Market Structures
High SchoolNot all markets are alike. A wheat farmer is a price-taker with no power at all; a patent-holding drug maker sets its own price. Between these poles lies a spectrum — perfect competition, monopolistic competition, oligopoly, monopoly — and where a firm sits determines its prices, profits, and how well it serves society.
At a glanceThe spectrum of competition
Structures differ along four axes: number of firms, freedom of entry, product differentiation, and price-setting power. Firms maximise profit where marginal revenue equals marginal cost (MR = MC).
| Structure | Firms | Entry | Product | Price power | Long-run profit |
|---|---|---|---|---|---|
| Perfect competition | Very many | Free | Identical | None (price-taker) | Normal only |
| Monopolistic competition | Many | Free | Differentiated | Some | Normal only |
| Oligopoly | Few | Barriers | Varies | Significant, interdependent | Can be supernormal |
| Monopoly | One | Blocked | Unique | Price-maker | Supernormal |
In perfect competition the firm faces a horizontal (perfectly elastic) demand curve at the market price, so price = MR. Free entry competes away any supernormal profit, leaving only normal profit in the long run — and it achieves both productive and allocative efficiency. A monopoly faces the whole downward market demand, so to sell more it must cut price on all units; hence MR < price. It restricts output and charges a higher price, earning supernormal profit protected by barriers to entry (economies of scale, patents, control of a resource, legal protection).
Why monopoly output is "too low"
Because a monopolist must lower the price on every unit to sell one more, its marginal revenue falls twice as fast as demand. Setting MR = MC therefore yields a quantity below the competitive level and a price above marginal cost. Society loses the mutually beneficial trades between the monopoly quantity and the competitive quantity — a deadweight loss. The monopolist also has weaker pressure to cut costs (X-inefficiency).
Oligopoly is defined by interdependence: each firm's best move depends on rivals' reactions. This breeds either collusion (overt cartels like OPEC, or tacit price leadership) to act like a monopoly, or fierce price wars. The kinked demand curve model explains sticky prices: rivals match price cuts (so demand below the current price is inelastic) but ignore price rises (elastic above), discouraging any change. Game theory's prisoner's dilemma shows why cartels are unstable — each member gains by secretly cheating.
Monopoly is not always bad: economies of scale may make one large firm cheaper than many small ones (a natural monopoly, e.g. water pipes), and monopoly profits can fund innovation (Schumpeter's "creative destruction"). Firms with power may also practise price discrimination — charging different prices to different buyers (student discounts, airline fares) to capture more surplus.
Worked example: a monopolist's profit-maximising choice
Q1Why does supernormal profit disappear in perfect competition but persist in monopoly?
The difference is entry. In perfect competition entry is free, so any supernormal profit attracts new firms, raising supply, lowering price, and competing profit down to the normal level. In monopoly, barriers to entry (patents, scale, legal protection) keep rivals out, so the incumbent can sustain supernormal profit indefinitely.
Q2What makes a cartel like OPEC inherently unstable?
It is a prisoner's dilemma. Collectively, members gain by restricting output to keep prices high. But individually, each member can earn more by secretly producing above quota and selling at the high price. Since every member has that incentive, cheating tends to spread, output rises, and the price collapses — unless the cartel can monitor and punish defectors, which is hard.
Q3Give the conditions required for successful price discrimination.
Three: (1) the firm must have market power (be a price-maker); (2) it must be able to separate buyers into groups with different price elasticities (e.g. students vs commuters); and (3) it must prevent resale between groups, or the cheap buyers would undercut it. When all three hold, the firm charges more to inelastic buyers and less to elastic ones, raising total profit above the single-price level.
Q4Is a natural monopoly best broken up into competing firms?
Usually no. A natural monopoly exists where economies of scale are so large that one firm supplies the whole market at lower average cost than several could (water distribution, rail track). Splitting it up would raise costs by duplicating expensive infrastructure. The standard remedy is to keep the single network but regulate its price (e.g. an RPI−X price cap) or bring it into public ownership, rather than force wasteful competition.
Q5How can monopolistic competition have many firms yet still let each set its own price?
Through product differentiation — branding, quality, location. Each café or hairdresser sells something slightly unique, giving it a small, downward-sloping demand curve and thus a little pricing power in the short run. But because entry is free, rivals imitate any success, demand for each firm shrinks, and long-run profit is competed back to normal. So it blends monopoly-like variety with competition-like zero long-run supernormal profit.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1A wheat farmer faces a market price of $8. Can she sell at $9?
- In perfect competition goods are identical and she is tiny.
- Buyers would simply buy from rivals at $8.
- So she must accept the market price.
Example 2A monopolist sells 10 units at $20 or 11 units at $19. What is marginal revenue of the 11th unit?
- Revenue at 10 units = 10 x 20 = 200.
- Revenue at 11 units = 11 x 19 = 209.
- MR = 209 - 200 = 9, below the $19 price.
Now you try
Work each one out first, then tap to reveal the worked answer.