Market structures

A firm maximises profit at the output where MC = MR: producing one more unit would add more to cost than to revenue, one fewer would forgo revenue that exceeded its cost. This holds in every structure. What differs is marginal revenue. A price-taker sells every unit at the market price, so MR = P and the demand curve it faces is horizontal. A price-maker must cut price to sell more, so MR lies below the demand (average revenue) curve — and price at the profit-maximising output is read off the demand curve above MC = MR, not at the intersection.

Profit is measured against average cost. Normal profit is the return just sufficient to keep the firm in the industry and is included in AC, so a firm earning normal profit has AR = AC. Supernormal (abnormal) profit is anything above that: AR > AC. This distinction is tested more than any other on the topic.

Assumptions: many buyers and sellers, none large enough to influence price; a homogeneous product; perfect information; no barriers to entry or exit. Each firm is a price-taker facing a perfectly elastic demand curve at the market price: P = AR = MR.

Short run: the firm produces where MC = MR (= P). If P > AC it earns supernormal profit; if P < AC it makes a loss but continues producing as long as P ≥ AVC, since revenue covers the variable costs and contributes to fixed costs already incurred. Below AVC it shuts down. The firm’s supply curve is its MC curve above AVC.

Long run: supernormal profit attracts entry, which shifts market supply right and lowers price until only normal profit remains; losses cause exit and the reverse. Long-run equilibrium is at the minimum of AC with P = MC = AC: the firm is allocatively efficient (P = MC — the price consumers pay equals the cost of the last unit) and productively efficient (producing at minimum average cost). It is the benchmark against which the other structures are judged.

Assumptions: a single seller (in practice, a firm with dominant market power), a product with no close substitutes, and high barriers to entry — legal (patents, licences), natural (economies of scale so large that one firm supplies the market at lowest cost), control of an essential resource, or brand loyalty and sunk costs.

The monopolist faces the downward-sloping market demand curve, so MR is below AR. It produces where MC = MR and charges the price on the demand curve at that output — above marginal cost. Because entry is blocked, supernormal profit persists into the long run. Output is lower and price higher than under perfect competition, and the units between the monopoly output and the competitive output — where consumers value the good above its marginal cost but it is not produced — are the deadweight welfare loss.

The case for monopoly, which the exam expects you to know as well as the case against: economies of scale can make its average cost lower than many small firms’ would be (a natural monopoly), and supernormal profit can fund research and development — dynamic efficiency — that price-taking firms cannot afford. A monopolist can also price discriminate — charge different groups different prices for the same product — if it can separate the groups, they have different elasticities, and resale between them is prevented.

Assumptions: many firms, low barriers to entry, and differentiated products — restaurants, hairdressers, clothing brands. Differentiation gives each firm a downward-sloping demand curve (it can raise price without losing every customer), but the many substitutes make it elastic.

Short run: like a small monopoly — MC = MR, price above MC, supernormal profit possible. Long run: entry is easy, so new firms take customers, each firm’s demand curve shifts left until it is tangent to AC, and only normal profit remains. At that tangency the firm is producing on the downward-sloping part of AC — excess capacity — so it is neither productively efficient nor, with P > MC, allocatively efficient. The consumer gets variety in exchange.

Assumptions: a few large firms dominate (a high concentration ratio), with significant barriers to entry, and — the defining feature — interdependence: each firm’s decisions affect, and depend on, its rivals’ reactions. There is no single model; the syllabus expects two approaches.

Paper 3 is 30 questions in 75 minutes, one mark each. Market-structure questions are rarely about definitions; they give a situation or a set of figures and ask which conclusion follows. Three in the paper’s shape — Quanta’s own, not Cambridge’s:

Every one of those turns on a single distinction. Practising the real papers on Quanta tags each wrong answer to its syllabus section, so after three sittings you know whether it is the shutdown rule or the kinked curve that keeps costing you. See all 9708 Paper 3 sittings →

1. 1.Normal profit read as zero profit

   Normal profit is a cost — the return that keeps the owner in the industry — and is inside AC. A firm with AR = AC is earning it, not making nothing.

2. 2.Price taken at MC = MR

   For any price-maker, MC = MR fixes the output; the price is on the demand curve directly above it. Reading price at the intersection understates it.

3. 3.Monopoly always inefficient

   Allocatively, yes (P > MC). But a natural monopoly can be the lowest-cost way to supply the market, and supernormal profit can buy dynamic efficiency. Questions test whether you know both sides.

4. 4.The kink the wrong way round

   Rivals follow cuts, not rises. So demand is elastic above the current price and inelastic below it — not the reverse.

5. 5.Monopolistic competition confused with monopoly

   Monopolistic competition has many firms and free entry; its long-run profit is normal. It shares with monopoly only the downward-sloping demand curve.