Same costs, different market. How the number of firms, product differentiation and barriers to entry decide price, output, profit and efficiency, from perfect competition to monopoly.
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Market structure is defined by four features: the number and size of firms, whether products are differentiated, how easily firms can enter and exit, and how much information buyers and sellers have. Those features fix how much control a firm has over price, and so what its revenue curves look like.
| Feature | Perfect competition | Monopolistic competition | Oligopoly | Monopoly |
|---|---|---|---|---|
| Firms | Very many, all small | Many | Few, large, interdependent | One (or one dominant) |
| Product | Homogeneous | Differentiated | Homogeneous or differentiated | Unique, no close substitute |
| Entry / exit | Free | Fairly free | Significant barriers | Very high barriers |
| Information | Perfect | Imperfect | Imperfect, rivals watched closely | Imperfect |
| Demand facing the firm | Horizontal: D = AR = MR | Downward, fairly elastic | Downward, kinked if rivals react | Market demand, downward |
| Price control | Price taker | Some | Considerable, but rivals react | Price maker |
A perfectly competitive firm takes the market price. It can sell as much as it likes at that price and nothing at a higher one, so it only chooses output. It expands until MC = MR = P. Whether that output earns supernormal, normal or subnormal profit depends on where the price line sits against ATC.
Drag price down slowly. Profit shrinks to zero when P touches the bottom of ATC, then turns negative. The firm keeps producing while P is above AVC, because each unit still contributes something to fixed costs. Once P drops below the lowest point of AVC the firm is better off closing: it loses only FC instead of FC plus the shortfall on variable costs.
Short run: stay open if P ≥ AVC (TR covers variable costs). Long run: stay only if P ≥ ATC, because every cost is variable and must be recovered.
The firm's supply curve is the part of MC above the minimum of AVC. Tick the box in the lab to highlight it.
A monopolist faces the whole market demand curve, so selling more means cutting the price on every unit. Its MR curve therefore sits below AR (= demand). It still maximises profit at MR = MC, but then reads the price off the demand curve, not the MR curve. That gap is what creates supernormal profit and the welfare loss.
A competitive industry with the same costs would produce where D = MC, at a lower price and a higher output. The monopolist restricts output to raise price, and the triangle between demand and MC over the lost output is the deadweight loss: units consumers value above their cost that never get made.
Large scale can mean lower unit costs than many small firms could reach, and supernormal profit can fund R&D (dynamic efficiency). A natural monopoly has LRAC still falling at the market's whole demand, so one firm is cheapest. Regulators then cap price rather than break the firm up.
Many firms sell differentiated products (restaurants, hairdressers, clothing brands). Each has some control over price because its product is not identical to rivals', but entry is easy. In the short run a firm looks like a small monopolist. In the long run, entry pulls its demand curve left and makes it flatter until it just touches ATC.
With a few large firms, each firm's best move depends on what rivals do. That interdependence produces two typical behaviours: prices that stay put for long periods, and a choice between competing and colluding. The kinked demand curve explains the first.
Assume the market price is P0. If the firm raises price, rivals do not follow, so it loses many customers: demand above P0 is elastic (flat). If it cuts price, rivals match to protect their share, so it gains few customers: demand below P0 is inelastic (steep). The result is a kink at P0, and a break in the MR curve directly below it. Costs can move anywhere within the gap in MR without changing the profit-maximising price.
Price cuts, promotions, price wars. Risky in oligopoly because rivals can match, leaving everyone with lower revenue and no extra share.
Branding, advertising, product quality, loyalty schemes, after-sales service, innovation. Hard for rivals to copy instantly, so firms compete here instead, at the cost of higher spending.
Firms in an oligopoly can agree to keep prices high (overt collusion, a formal cartel) or move in step without an agreement (tacit collusion). Both are unstable, because each firm gains by cheating while the other holds the agreement. Game theory shows why with a payoff matrix.
A market is contestable when entry and exit are costless in practice: the threat of a rival entering matters more than the number of firms currently there. Its key feature is low sunk costs (money that cannot be recovered on exit), which allows hit-and-run entry.
Even a monopolist behaves almost competitively if entry is easy: it holds price near ATC (limit pricing), keeps costs down and takes only normal profit, because supernormal profit would attract a rival immediately. Airline routes and online retail are common examples.
High sunk costs (advertising, specialised plant, R&D) mean a failed entrant loses everything. Incumbents can also threaten predatory pricing or use loyalty schemes, so the market becomes less contestable however many firms are present.
| Outcome | Perfect comp. | Monopolistic comp. | Oligopoly | Monopoly |
|---|---|---|---|---|
| Short-run profit | Any of super, normal, sub | Any | Usually supernormal | Usually supernormal |
| Long-run profit | Normal only | Normal only | Supernormal can persist | Supernormal can persist |
| Price vs MC | P = MC | P > MC | P > MC | P > MC |
| Productive efficiency | Yes (P = min ATC) | No (excess capacity) | Usually not | Usually not, unless scale economies are big |
| Allocative efficiency | Yes | No | No | No |
| Dynamic efficiency | Weak: no supernormal profit to invest | Some product variety | Often strong: profit funds R&D | Possible, but little pressure to innovate |
| Non-price competition | None | Central | Central | Limited |
Press Play, or use the numbered steps to jump around. The firm takes the price from the market and produces where MC = P.
