Difficulty: Intermediate | Prerequisites: Ch. 14 (competitive markets, profit maximisation via MR = MC).
This chapter moves from the competitive extreme to the opposite end of the spectrum: a market with a single seller. Where competitive firms are price takers, a monopolist is a price maker. You will see that the same MR = MC rule applies, but because a monopolist faces the entire downward-sloping market demand curve, its marginal revenue is always less than its price. This leads to higher prices, lower output, and deadweight loss compared to competition. The chapter also introduces price discrimination and the policy tools governments use to deal with monopoly power.
A monopolist is the sole seller of a product with no close substitutes. It maximises profit where MR = MC, then charges the highest price consumers will pay for that quantity (read off the demand curve). The result is a price above marginal cost, output below the socially efficient level, and a deadweight loss. Price discrimination can reduce or eliminate this inefficiency, but it transfers surplus from consumers to the monopolist.
Monopoly
A firm that is the sole seller of a product without any close substitutes.
In simple terms, if you want the product, there is only one place to get it.
Market power
The ability of a firm to influence the market price of the good it sells, typically by adjusting the quantity it supplies.
Natural monopoly
A monopoly that arises because a single firm can supply the entire market at a lower cost than two or more firms could. This happens when there are economies of scale over the relevant range of output, so average total cost keeps falling.
Think of it as a market where one big producer is cheaper than several small ones, like a local water utility.
Barriers to entry
Factors that prevent other firms from entering a monopolist's market. Three main sources: ownership of a key resource, government regulation (patents, licences), and economies of scale (natural monopoly).
Price discrimination
The business practice of selling the same good at different prices to different customers, where the price difference is not due to differences in cost.
Think of it as charging each customer closer to what they are willing to pay.
Perfect price discrimination
A situation in which the monopolist knows each customer's exact willingness to pay and charges them that amount. Eliminates deadweight loss but transfers all surplus to the producer.
Deadweight loss (in monopoly context)
The reduction in total surplus that results from the monopolist producing less than the socially efficient quantity. Measured as the triangle between the demand curve and the MC curve, from the monopoly quantity to the efficient quantity.
Antitrust laws
A group of statutes aimed at curbing monopoly power, preventing anti-competitive mergers, and prohibiting collusive behaviour among firms.
Monopoly resources: A single firm owns a key resource with no close substitutes (rare in practice because most goods are traded globally)
Government regulation: Patents (20 years exclusive right), copyrights, licences. These create monopoly intentionally, trading higher prices for incentives to innovate or create.
The production process (natural monopoly): Economies of scale make one firm cheaper than many. ATC keeps falling over the relevant output range. Classic examples: water utilities, bridges, power distribution.
A competitive firm faces a horizontal demand curve (perfectly elastic) because it sells a product identical to many others
A monopolist faces the entire market demand curve, which slopes downward. To sell more, it must lower its price on all units.
Total revenue = P x Q (same as any firm)
Average revenue = P (same as any firm)
Marginal revenue < P, because selling one more unit requires lowering the price on all previous units too
Two effects when a monopolist sells one more unit:
Output effect: more units sold, which raises revenue
Price effect: lower price on all units, which reduces revenue
The MR curve lies below the demand curve and can become negative (when the price effect dominates)
Same rule: produce where MR = MC
Then read the price off the demand curve at that quantity (not off the MC curve)
Key difference from competition: P > MR = MC, so price exceeds marginal cost
Derive the MR curve from the demand curve
Find Q where MR = MC
On the demand curve, find P at which consumers will buy that Q
If P > ATC, the monopoly earns a profit. Profit = (P - ATC) x Q
Price stays the same (MC is unchanged, so the profit-maximising Q and P do not change)
Profit decreases (ATC rises, so the gap between P and ATC shrinks)
The socially efficient quantity is where the demand curve intersects MC (value to consumers = cost of production)
A monopolist produces where MR = MC, which is less than the efficient quantity
The monopolist charges P > MC, so consumers who value the good above its cost but below the monopoly price do not buy it
The deadweight loss triangle sits between the demand curve and the MC curve, from the monopoly quantity to the efficient quantity
This is similar to the wedge created by a tax, except the revenue goes to the firm as profit rather than to the government
The profit transfer from consumers to the monopolist does not reduce total surplus on its own (it is a redistribution, not a loss)
The social cost is the deadweight loss from reduced output, not the profit itself
Requires market power (not possible under perfect competition) and the ability to separate customers by willingness to pay
Three degrees:
First degree (perfect): charge each customer their exact willingness to pay. Eliminates deadweight loss, but all surplus goes to the producer. Example: university tuition with financial aid.
Second degree: charge different prices based on quantity purchased (bulk discounts).
Third degree: charge different prices to different customer segments (student discounts, senior tickets, airline pricing).
Arbitrage can undermine price discrimination: if customers who pay less can resell to those who would pay more, the price difference collapses
Price discrimination can raise economic welfare by getting the good to consumers who would not buy at a single monopoly price
Antitrust laws: Break up monopolies or prevent anti-competitive mergers. Horizontal mergers (same market) face more scrutiny than vertical mergers (different stages of production). Key US laws: Sherman Act, Clayton Act.
Regulation: Government sets the price. Marginal-cost pricing is efficient but can cause losses for natural monopolies (where MC < ATC). Average-cost pricing covers costs but removes incentives to reduce them.
