4.2 Monopolies

A monopoly is a single firm that controls an entire market because barriers to entry block competitors. It maximizes profit where MR = MC, then sets price up on the demand curve, so price ends up greater than marginal cost.

Why This Matters for the AP Microeconomics Exam

Monopoly is one of the most graph-heavy topics in AP Microeconomics, and it shows up in both multiple-choice questions and free-response prompts. You should be able to draw and label a correct monopoly graph, find the profit-maximizing quantity and price, and shade areas for consumer surplus, producer surplus, profit or loss, and deadweight loss.

This topic also builds the comparison skills you need across all of Unit 4. Once you can explain why price is greater than marginal cost in a monopoly, you can apply the same reasoning to monopolistic competition and oligopoly, and you can contrast all of them with the efficient perfect competition model from Unit 3.

Key Takeaways

• A monopoly exists because of barriers to entry such as high start-up costs, legal protections like patents, or exclusive control of a key resource.
• The profit-maximizing quantity is where MR = MC. Price is found by going straight up from that quantity to the demand curve, so price is greater than marginal cost.
• The marginal revenue curve always lies below the demand curve because a single-price monopoly must lower price on every unit to sell one more.
• A monopoly is allocatively inefficient (P does not equal MC) and underproduces compared to the socially optimal output, which creates deadweight loss.
• Use ATC to show profit or loss: compare ATC to price at the profit-maximizing quantity and shade the rectangle.
• A natural monopoly happens when one firm has falling long-run average cost across the entire market demand, so one firm can supply the market at a lower cost than several firms could.

Characteristics of Monopolies

• One firm is the entire industry: A monopoly is a single seller, so the firm's demand curve is the market demand curve.
• The firm is a price maker: A monopoly can choose its price along the demand curve instead of accepting a market price.
• High barriers to entry: These barriers can be economic (high fixed or start-up costs), legal (patents, licenses, government franchise), or based on exclusive ownership of a key resource. Barriers are what keep the monopoly in place.
• Long-run economic profits are possible: Because no new firms can enter, a monopoly can earn positive economic profit in the long run. There is no entry-driven adjustment to zero profit like in perfect competition.
• No close substitutes: The product has no good substitutes, which is part of why the firm has market power.

Graphing Monopolies

In a monopoly graph, the demand curve lies above the marginal revenue curve. This happens because a single-price monopoly has to lower its price to sell each additional unit. Since the lower price applies to all units sold, not just the last one, the extra revenue from one more unit is always less than its price.

For example, if you can sell 5 units at 10 dollars each but must drop to 8 dollars to sell a 6th unit, all 6 units now sell for 8 dollars, not just the last one. That is why marginal revenue is always below demand for a single-price monopoly.

The profit-maximizing output is where MR = MC. To find the price, go straight up from that quantity to the demand curve.

The graph above shows a standard monopoly with demand above MR. The profit-maximizing output is where MR = MC at Q1. Find the price by going up from Q1 to the demand curve and reading off P1. You go to the demand curve, not to MR, because a monopoly has market power and can charge the highest price buyers are willing to pay at that quantity, which is what the demand curve shows.

Profit and Loss on a Monopoly Graph

When a problem asks you to show a profit or loss, use the ATC curve. To show a profit, ATC sits below price at the profit-maximizing quantity. Shade the rectangle between price and ATC.

Monopoly Graph Showing a Profit

To show a loss, ATC sits above price at the profit-maximizing quantity. Shade that rectangle.

Monopoly Graph Showing a Loss

Finding this rectangle works much like it does in perfect competition: locate the price, go up or down to ATC, and complete the rectangle. That rectangle is your profit or loss. You can also find the total revenue rectangle and the total cost rectangle, then take the difference. The leftover is either profit (extra revenue) or loss (extra cost).

Calculating a Monopoly's Profit

In this graph, total revenue is 1200 dollars, found by multiplying price by the profit-maximizing output. Total cost is ATC times that same output (2 dollars x 200 = 400 dollars). Profit is total revenue minus total cost (1200 dollars - 400 dollars = 800 dollars).

Calculating a Monopoly's Loss

In this graph, total revenue is 500 dollars, found by multiplying price by the profit-maximizing output. Total cost is ATC times that output (9 dollars x 100 = 900 dollars). The loss is total revenue minus total cost (500 dollars - 900 dollars = -400 dollars).

Monopoly and Efficiency

In perfect competition, firms reach both allocative and productive efficiency, and consumer and producer surplus are maximized. A monopoly does not reach allocative efficiency because it charges a price above marginal cost and produces less than the socially optimal quantity.

The socially optimal quantity is where demand equals marginal cost (P = MC). A monopoly produces less than this and charges more, so some mutually beneficial trades never happen. That lost surplus is the deadweight loss triangle.

