Market Power and Market Structures: IB Economics 2.11

The four market structures

Market structures sit on a spectrum defined by the number of firms, freedom of entry, and product differentiation. Perfect competition has many small firms selling identical products with free entry, so each firm is a price taker with zero market power. Monopoly is a single seller protected by high barriers to entry, holding the most market power.

Between them sit monopolistic competition, with many firms selling slightly differentiated products and easy entry, and oligopoly, where a few large firms dominate and interdependence matters. As you move from perfect competition to monopoly, market power rises and the firm's demand curve becomes steeper, giving it more control over price.

You can build and compare the extreme cases interactively at /sandbox/perfect-competition and /sandbox/monopoly. This IB topic overlaps closely with the AP Microeconomics unit on market structures, so AP-depth practice on those diagrams transfers directly.

Profit maximisation: MC equals MR worked example

Every firm maximises profit where marginal cost equals marginal revenue (MC = MR). For a monopoly, marginal revenue lies below the demand curve because cutting price to sell one more unit lowers the price on all units. Suppose demand is P = 100 minus Q, so total revenue is 100Q minus Q squared and marginal revenue is 100 minus 2Q. Let marginal cost be constant at 20.

Setting MR = MC gives 100 minus 2Q = 20, so Q = 40, and the price from the demand curve is P = 100 minus 40 = 60. If instead this were a perfectly competitive industry pricing where P = MC, output would solve 100 minus Q = 20, giving Q = 80 at a price of 20. The monopoly restricts output from 80 to 40 and raises price from 20 to 60.

The welfare cost is the deadweight loss triangle between the two quantities. Its area is one half times the lost output (80 minus 40 = 40) times the price gap (60 minus 20 = 40), which equals 0.5 times 40 times 40 = 800 units of welfare lost.

Short run and long run outcomes

In perfect competition and monopolistic competition, firms can earn abnormal (supernormal) profit in the short run, but because entry is free, new firms enter, driving price down until only normal profit remains in the long run. The difference is that the monopolistically competitive firm keeps a downward-sloping demand curve, so its long-run equilibrium sits where price exceeds marginal cost and it produces below the minimum of average cost.

Monopoly and oligopoly are protected by barriers to entry, so abnormal profit can persist into the long run. This persistence of profit is the defining consequence of market power and is what policy often targets.

Efficiency comparison

Allocative efficiency occurs where price equals marginal cost, so the last unit's value to consumers equals its cost. Only perfect competition achieves this in the long run. Productive efficiency occurs at the minimum of average cost, again reached only by perfect competition in the long run.

Monopoly is both allocatively inefficient, because price exceeds marginal cost, and productively inefficient, because it does not produce at minimum average cost. Monopolistic competition is inefficient on both counts too, though less severely, and it carries excess capacity from producing below the efficient scale. The trade-off defenders of large firms raise is dynamic efficiency: monopoly profits can fund research and development, as seen in pharmaceuticals.

Price discrimination

Price discrimination is charging different consumers different prices for the same good for reasons not based on cost. Three conditions must all hold: the firm must have market power, it must be able to separate consumers by their price elasticity of demand, and it must prevent resale between the groups.

Airlines are the standard example: business travellers with inelastic demand pay high last-minute fares, while tourists with elastic demand who book early pay less, and a plane ticket cannot be resold. Under third-degree price discrimination the firm charges more in the inelastic market and less in the elastic one, raising total revenue and profit while capturing consumer surplus.

Oligopoly, collusion and game theory

Oligopoly is defined by interdependence: each firm's best move depends on what rivals do. Firms may collude, either formally through a cartel such as OPEC setting oil quotas, or tacitly through price leadership, to act like a monopoly and share higher profits. Collusion is illegal in most jurisdictions because it harms consumers.

Game theory shows why collusion is unstable. In the prisoner's dilemma, two firms each face the choice to keep prices high or undercut. Whatever the rival does, each firm gains individually by cheating, so the dominant strategy is to cut price. Both firms cheat and end up worse off than if they had cooperated, which explains why cartels frequently break down.

Natural monopoly and policy responses

A natural monopoly arises when huge fixed costs and continuously falling average cost mean a single firm can supply the whole market more cheaply than several could. Water networks, electricity grids, and rail track are examples, because duplicating the infrastructure would waste resources. Splitting a natural monopoly into competitors would raise average cost, so competition policy usually keeps one provider and regulates it instead.

Governments respond to market power in several ways: competition or antitrust law that blocks mergers and cartels, price regulation such as capping the price of a regulated utility, public ownership, or removing barriers to entry to invite competition. Each is evaluated against the intervention key concept, weighing lower prices against regulatory cost and the risk of regulatory capture.

Common Paper mistakes

Always draw marginal revenue below and twice as steep as a straight-line demand curve for a monopoly; a common error is placing MR on the demand curve. Mark the profit-maximising output from where MC crosses MR, then read price up on the demand curve, not on MR.

State the three price discrimination conditions in full, since dropping the no-resale condition loses marks. When evaluating monopoly, do not claim it is always bad: bring in economies of scale, natural monopoly, and dynamic efficiency to balance the answer.

How this is examined

• This HL-only topic is diagram-heavy. On Paper 1 a clear, labelled monopoly diagram showing MC = MR, the price read off demand, and the deadweight loss earns analysis marks fast.
• Paper 3 can ask for calculations from a demand and cost function: find MR, set MR = MC, solve for quantity, then read price off demand. Practise the arithmetic shown in the worked example.
• For evaluation, avoid a one-sided attack on monopoly. Balance allocative and productive inefficiency against economies of scale, natural monopoly, and dynamic efficiency for the top mark band.
• When a game theory question appears, set up a payoff matrix, identify each firm's dominant strategy, and state the Nash equilibrium to explain why collusion is unstable.

Key terms

monopolyperfect competitionmonopolistic competitionoligopolyprice discriminationnatural monopoly