the long run, even though they have some pricing power?
Answer: Low barriers to entry allow new firms to enter when profits exist. Entry shifts the incumbent firm’s demand curve left (fewer customers at each price), reducing MR and the profit-maximizing quantity. Entry continues until P = AC — zero economic profit. The firm still has pricing power (downward-sloping demand), but that power is insufficient to sustain profits because competitors keep entering with differentiated products.
Q5. Monopolistic competition achieves zero long-run profit just like perfect competition. Why do economists still say it is inefficient?
Answer: Two inefficiencies persist even at zero profit:
- Not productively efficient: The firm produces on the downward-sloping portion of AC, not at the minimum. Average cost could be lower with more output.
- Not allocatively efficient: P > MC (because MR = MC and P > MR), so society values more output than is produced. The marginal benefit exceeds the marginal cost, but the firm doesn’t expand.
Q6. What is the “trade-off” of monopolistic competition? Is the inefficiency worth it?
Answer: The trade-off is efficiency vs. variety. Monopolistic competition produces at higher average cost and charges higher prices than perfect competition, but consumers get a rich variety of products, styles, and innovations. Most people prefer a world with many types of clothing, food, and cars over a world where everyone wears the same blue jeans. Whether this variety is “optimal” is an ongoing debate.
Q7. Explain how the prisoner’s dilemma applies to oligopolistic firms.
Answer: Two oligopolists could both earn high profits by cooperating (holding down output like a monopoly). But each firm has an incentive to cheat — secretly increase output to grab more market share and higher individual profit. If one cooperates while the other cheats, the cooperator earns very little. So both firms’ dominant strategy is to increase output, even though this makes them both worse off ($400 each instead of $1,000 each). The individual incentive to cheat undermines the collective benefit of cooperation.
Q8. What is a kinked demand curve, and how does it enforce cooperation among oligopolists?
Answer: The kinked demand curve describes a situation where competitors match price cuts but don’t match price increases. If a firm cuts its price, all competitors match → very little sales increase (inelastic below the kink). If a firm raises its price, competitors don’t follow → the firm loses many customers (elastic above the kink). Result: neither raising nor lowering price is attractive, so each firm stays at the agreed price/quantity — enforcing tacit cooperation without a formal agreement.
Q9. Why are cartels inherently unstable, even when they successfully raise prices?
Answer: Each cartel member faces a prisoner’s dilemma. Once the cartel raises prices, each individual firm can earn even higher profits by secretly increasing output while others hold back. The more members cheat, the lower the price falls — eventually collapsing the cartel. The lysine and French soap cartels both broke down due to members cheating. Additionally, collusion is illegal in most countries, making enforcement impossible through courts.
Q10. Fill in the market structure comparison table:
| Feature | Perfect Competition | Monopolistic Competition | Oligopoly | Monopoly |
|---|---|---|---|---|
| Number of firms | ? | ? | ? | ? |
| Product type | ? | ? | ? | ? |
| Barriers to entry | ? | ? | ? | ? |
| Long-run profit | ? | ? | ? | ? |
| Efficiency | ? | ? | ? | ? |
Answer:
| Feature | Perfect Competition | Monopolistic Competition | Oligopoly | Monopoly | |———|——————-|————————|———–|———-| | Number of firms | Many | Many | Few | One | | Product type | Identical | Differentiated | Identical or differentiated | Unique, no substitutes | | Barriers to entry | None | Low | High | Very high | | Long-run profit | Zero | Zero | Positive (possible) | Positive | | Efficiency | Both productive & allocative | Neither | Neither | Neither |
Q11. OPEC has survived for over 40 years despite the prisoner’s dilemma incentive to cheat. Why?
Answer: Several factors help OPEC persist: (1) The dominant member (Saudi Arabia) can punish cheaters by flooding the market with cheap oil. (2) Members share geographic and political ties that facilitate communication and diplomacy. (3) Oil production is observable — it’s hard to secretly increase output without being detected. (4) Repeated interactions make long-term cooperation more attractive than short-term cheating. (5) OPEC operates under international law, where explicit price-fixing agreements are in a gray area and are not easily enforced against.
Q12. Calculate the HHI for an industry with four firms having market shares of 40%, 30%, 20%, and 10%. Is it concentrated? What happens to the HHI if the top two firms merge?
Answer: Before merger: $HHI = 40^2 + 30^2 + 20^2 + 10^2 = 1600 + 900 + 400 + 100 = 3{,}000$ → Highly concentrated
After merger (70%, 20%, 10%): $HHI = 70^2 + 20^2 + 10^2 = 4900 + 400 + 100 = 5{,}400$
Increase = $5400 - 3000 = 2{,}400$ points. The DOJ would almost certainly challenge this merger (any increase > 200 in a concentrated market raises concerns).
Q13. In a Cournot duopoly with demand $P = 80 - Q$ and $MC = 8$ for both firms, find each firm’s output, the market price, and total industry profit.
Answer: Reaction function: $q_i = \frac{80 - 8 - q_j}{2} = \frac{72 - q_j}{2} = 36 - 0.5q_j$
Symmetric equilibrium: $q = 36 - 0.5q \implies 1.5q = 36 \implies q^* = 24$ each.
$Q = 48$, $P = 80 - 48 = $32$
Profit per firm: $(32 - 8) \times 24 = $576$. Total industry profit: $1,152.
Compare: Monopoly ($Q = 36$, $P = 44$, profit $= 1,296$); Competition ($Q = 72$, $P = 8$, profit $= 0$). Cournot is right in between.
Q14. Explain the “excess capacity theorem” in monopolistic competition. A coffee shop in your neighborhood has seating for 80 but usually serves 50 customers. Is this an example?
Answer: The excess capacity theorem states that monopolistic competitors produce below the output that minimizes ATC. They operate on the downward-sloping portion of their ATC curve, meaning they could serve more customers at a lower average cost. The coffee shop with 80 seats serving 50 customers is a classic example — it has 30 seats of excess capacity. But this is the price of product differentiation: the shop’s unique atmosphere, location, and menu justify its existence even though a larger, more efficient operation could serve coffee more cheaply.
Q15. The U.S. streaming market has Netflix (23%), YouTube Premium (20%), Amazon Prime (12%), Disney+ (11%), Hulu (10%), HBO Max (9%), and others (15%). Calculate the HHI. Compare this market structure to perfect competition and monopoly.
Answer: $HHI = 23^2 + 20^2 + 12^2 + 11^2 + 10^2 + 9^2 + 15^2 = 529 + 400 + 144 + 121 + 100 + 81 + 225 = 1{,}600$
This is moderately concentrated (1,500–2,500 range), consistent with an oligopoly with some monopolistic competition characteristics. Firms offer differentiated products (exclusive content), have some pricing power, but face competitive pressure from close substitutes. It is far from monopoly (HHI = 10,000) and also far from perfect competition (HHI ≈ 0).