BEO6600 · Business Economics
Market Structures
Having taken the supply curve as given, this session opens the firm and asks how much it should produce and what price it can charge. The answer turns on market power, which places firms on a spectrum: perfect competition → monopolistic competition → oligopoly → monopoly. Every firm maximises profit the same way — produce where MR = MC — but a competitive price taker has P = MR = MC (efficient, no markup, no deadweight loss), while a firm with market power sets P > MC, creating a markup and a welfare cost. A monopolist finds quantity at MR = MC then reads the price up to demand; monopolistic competition adds free entry that competes long-run profit to zero (P = ATC) but keeps a markup and excess capacity; and oligopoly is a prisoner's dilemma where the dominant strategy drives both firms to defect, which is why cartels are unstable. Know the four-structure comparison table cold.
What this chapter covers
- 016.1 The competition spectrum and the revenue concepts (TR, AR, MR)
- 02The universal profit-max rule: MR = MC
- 036.2 Perfect competition: P = MR = MC; the shut-down rule
- 046.3 Monopoly: MR < P, then read price off demand (P > MC)
- 056.4 The welfare cost of monopoly (transfer + deadweight loss)
- 066.5 Monopolistic competition: zero long-run profit, excess capacity, markup
- 076.6 Oligopoly and the prisoner's dilemma (one 2x2 grid)
- 086.7 The four-structure comparison table
Worked example: the four-structure comparison and the monopoly trap
- +1(a) The universal rule is MR = MC for both. The competitive firm is a price taker, so P = AR = MR; substituting gives P = MC — efficient, no markup. The monopolist faces market demand, so MR < P; it still uses MR = MC but ends with P > MC — a markup.
- +1(b) Step 1: set MR = MC to find the profit-maximising quantity Q*. Step 2: go straight up to the demand curve at Q* to read the price. Because demand lies above MR, the price exceeds MC.
- +1The common error: setting price at MR = MC, or reading price off the MC curve. The monopolist uses MR = MC only for the quantity; the price always comes from demand. A monopoly has no supply curve.
- +1(c) The dominant-strategy outcome is (High, High): each firm plays High whatever the rival does, so both land there. It is a Nash equilibrium — no firm can do better by deviating alone.
- +1Why it is worse than colluding: if both restricted output (Low, Low) they would behave like a single monopoly and each earn more, but the temptation to cheat and expand pulls them to (High, High). That is exactly why oligopoly cartels are hard to hold together.
Key terms
- Profit-maximising rule (MR = MC)
- Every firm, in any market structure, maximises profit by producing where marginal revenue equals marginal cost. If MR > MC the next unit adds more revenue than cost (make it); if MR < MC the last unit cost more than it earned (cut it). What changes across structures is the relationship between MR and price.
- Price taker
- A firm so small relative to the market that it cannot move the price — the defining feature of perfect competition. It faces a horizontal demand line at the market price, so P = AR = MR, and the profit-max rule reduces to P = MC. This is the efficiency benchmark, with no markup and no deadweight loss.
- Monopoly
- A market with a single seller of a product with no close substitute, protected by barriers to entry (control of a key resource, government grants like patents, or a natural monopoly from large economies of scale). Facing the whole downward-sloping market demand, MR < P; it sets quantity at MR = MC then reads price up to demand, so P > MC.
- Monopolistic competition
- Many firms selling differentiated products with low entry barriers. Each has a sliver of market power (a downward but elastic demand), so it behaves like a mini-monopoly in the short run. Free entry competes long-run profit to zero where demand is tangent to ATC, leaving excess capacity and a persistent markup (P > MC).
- Prisoner's dilemma
- A game in which each player's dominant strategy leads to an outcome worse for both than cooperation. In an oligopoly, both firms expanding output is the dominant strategy and the Nash equilibrium, even though restricting output (colluding) would earn each more — which is why cartels are unstable.
Market Structures FAQ
How does a monopolist set its price and quantity?
In two steps. First, find the profit-maximising quantity by setting MR = MC. Second, go straight up from that quantity to the demand curve to read the price. Because demand lies above marginal revenue for a monopolist, the price exceeds marginal cost — a markup. The favourite trap is setting the price at MR = MC or reading it off the MC curve; the price always comes from demand, and a monopoly has no supply curve.
Why does monopoly create a deadweight loss but perfect competition does not?
A competitive market trades where P = MC, so every unit worth more than it costs is produced — total surplus is maximised. A monopolist restricts output below the competitive quantity and charges a higher price. Part of the effect is a transfer of surplus from consumers to the firm (not a loss to society), but the units that are valued above their cost yet never produced are pure deadweight loss — destroyed, gained by no one.
How is monopolistic competition different from perfect competition in the long run?
Both end with zero long-run economic profit because of free entry. The difference is the price-cost relationship: a competitive firm settles at P = MC = minimum ATC, while a monopolistically competitive firm settles where demand is merely tangent to ATC on its falling arm, so P = ATC > MC. That leaves a markup and excess capacity (output below the minimum-ATC scale) — the price of product variety.
Why are oligopoly cartels unstable?
Because of the prisoner's dilemma. Together, the firms would earn the most by colluding and restricting output like a single monopoly. But each firm is individually tempted to cheat and expand — expanding is its dominant strategy. When every firm follows its dominant strategy, they all expand, landing on a Nash equilibrium that is worse for everyone than the collusive outcome. That temptation to defect is what makes collusion hard to sustain.
Exam move
The exam loves a 'complete the table' item, so memorise the four-structure comparison cold — number of firms, product type, barriers, the price-cost relationship, and long-run profit — reading it left to right as rising market power. Anchor everything on the universal rule MR = MC, then track how P relates to MC in each structure: P = MC (perfect competition, efficient), P > MC (the other three). For monopoly, drill the two-step method (Q at MR = MC, then price up to demand) and the deadweight-loss triangle versus the competitive benchmark. For monopolistic competition, draw the long-run tangency on the falling arm of ATC (not its minimum). For oligopoly, practise spotting the dominant strategy in a single 2x2 grid and reading off the Nash equilibrium. Firm theory stays at the MR = MC and diagram level — you are not asked to derive cost schedules from a table.
