1 Economic Concepts · 1.1 Microeconomics: demand, production, markets and factor pricing
1.1.5 Market structures and price determination: perfect competition, monopoly, monopolistic competition and oligopoly
Every profit-maximising firm produces where MR = MC (with MC rising); what differs across market structures is the number of sellers, the kind of product, entry conditions and so whether price equals MC and whether supernormal profit survives in the long run.
- APPSC Paper I
- Advanced level
- 12 min read
- Free
Core facts
- Profit maximisation: (1) MR = MC (first-order condition); (2) MC cuts MR from below (MC rising, second-order condition).
- Perfect competition: very many buyers and sellers, homogeneous product, free entry and exit, perfect knowledge, perfect mobility. Firm is a price taker; industry fixes price by demand and supply.
- Perfect competition firm: P = AR = MR; equilibrium P = MC.
- Short run: a competitive firm may earn supernormal profit, normal profit or a loss. It keeps producing as long as P ≥ minimum AVC.
- Shutdown point = minimum AVC. Break-even point = minimum ATC.
- Short-run supply curve of the firm = the part of the MC curve above minimum AVC.
- Long run (perfect competition): P = MR = AR = MC = minimum LAC; only normal profit; firm works at the optimum size.
- Monopoly: single seller, no close substitutes, strong barriers to entry; price maker; AR slopes down, MR < AR.
- Monopoly equilibrium: MR = MC, so P > MC. Supernormal profit can last in the long run. A monopolist has no supply curve.
- A monopolist (with positive MC) always produces on the elastic part of demand (e > 1).
- Price discrimination (A.C. Pigou's three degrees): first degree (each unit at the buyer's maximum price), second degree (blocks/slabs), third degree (separate markets). Higher price in the market with less elastic demand.
- Monopolistic competition (E.H. Chamberlin, 1933): many sellers, differentiated products, free entry, selling costs (advertising). Demand slopes down but is fairly elastic. Long run: normal profit with excess capacity.
- Oligopoly: few sellers, interdependence, barriers to entry. Duopoly = two sellers. Kinked demand curve (P.M. Sweezy, 1939; also Hall and Hitch) explains price rigidity.
- Marshall's time periods: market period (very short; supply fixed, demand rules price), short period, long period, secular period. Marshall compared demand and supply to the two blades of a pair of scissors.
- Buyer side: monopsony = single buyer; oligopsony = few buyers; bilateral monopoly = single seller facing a single buyer.
Explained
1. Comparing the four structures.
| Feature | Perfect competition | Monopolistic competition | Oligopoly |
|---|---|---|---|
| Sellers | Very many | Many | Few |
| Product | Homogeneous | Differentiated | Homogeneous or differentiated |
| Entry | Free | Free | Restricted |
| Demand for firm | Perfectly elastic | Elastic, downward | Kinked or uncertain |
| Feature | Monopoly |
|---|---|
| Sellers | One |
| Product | No close substitute |
| Entry | Blocked (patents, licences, control of a raw material, scale) |
| Demand for firm | Industry demand, downward sloping |
2. Market price under perfect competition. Price is set where market demand equals market supply.
Example 1. Qd = 100 − 5P and Qs = 20 + 3P. Find the equilibrium price and quantity. Solution. 100 − 5P = 20 + 3P → 8P = 80 → P = 10. Q = 100 − 50 = 50 (check: 20 + 30 = 50). Answer: P = ₹10, Q = 50
- An increase in demand (supply unchanged) raises both price and quantity; an increase in supply (demand unchanged) lowers price and raises quantity.
- In Marshall's market period supply is fixed, so price depends on demand alone; in the long period supply adjusts fully, so cost of production matters most.
3. The competitive firm.
Example 2. A firm in perfect competition has TC = Q³ − 6Q² + 15Q + 10. The market price is ₹15. Find its output and profit, and its shutdown price. Solution.
