Market Power I: Monopoly
When big becomes power — and what power can do
Imagine two firms. One becomes huge because millions of consumers voluntarily choose it every day. The other becomes huge because it controls the only bridge, the only operating system, the only hospital in town, the only legal license, or the only pipeline into a market.
Both are big. Only one may have dangerous market power.
The question is not whether a firm is large, famous, or profitable. The question is whether consumers are trapped. When is size the reward for creating value, and when does size become the power to restrict output and raise price? And once a firm has that power, what is the most it can do with it?
Monopoly and Market Power
Big is not the same as powerful.
A monopoly is a single seller of a good with no close substitutes. Market power is the ability to influence price. These ideas are related, but they are not the same as ordinary success. A firm can become large by making something consumers love. The economic concern begins when consumers lack alternatives and the firm can restrict output.
In perfect competition, each firm is a price taker. It faces a horizontal demand curve for its own output and produces where price equals marginal cost in the efficient benchmark. A monopolist faces the entire market demand curve. It is a price maker, but not all-powerful. A monopolist can choose price or quantity, but not both independently. Demand still pushes back.
| Feature | Perfect competition | Monopoly |
|---|---|---|
| Sellers | Many | One |
| Firm demand | Horizontal | Downward-sloping market demand |
| Price role | Price taker | Price maker constrained by demand |
| Output rule | P = MC | MR = MC, then price from demand |
Why Marginal Revenue Lies Below Demand
Selling one more unit can lower the price on units already being sold.
Demand tells the monopolist the price consumers will pay. Marginal revenue tells the monopolist what selling one more unit actually adds. For a single-price monopolist, marginal revenue is below price because selling more requires lowering the price on all units sold, not just the new unit.
If the monopolist can sell one unit for $10, total revenue is $10. To sell two units, suppose it must lower the price to $9. Total revenue becomes $18. The second unit does not add $9 of revenue. It adds $8, because the first unit also sells for $1 less.
| Quantity | Price | Total revenue | Marginal revenue |
|---|---|---|---|
| 1 | $10 | $10 | $10 |
| 2 | $9 | $18 | $8 |
| 3 | $8 | $24 | $6 |
| 4 | $7 | $28 | $4 |
Retrieval practiceIf price is $8 and quantity is 3, what is total revenue?▸
TR = P × Q = $8 × 3 = $24.
Profit Maximization for a Monopolist
Find quantity with MR = MC. Find price on demand.
The AP monopoly graph has one move students must master: the monopolist produces where MR = MC, then charges the price consumers are willing to pay for that quantity on the demand curve. The monopolist does not choose the point where demand intersects marginal cost. That point is the efficient benchmark, not the profit-maximizing monopoly quantity.
Quick checkWhy does the monopolist not charge the price at MR = MC?▸
Monopoly Inefficiency and Deadweight Loss
The deeper loss is the trade that never happens.
Monopoly creates inefficiency because output is restricted below the socially efficient quantity. The efficient benchmark is where price equals marginal cost, or where demand intersects MC. At that point, every unit whose value exceeds its marginal cost is produced.
A monopolist instead produces where MR = MC. Since MR lies below demand, that quantity is smaller than the efficient quantity. Price is higher, output is lower, and some consumer surplus is transferred to the producer as profit. But that transfer is not the social cost. A transfer just moves surplus from one pocket to another; the economy still has it. The genuine loss — the deadweight loss — is the surplus that is destroyed, because some mutually beneficial trades, units whose value exceeds their marginal cost, simply never happen. Nobody captures that value. It is gone.
| Area | How to find it on the monopoly graph |
|---|---|
| Consumer surplus | Above Pm and below demand, up to Qm. |
| Producer surplus | Below Pm and above the relevant cost measure over units sold. |
| Economic profit | (Pm − ATC at Qm) × Qm, shown as the profit rectangle when Pm > ATC. |
| Deadweight loss | The lost-surplus triangle between Qm and Qe. |
Price Discrimination: The Same Power, Used to Sort
The phrase sounds like a moral accusation. In economics, it is first a description.
Price discrimination occurs when a seller charges different buyers different prices for reasons not fully explained by cost differences. The careful wording matters. If overnight shipping costs more because it requires faster transport and tighter logistics, the higher price is not price discrimination — it reflects a real cost. If a lender charges more because a borrower is more likely to default, part of the higher price may reflect a real cost too.
But if two students sit in the same lecture hall, hear the same professor, and take the same exam, while one pays close to the sticker price and another pays a far lower net price after aid, the cost of teaching one rather than the other does not explain the gap. The gap is explained by sorting: who can pay, who is likely to enroll, and who needs a discount to show up. This is why price discrimination hides inside friendlier words — discount, scholarship, coupon, loyalty reward, special offer. The label does not settle the economics. Generosity, recruiting, and price discrimination can overlap.
Three things must hold. A seller cannot simply wish price discrimination into existence.
| Condition | What it means | Example |
|---|---|---|
| Market power | The seller can influence price, not just accept it. | A differentiated college, an airline on a route. |
| Sorting | The seller can tell buyers or groups apart by willingness to pay. | Aid forms, age, timing, a coupon that rewards searching. |
| Limited resale | Low-price buyers cannot resell to high-price buyers. | A discounted seat or ticket is tied to one person. |
Elasticity is escape power. The rule underneath all of it: charge higher prices to groups with less elastic demand and lower prices to groups with more elastic demand. Buyers with good substitutes, flexible timing, or lower incomes can walk away more easily, so they are charged less. Buyers with urgent needs and few substitutes are easier to charge more. A business traveler whose meeting is Tuesday has less elastic demand than a vacationer who can shift dates, drive, or skip the trip. A coupon-clipper reveals price sensitivity by the very act of clipping.