Public ownership: Government runs the monopoly itself. Economists generally prefer private ownership because private firms have stronger cost-reduction incentives.
Do nothing: If the market failure is small relative to the imperfections of government intervention, inaction may be the best option.
\text{Profit} = (P - ATC) \times Q\text{MR} < P \quad \text{(always, for a monopolist)}\text{Profit-maximising output: } MR = MC\text{Then price is read from the demand curve at } Q_{MAX}Competition vs monopoly comparison:
Feature | Competition | Monopoly |
|---|---|---|
Number of firms | Many | One |
Marginal revenue | MR = P | MR < P |
Pricing rule | P = MC | P > MC |
Long-run profit | Zero (entry/exit) | Possible (barriers) |
Efficient output? | Yes | No (produces too little) |
Price discrimination? | No | Yes |
Students often think a monopolist can charge any price it wants. It cannot. The monopolist is constrained by the demand curve: a higher price means fewer units sold. It chooses the price-quantity combination that maximises profit.
Students sometimes believe the monopolist's profit is the social cost. The profit is a transfer from consumers to the producer, not a loss to society. The social cost is the deadweight loss from producing too little.
Students often think an increase in fixed costs will cause the monopolist to raise its price. Fixed costs do not affect MC, so the profit-maximising price and quantity are unchanged. Only profit decreases.
Students sometimes confuse the monopolist's supply curve with the competitive firm's. A monopolist does not have a supply curve in the traditional sense, because its output decision depends on the shape of the demand curve, not just the price.
The relationship P > MR = MC is tested heavily. Be able to draw and label the monopoly graph: demand, MR, MC, ATC, the profit-maximising Q, the monopoly price, and the profit rectangle.
Deadweight loss identification: know how to shade the triangle and explain why it represents lost trades.
Price discrimination questions often ask you to compare welfare outcomes (single price vs perfect discrimination vs no discrimination).
Fixed cost changes: remember that MC is unaffected, so price stays the same, only profit changes.
Natural monopoly regulation: know the problem with marginal-cost pricing (MC < ATC means losses) and why average-cost pricing is a common compromise.
Real-world application: Pharmaceutical patents are a textbook example. A drug company gets 20 years of monopoly pricing to recoup R&D costs. During that period, the drug is priced well above marginal cost. Once the patent expires, generic competition drives the price toward MC, and the market behaves more like perfect competition.
True or false: A monopolist's marginal revenue equals the price of its good. (False, MR < P)
Fill in the blank: The deadweight loss of monopoly arises because the monopolist produces ____ than the socially efficient quantity. (less)
True or false: If a monopolist's fixed costs rise, its price will increase. (False, price stays the same)
Fill in the blank: Perfect price discrimination eliminates deadweight loss but transfers all surplus to the ____. (producer)
True or false: A natural monopoly has increasing average total cost. (False, it has decreasing ATC)
Q: For a profit-maximising monopoly that charges a single price, what is the relationship between P, MR, and MC?
A: P > MR and MR = MC. The monopolist sets output where marginal revenue equals marginal cost, but charges a price (read from the demand curve) that exceeds both.
Q: A monopolist currently earns positive profit. The government imposes a lump-sum tax (a fixed amount regardless of output). What happens to the monopolist's price, output, and profit?
A: Price and output stay the same, because a lump-sum tax is a fixed cost and does not affect marginal cost. Profit decreases by exactly the amount of the tax.
Q: Why does marginal-cost pricing cause problems for natural monopolies?
A: Natural monopolies have declining ATC, which means MC < ATC throughout the relevant range. Setting P = MC forces the firm to sell below its average cost, producing a loss. The firm would need a subsidy to stay in business, or regulators must allow average-cost pricing instead.
Q: Explain how perfect price discrimination affects deadweight loss and the distribution of surplus.
A: Under perfect price discrimination, the monopolist charges each consumer exactly their willingness to pay. Every unit where willingness to pay exceeds MC is sold, so the quantity produced equals the efficient quantity and deadweight loss is zero. However, the entire surplus goes to the producer; consumer surplus is zero.
Q: A publisher sells a novel at $30 to 100,000 fans and could alternatively sell at $5 to 500,000 readers. Which price maximises profit if the fixed cost of publishing is $2 million and marginal cost is zero?
A: At $30: revenue = $3m, profit = $1m. At $5: revenue = $2.5m, profit = $0.5m. The $30 price maximises profit. The deadweight loss is the surplus lost by the 400,000 readers who value the book above $0 (MC) but below $30.
Monopoly is the polar opposite of perfect competition (Ch. 14). Both types of firms maximise profit where MR = MC, but the competitive firm has MR = P while the monopolist has MR < P. Monopolistic competition (Ch. 16) borrows from both: firms have some market power (downward-sloping demand) but free entry drives long-run profit to zero. Oligopoly (Ch. 17) sits between monopoly and competition, where the key tension is whether firms cooperate (act like a monopoly) or compete.
Monopoly, monopolist, price maker, barriers to entry, natural monopoly, economies of scale, market power, deadweight loss, price discrimination, first-degree second-degree third-degree price discrimination, perfect price discrimination, antitrust laws, Sherman Act, Clayton Act, marginal-cost pricing, average-cost pricing, public ownership, patent, copyright, MR less than price, welfare cost of monopoly, Mankiw Ch. 15