Five labs, one for each market structure plus the pricing game. Drag the sliders and read the verdict under each diagram. Curves are illustrative models with fixed shapes.
Drag each description into the market structure it best fits. Tap an item, then tap a box if dragging is awkward.
Enter numbers only.
Sketch each diagram in your notebook, then explain it in your own words and apply it to a real market. Answers save in this browser only.
Examples are explained in general terms so they stay true. Look up current figures before quoting numbers in an exam.
Many small farmers selling near-identical wheat or rice, or traders in a deep currency market, come close to perfect competition: lots of sellers, a standard product, and prices set by the market as a whole. No single seller is likely to be able to change the price.
Laying a second set of pipes or rails alongside the first would be very costly, so one provider can usually supply the whole market at lower average cost than two. These are often regulated, with price caps to stop the provider using its market power.
Lots of firms sell products that are similar but not identical, and new ones can open easily. A fashionable restaurant may earn supernormal profit for a while, but rivals are likely to copy the idea and draw customers away.
A few large firms hold most of the sales. Each watches the others' prices and promotions closely, and prices often stay similar for long periods while firms compete on branding, loyalty cards and service instead.
When one airline cuts fares on a route, rivals are likely to match, and all end up with lower revenue and little change in market share. Each would be better off if all held fares, but each is tempted to cut.
Groups of producers have at times agreed to limit output to support a higher price. Each member may be tempted to produce more than its quota, which is one reason such agreements can be hard to sustain.
If an incumbent airline earns high profit on a route, a low-cost rival with planes it can move elsewhere may be able to enter quickly and leave if it fails. The threat of entry can push the incumbent to keep fares closer to cost.
Open each question for the reminders, the model answer and the common mistakes. Model answers are colour-coded to the mark scheme.
A firm in monopolistic competition sells a differentiated product and faces fairly free entry, so in the short run it may earn supernormal profit where MR = MC and price is above ATC. This profit is likely to attract new firms, because entry is easy.
As new firms enter and offer similar products, they take customers from existing firms, so the demand curve facing each firm shifts left and becomes more elastic. Entry is likely to continue until demand just touches ATC at the profit-maximising output, where price equals ATC and only normal profit is earned. A new fashionable café format could follow this path once rivals copy it. If the firm has strong brand loyalty, entry may take longer and supernormal profit may last for some time.
In an oligopoly, firms are interdependent, so each must consider how rivals will react to a price change. The kinked demand curve assumes that if a firm raises its price, rivals are unlikely to follow, so demand above the current price P₀ is elastic and the firm may lose many customers. If the firm cuts its price, rivals are likely to match the cut to protect their share, so demand below P₀ is inelastic and the firm gains few extra sales.
This gives a kink in the demand curve at P₀ and a vertical gap in the marginal revenue curve below it. A rise or fall in marginal cost that stays within this gap leaves MC = MR at the same output, so the profit-maximising price is unchanged. This could help explain why prices in markets such as supermarkets can stay similar for long periods.
The model only explains why price is sticky; it does not explain how P₀ was set in the first place. Real oligopolies may also have a price leader that the other firms follow, so prices can change when the leader moves.
Perfect competition has many small firms, identical products, free entry and perfect information, so every firm is a price taker producing where P = MC. A monopoly is a single seller protected by barriers to entry, which faces the whole market demand curve and can choose its price.
Consumers are likely to benefit more from perfect competition because price is lower and output is higher. A competitive industry produces where demand equals MC, which is allocatively efficient, and in the long run price falls to the lowest point of ATC, so it is also productively efficient. A monopolist restricts output to the point where MR = MC and charges the higher price on the demand curve, which creates supernormal profit and a deadweight loss. A regulated water provider with no competitor could charge more without price controls. However, this comparison assumes both industries have the same costs, which may not be true in practice. On price and output, therefore, consumers are likely to be better off under perfect competition.
Consumers could benefit more from monopoly if it has much lower costs through economies of scale. If the monopoly is a natural monopoly, one large firm may have lower average costs than many small ones, and supernormal profit can fund research and development, which could give better products over time (dynamic efficiency). Pharmaceutical firms with patents may invest heavily in new drugs for this reason. But a monopoly with little competition may also suffer from X-inefficiency, so cost savings may not be passed on to consumers. Monopoly could therefore benefit consumers in some cases.
Overall, consumers are likely to benefit more from perfect competition when costs are similar, because price is lower and output higher. However, if there are large economies of scale and the monopoly is regulated or faces the threat of entry, consumers may do as well or better. The result mainly depends on how big the cost savings are and how much pressure the monopoly faces.
| Command word | What it asks for |
|---|---|
| Define / State | A precise meaning in one or two ideas. |
| Calculate | Formula, substitution, answer with units. |
| Explain | A reason with a developed link, usually with an example or diagram. |
| Analyse | A chain of cause and effect applied to the context. |
| Discuss / Assess | Arguments on more than one side and a justified judgement. |
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