Key Points on a Monopoly Graph

There are several reference points you should be able to identify on a monopoly graph (see the graph below).

• Profit-maximizing price and output (sometimes called loss-minimizing): Output is where MR = MC, then go up to the demand curve and over to find the price. On the graph below this is Q1 and P4. This is the point described earlier.
• Socially optimal (allocatively efficient) price and output: This is where P = MC. Producing here usually requires government intervention, often a price ceiling that lowers price, increases output, and removes deadweight loss. On the graph below this is Q3 and P2. The catch is that this point can leave the firm with a loss if ATC is higher than price there.

• Fair-return price and output: This is where P = ATC. Do not confuse it with the productively efficient point of P = minimum ATC. Here total revenue equals total cost, so the monopoly earns only a normal profit. It is often set with a price ceiling. On the graph below this is Q4 and P1. Some deadweight loss may remain. This is a common compromise between the profit-maximizing point and the socially optimal point.
• Price and output that maximizes total revenue: This is where MR = 0. This quantity also separates the elastic and inelastic parts of the demand curve. On the graph below this is Q2 and P3.

Natural Monopoly

A natural monopoly happens when one firm has economies of scale across the entire range of market demand, meaning its long-run average cost keeps falling as output rises. Because a single firm can serve the whole market at a lower cost than several smaller firms could, it makes sense for just one firm to supply the market.

This is why natural monopolies often face regulation rather than being broken up. Regulators may use fair-return pricing (P = ATC) so the firm covers its costs, or socially optimal pricing (P = MC), which can require a subsidy because the firm would otherwise take a loss at that point.

Monopoly Graph and Elasticity

The demand curve on a monopoly graph has elastic, inelastic, and unit elastic sections. Use the quantity where MR = 0 to separate them. Draw a line from that quantity straight up to the demand curve. The point where it touches demand is the unit elastic point. The section above it is the elastic region, and the section below it is the inelastic region.

In the elastic region, a monopoly can lower price and still raise total revenue. In the inelastic region, lowering price reduces total revenue (this is the Total Revenue Test). A monopoly will never willingly produce in the inelastic region, because marginal revenue is negative there while marginal cost stays positive, which would shrink profit. The firm only ends up producing there if forced by regulation, such as a price ceiling, or supported by a subsidy.

How to Use This on the AP Microeconomics Exam

Free Response

Draw the monopoly graph cleanly with demand above MR, and an MC curve and ATC curve. Label the profit-maximizing quantity where MR = MC, then go up to the demand curve for the price. Graders look for correct labels and the right vertical alignment between quantity, price, and cost curves.

When asked about profit or loss, place ATC correctly at the profit-maximizing quantity and shade the rectangle. When asked about efficiency, identify the socially optimal output where P = MC and shade the deadweight loss triangle between the monopoly quantity and the socially optimal quantity.

Problem Solving

To calculate profit or loss, find total revenue (P x Q) and total cost (ATC x Q), then subtract. You can also use (P - ATC) x Q directly. Watch your signs: a negative result is a loss.

To find areas from a graph, use triangle and rectangle formulas. Consumer surplus is the triangle under demand and above price up to the monopoly quantity. Deadweight loss is the triangle between the demand and MC curves, from the monopoly quantity out to the socially optimal quantity.

Common Trap

Do not set price at the MR = MC point. That intersection only gives you the quantity. Always read price off the demand curve directly above that quantity, because a monopoly charges the most buyers will pay at the quantity it produces.

Common Misconceptions

• A monopoly does not charge the highest price it can imagine. It is limited by the demand curve. It picks the quantity where MR = MC and then charges what buyers will pay for that quantity.
• MR = MC gives quantity, not price. Students often stop at that intersection and read off the price there. Go up to the demand curve for the price.
• Profit is not the gap between price and marginal cost. Profit per unit is price minus ATC, not price minus MC. Use ATC for the profit or loss rectangle.
• A monopoly can still take a loss. Having market power does not guarantee profit. If ATC is above price at the profit-maximizing quantity, the firm loses money.
• A natural monopoly is not just a big firm. It specifically means one firm has falling long-run average cost across the whole market, so one firm supplies the market more cheaply than several could.
• Deadweight loss comes from underproduction, not from the firm earning profit. The inefficiency is that the monopoly produces less than the socially optimal quantity where P = MC.

Related AP Microeconomics Guides

• 4.1 Introduction to Imperfectly Competitive Markets
• 4.4 Monopolistic Competition
• 4.3 Price Discrimination
• Unit 4 Overview: Imperfect Competition
• 4.5 Oligopoly and Game Theory