- MC = 3Q² − 12Q + 15. Set P = MC: 15 = 3Q² − 12Q + 15 → 3Q(Q − 4) = 0 → Q = 0 or 4. At Q = 0 MC is falling, so the second-order condition fails; Q = 4.
- TR = 15 × 4 = 60. TC = 64 − 96 + 60 + 10 = 38. Profit = 60 − 38 = ₹22 (ATC = 9.5, so profit per unit 15 − 9.5 = 5.5).
- AVC = Q² − 6Q + 15, minimum at Q = 3, value 6. Shutdown price = ₹6.
- Answer: Q = 4, profit ₹22, shutdown price ₹6. (Break-even price = minimum ATC ≈ ₹9.10.)
Short-run cases for a competitive firm:
| Price | Result |
|---|---|
| P > minimum ATC | Supernormal profit |
| P = minimum ATC | Normal profit (break-even) |
| min AVC < P < min ATC | Loss, but keeps producing (covers variable cost and part of fixed cost) |
| P < minimum AVC | Shuts down (loss = TFC) |
4. Monopoly.
Example 3. A monopolist faces P = 100 − 2Q and has a constant MC = AC = ₹20. Find output, price and profit. Solution. TR = 100Q − 2Q², MR = 100 − 4Q. MR = MC: 100 − 4Q = 20 → Q = 20. P = 100 − 40 = 60. Profit = (60 − 20) × 20. Answer: Q = 20, P = ₹60, profit ₹800
- Under perfect competition (P = MC = 20), output would be 40. Monopoly gives less output at a higher price.
- Elasticity at the monopoly point: e = P/(bQ) = 60/40 = 1.5 (> 1, the elastic range).
5. Monopolistic competition. Each firm sells a slightly different brand (soap, toothpaste, restaurants), so it has some control over price. In the short run it may earn supernormal profit; new firms then enter, each firm's demand shifts left until AR is tangent to LAC. At this point P = LAC (normal profit) but on the falling part of LAC, so output is below the minimum-cost output: excess capacity. Selling costs (advertising) shift demand and are a feature of this market.
6. Oligopoly. Because each firm's sales depend on rivals' reactions, there is no single model.
- Kinked demand curve: rivals follow a price cut but do not follow a price rise. Demand is elastic above the current price and inelastic below it, so the MR curve has a vertical gap at the kink; MC can shift within the gap without changing price. This explains sticky prices.
- Cartel: firms collude and act as a joint monopoly (e.g. OPEC). Price leadership: a dominant or low-cost firm sets the price, others follow.
- Cournot duopoly (1838): each firm chooses output taking the other's output as given.
Deeper
1. Lerner index of monopoly power. L = (P − MC) ÷ P = 1/e at the profit-maximising point.
- Example 3: L = (60 − 20)/60 = 2/3 = 1/1.5. ✓ L = 0 under perfect competition (P = MC).
2. Welfare loss of monopoly. Compared with competition, the lost consumer and producer surplus (deadweight loss) is the triangle between demand and MC from the monopoly output to the competitive output.
- Example 3: ½ × (60 − 20) × (40 − 20) = ₹400.
3. Third-degree price discrimination. Conditions: separate markets (no resale), different elasticities, some monopoly power. Rule: MR₁ = MR₂ = MC; since MR = P(1 − 1/e), the price is higher where e is lower.
Example 4. MC = ₹10 (constant). Market 1: P₁ = 50 − 2Q₁. Market 2: P₂ = 30 − Q₂. Find prices. Solution.
- Market 1: MR₁ = 50 − 4Q₁ = 10 → Q₁ = 10, P₁ = 30. Elasticity e₁ = 30/(2 × 10) = 1.5.
- Market 2: MR₂ = 30 − 2Q₂ = 10 → Q₂ = 10, P₂ = 20. Elasticity e₂ = 20/(1 × 10) = 2.
- Check: 30(1 − 1/1.5) = 10 and 20(1 − 1/2) = 10.
- Answer: P₁ = ₹30 (less elastic market), P₂ = ₹20 (more elastic market). Profit = 200 + 100 = ₹300.
