Market Structures

Market Structures

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Market structures classify markets by degree of competition, determined by number of sellers, product type, entry barriers, and price control.

Four structures: Perfect competition (many firms, homogeneous product, price taker) → Monopolistic competition (many firms, differentiated product, some price control) → Oligopoly (few firms, interdependence) → Monopoly (single firm, price maker, barriers to entry).

Key formula: MR = MC (profit maximization for all structures). In perfect competition specifically, AR = MR = P (price equals both average and marginal revenue).

Lerner Index measures market power: L = (P − MC)/P = 1/|ed|, where P = price, MC = marginal cost, ed = elasticity of demand. Higher L = more monopoly power.

RBI-specific: Expect 1–2 MCQ questions. Common traps: conflating AR = MR as universal (only in perfect competition), forgetting that monopolistic competition earns zero economic profit in LR (P = AC), and using wrong MR formula for different structures.

---

🟡 Standard — Regular Study (2d–2mo)

Four Market Structures Compared

Structure	Sellers	Product	Entry	P vs MC	LR Profit
Perfect Competition	Many	Homogeneous	Free	P = MC	Zero (normal)
Monopolistic Competition	Many	Differentiated	Free	P > MC	Zero (P = AC)
Oligopoly	Few	Homog./Differentiated	Restricted	P > MC	Can be positive
Monopoly	One	Unique	Blocked	P > MC	Supernormal

Profit Maximization Condition

The rule MR = MC applies universally across all market structures. Firms produce where MR crosses MC from below.

Derivation for monopoly/monopolistic competition: TR = P × Q, but since P falls as Q rises: MR = dTR/dQ = P + Q(dP/dQ) = P[1 + 1/ed]. Because ed is negative, MR < P.

In perfect competition: since firms are price takers, P = AR = MR (demand curve is perfectly elastic). Therefore P = MC at equilibrium.

Social Cost of Monopoly

Monopoly creates deadweight loss (DWL): the triangle between P = MC and P = monopoly price. This represents allocative inefficiency — society loses surplus because output is below the socially optimal level.

Lerner Index (L) = (P − MC)/P = 1/|ed|. Higher values indicate greater market power. For a perfectly competitive firm where P = MC, L = 0.

Concentration Measures

• CRn: Sum of market shares of top n firms. CR4 > 40% suggests oligopoly.
• HHI: Σ(si)², where si = market share of firm i in %. HHI > 2500 indicates high concentration (US DOJ threshold for anti-competitive concern).

Exam Pattern for RBI Grade B

RBI Grade B asks 3–4 questions from this topic. Questions often require:

• Identifying market structure from a scenario
• Calculating Lerner Index
• Comparing efficiency across structures
• Understanding why monopoly produces less than socially optimal output

---

🔴 Extended — Deep Study (3mo+)

Price Discrimination

A monopolist with market power may charge different prices to different consumers. Three types exist:

• First-degree (perfect discrimination): Charge each consumer their maximum willingness to pay. MR = P (no DWL), but practically difficult to implement.
• Second-degree: Different prices for different quantities consumed (e.g., volume discounts).
• Third-degree (most common): Separate prices for different market segments (e.g., student discounts, senior citizen fares). Requires identifiable groups with different elasticities.

The condition: MR in each segment = MC for overall profit maximization. Consumers with inelastic demand pay higher prices.

Oligopoly and Game Theory

Oligopoly firms are interdependent — each firm’s decision depends on rivals’ responses. This makes game theory essential:

• Nash Equilibrium: Each firm chooses its best strategy given rivals’ strategies. No firm can gain by unilaterally changing its choice.
• Collusion/Cartel: Firms coordinate to behave like a monopoly, sharing monopoly profit. Unstable — incentive to cheat. Cartels often collapse (OPEC is a notable exception).
• Prisoner’s Dilemma: Individual rational choices lead to inferior collective outcomes. Both firms would earn more by colluding, but each has incentive to cheat.

Kinked demand curve model: If a rival matches price cuts but ignores price increases, a firm’s demand curve has a kink at the current price. This explains price rigidity in oligopoly.

Common Mistakes to Avoid

1. AR = MR is NOT universal. This holds only in perfect competition where price is constant. In monopoly and monopolistic competition, MR < AR (price) because firms face downward-sloping demand.

2. Long-run profit confusion. Perfect and monopolistic competition yield zero economic profit in LR (P = AC) due to free entry. Monopoly retains supernormal profit because barriers prevent entry.

3. HHI calculation: Market shares must be squared, not summed directly. A duopoly with 50% each: HHI = 2500 + 2500 = 5000 (highly concentrated).

4. Monopolistic competition efficiency: LR equilibrium has excess capacity — firms produce below the minimum AC point, meaning productive inefficiency exists alongside allocative inefficiency.

Connections to Adjacent Topics

• Market failure: Monopoly’s DWL is a textbook example of market failure requiring regulation.
• Imperfect information: Affects all structures — oligopoly pricing strategies often exploit information asymmetry.
• Factor markets: Monopsony (single buyer) parallels monopoly; LR equilibrium condition P = AC extends to labor markets.

Practice Prompts

1. A monopoly faces demand P = 100 − Q and MC = 20. Calculate profit-maximizing output, price, and Lerner Index. (Answer: Set MR = MC → MR = 100 − 2Q = 20 → Q = 40; P = 60; L = (60−20)/60 = 0.67)

2. Two firms in a duopoly simultaneously choose prices. Game theory predicts equilibrium outcome differs from monopoly outcome. Explain using Nash equilibrium concept.

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.