Revise Market structures: perfect competition and monopoly for IB Economics (2.10)

Perfect competition is a theoretical model built on four strict assumptions. No real market meets them all, but some (wholesale wheat, foreign exchange) come close, and the model's predictions about long-run price, profit and efficiency are what make it so useful as a benchmark.

Together these make every firm a price taker: it must accept the price set by total market supply and demand. Its own demand curve is therefore a horizontal (perfectly elastic) line at that price. Because each extra unit sells at the same unchanged price, Price = Average Revenue = Marginal Revenue (P = AR = MR). This single fact — that P, AR and MR are all equal and constant — is the source of every efficiency result that follows, and it is what distinguishes perfect competition from every other structure.

Whatever the market structure, a firm maximises profit at the output where marginal cost equals marginal revenue (MC = MR), with MC rising through MR. The logic is simple: if MR > MC, the last unit added more to revenue than to cost, so producing it raised profit — keep expanding. If MR < MC, the last unit cost more than it earned — cut back. Profit is therefore greatest exactly where MC = MR. What changes between structures is not the rule but the height of MR relative to price.

In the SHORT run a perfectly competitive firm can be in any of three positions, depending only on where the market price sits relative to its ATC. If price (AR) is above ATC it earns abnormal profit (AR > ATC); if price equals ATC it earns normal profit; if price is below ATC it makes an economic loss (and shuts down entirely if price falls below average variable cost). Remember that normal profit — the entrepreneur's opportunity cost — is already built into the ATC curve, so 'AR = ATC' means the firm is doing exactly as well as in its next best alternative.

In the LONG run the no-barriers assumption takes over and forces the industry to a unique outcome. Suppose firms are earning abnormal profit. Because entry is free, new firms are attracted in; the extra output shifts market supply to the right, and the market price falls. Entry continues until the price has fallen to the level where AR just equals ATC — normal profit — and the incentive to enter disappears. If instead firms were making losses, some would exit, market supply would shift left, and the price would rise back up to normal profit. Either way the long-run equilibrium is the same: every firm earns exactly normal profit, producing where P = MR = MC = ATC at the minimum point of ATC.

The long-run equilibrium of perfect competition is prized because it delivers BOTH types of static efficiency simultaneously — the reason economists use it as the welfare benchmark.

A monopoly exists when a single firm is the sole supplier of a good with no close substitutes. Its market power comes entirely from barriers to entry — the obstacles that stop rivals from competing the profit away. Because it is the only seller, the monopolist faces the WHOLE market demand curve, so its demand curve (its AR curve) slopes downward. That downward slope is the root of everything that makes monopoly different from perfect competition.

Why the monopolist's MR lies below its AR. Facing the downward-sloping demand curve, the monopolist can only sell an extra unit by lowering the price — and, selling a single homogeneous product, it must lower the price on ALL units, not just the last one. So the marginal revenue from the extra unit is its price MINUS the revenue lost by marking down every earlier unit. Marginal revenue is therefore always less than average revenue (price), and the MR curve lies below the AR curve, falling twice as steeply. This is the decisive contrast with perfect competition, where MR = AR.

Profit maximisation and long-run abnormal profit. The monopolist still obeys MC = MR: it finds the output where its (below-AR) marginal revenue curve cuts marginal cost, then charges the price read UP on the demand curve above that output. Because MR < AR, this price sits above marginal cost (P > MC). And because barriers to entry block new firms, no extra supply arrives to erode the profit — so a monopolist can sustain abnormal profit in the long run, the very thing free entry destroys under perfect competition.

Comparing a monopoly with a perfectly competitive industry that has the same costs exposes the welfare cost of market power. The competitive industry produces where P = MC (allocatively efficient). The monopolist, restricting output to where MR = MC, produces LESS and charges MORE.

Natural monopoly is the important exception. Where economies of scale are so vast that average cost falls right across the market — water pipes, electricity grids, rail track — a single firm genuinely produces at lower average cost than several duplicated networks could. Splitting it up would RAISE costs, so the efficient answer is one firm subject to government regulation (price caps forcing it toward the competitive outcome) rather than enforced competition. This is a direct application of the key concept of intervention.

Price discrimination (briefly). A monopolist with price-setting power can sometimes charge different consumers different prices for the SAME good — cheaper off-peak rail fares, student discounts, higher prices in less price-sensitive markets. It works only if the firm can (i) set prices, (ii) separate consumers by their price elasticity of demand, and (iii) prevent resale between the groups. By charging each group closer to its willingness to pay, the monopolist converts consumer surplus into extra profit; output can actually be higher than under a single price, though the distribution of surplus shifts toward the firm.

This HL lesson advances two of the course's key concepts. Efficiency is the thread throughout — perfect competition achieves both allocative (P = MC) and productive (min ATC) efficiency, while monopoly achieves neither and imposes a deadweight welfare loss. Intervention follows directly: because monopoly power harms welfare (yet natural monopoly and innovation complicate the picture), governments regulate — price caps, competition law, breaking up cartels. Everything here feeds the later HL analysis of market power, regulation and the wider case for and against government intervention.