- Indian examples: railway fares by class, electricity tariffs for domestic and commercial users, lower fees for students.
4. Cournot duopoly with numbers.
Example 5. Market demand P = 90 − Q, both firms have zero cost. Find the Cournot outcome. Solution. Firm 1 maximises (90 − q₁ − q₂)q₁: 90 − 2q₁ − q₂ = 0. By symmetry q₁ = q₂ = q: 90 − 3q = 0 → q = 30. Total 60, P = 30, each profit 900.
- Compare: monopoly Q = 45, P = 45; perfect competition Q = 90, P = 0. Cournot output (60) is two-thirds of the competitive output; with n firms it is n/(n + 1) of it.
5. Long-run industry supply under perfect competition. Constant-cost industry: horizontal LR supply; increasing-cost industry: upward sloping; decreasing-cost industry: downward sloping (external economies).
6. Other points examiners like.
- Natural monopoly: LAC keeps falling over the whole market demand (railways, water supply, power grid), so one firm is cheapest.
- Bilateral monopoly: price is indeterminate within limits; it depends on bargaining.
- Game theory (von Neumann and Morgenstern, 1944) and the Nash equilibrium describe oligopoly strategy; the prisoner's dilemma shows why cartels tend to break down.
- Bertrand model: duopolists compete on price; with identical products and costs, price falls to MC.
- Sales maximisation (Baumol): oligopoly firms may maximise revenue subject to a minimum profit.
- Joan Robinson (1933) built the theory of imperfect competition and monopsonistic exploitation; Chamberlin (1933) built monopolistic competition, stressing product differentiation and selling costs.
Common traps
- ✗ "A competitive firm shuts down when price falls below ATC." ✓ It shuts down only when price falls below minimum AVC.
- ✗ "A monopolist charges the highest possible price." ✓ It sets the price where MR = MC, on the elastic part of demand.
- ✗ "A monopolist has an upward-sloping supply curve." ✓ A monopolist has no supply curve.
- ✗ "Under price discrimination the higher price goes to the more elastic market." ✓ Higher price in the less elastic market.
- ✗ "Monopolistic competition was developed by Joan Robinson." ✓ Chamberlin (monopolistic competition); Joan Robinson wrote on imperfect competition (both 1933).
- ✗ "Kinked demand explains price discrimination." ✓ It explains price rigidity in oligopoly (Sweezy).
- ✗ "Monopsony means one seller." ✓ Monopsony = one buyer; monopoly = one seller.
- ✗ "In the long run a firm in monopolistic competition earns supernormal profit." ✓ Free entry leaves normal profit, with excess capacity.
Quick revision
- Equilibrium: MR = MC and MC rising.
- Perfect competition: P = AR = MR = MC; long run P = min LAC, normal profit.
- Shutdown = min AVC; break-even = min ATC.
- Firm's supply = MC above min AVC.
- Monopoly: P > MC, no supply curve, elastic range only.
- Lerner index (P − MC)/P = 1/e.
- Price discrimination: higher price where demand is less elastic; MR₁ = MR₂ = MC.
- Monopolistic competition: Chamberlin; differentiation, selling costs, excess capacity.
- Oligopoly: interdependence; kinked demand (Sweezy) → sticky prices; Cournot, cartel.
- Marshall: scissors; market period price set by demand.
Go deeper: Mains writing (beyond what APPSC Paper I usually asks)
Mains writing Mains writing
- Frame: market structure decides price, output, efficiency and the need for regulation.
- Analysis:
- Perfect competition gives allocative efficiency (P = MC) and productive efficiency (minimum LAC); monopoly gives neither.
- Monopolistic competition gives variety at the cost of excess capacity and advertising.
- Oligopoly (telecom, cement, aviation in India) risks collusion; the Competition Commission of India (Competition Act, 2002) checks cartels and abuse of dominance.
- Way forward: regulate natural monopolies (tariff commissions), keep entry open, and use competition law against cartels.
Practise this topic. Questions on Production, costs & market structures, with answers explained.
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