Economists sort price discrimination by how precisely the seller can read willingness to pay.
| Type | How price differs | Common example |
|---|---|---|
| First degree | Each buyer pays their maximum willingness to pay. | A theoretical, perfectly informed seller. |
| Second degree | Buyers self-select by quantity, version, or package. | Basic, pro, and enterprise software tiers. |
| Third degree | Identifiable groups pay different prices. | Student discounts, senior discounts, leisure vs. business fares. |
Quick checkA firm charges more for a larger package because it contains twice as much product. Is that price discrimination?▸
The Welfare Turn: When Sorting Expands the Pie
Sorting buyers is not only a way to take more. It can be a way to serve more.
Return to Concept 4. The single-price monopolist charges one high price, and in doing so it excludes every buyer whose willingness to pay sits above marginal cost but below that price. Those were mutually beneficial trades. Because they do not happen, deadweight loss appears.
Price discrimination changes the picture. If the firm can quietly lower the price for more elastic buyers while holding a high price for less elastic ones, it can sell to people who would have been priced out under a single posted price. Output rises. Deadweight loss shrinks. In the extreme of perfect price discrimination, the firm serves every buyer whose willingness to pay exceeds marginal cost, and the deadweight loss disappears entirely — the same efficient quantity as perfect competition.
So is price discrimination good? Here is the uncomfortable part. The pie can grow, but the firm keeps most of the new slice. Consumer surplus falls, because buyers now pay closer to their maximum, and the surplus they used to enjoy is captured as profit. Price discrimination can be efficient and distributionally unpopular at the same time. That is the honest verdict, and it is why monopoly is not a one-note villain: the very power that destroys surplus by restricting output can, turned the other way, recover it — while changing who ends up holding it.
A college has four potential students. The marginal cost of enrolling one more is $8,000.
| Student | Willingness to pay (one year) |
|---|---|
| A | $40,000 |
| B | $30,000 |
| C | $20,000 |
| D | $10,000 |
- Best single price: at $30,000, students A and B enroll. Revenue = $30,000 × 2 = $60,000.
- Producer surplus above marginal cost: ($30,000 − $8,000) × 2 = $44,000. Students C and D are priced out, even though each values a seat above its $8,000 cost — that is the deadweight loss.
- Perfect price discrimination: charge each student their willingness to pay. Revenue = $40,000 + $30,000 + $20,000 + $10,000 = $100,000.
- Producer surplus above marginal cost: ($40,000 − $8,000) + ($30,000 − $8,000) + ($20,000 − $8,000) + ($10,000 − $8,000) = $32,000 + $22,000 + $12,000 + $2,000 = $68,000.
Quick checkWhy can price discrimination reduce deadweight loss even while making consumers feel worse off?▸
The moves you can now make
Three application checks — revenue, the graph, and the welfare turn. Try each before revealing.
Revenue table
A monopolist sells 3 units at $8 each and 4 units at $7 each. What is the marginal revenue from the fourth unit?
Reveal answer▸
TR at 3 units = $24. TR at 4 units = $28. MR = $28 − $24 = $4. The fourth unit sells for $7, but the price cut applies to the first three as well, so MR falls below price.
Graph move
On a monopoly graph, how do you find Qm and Pm, and where is the deadweight loss?
Reveal answer▸
Find Qm where MR = MC. Go straight up from Qm to the demand curve to read Pm. Deadweight loss is the triangle between Qm and Qe (where D = MC), bounded above by demand and below by MC.
Welfare reasoning
A monopolist begins charging a lower price to a more elastic group while holding a higher price for a less elastic group. State one efficiency effect and one distributional effect.
Reveal answer▸
Efficiency: output may rise because buyers who were priced out under one price now buy, reducing deadweight loss.
Distribution: consumer surplus may fall, because the firm captures more of each buyer's willingness to pay as producer surplus.
From single price to price discrimination
A single-price monopolist faces downward-sloping market demand and earns positive economic profit. Later, consider the same firm if it could perfectly price discriminate.
- Draw a correctly labeled monopoly graph with D, MR, MC, and ATC. 1 pt
- Identify the profit-maximizing quantity, Qm. 1 pt
- Identify the price, Pm. 1 pt
- Identify the efficient quantity, Qe. 1 pt
- Shade or identify the profit rectangle. 1 pt
- Shade or identify the deadweight loss. 1 pt
- Explain why marginal revenue lies below demand. 1 pt
- Explain why deadweight loss is a true social cost, not just a transfer. 1 pt
- If the firm perfectly price discriminates, state what happens to output and to deadweight loss. 1 pt
- State who gains and who loses when the firm moves from one price to perfect price discrimination. 1 pt
Reveal scoring guide▸
- 1 ptD, MR, MC, and ATC correctly labeled.
- 1 ptQm found at MR = MC.
- 1 ptPm found on demand directly above Qm.
- 1 ptQe found where D = MC.
- 1 ptProfit rectangle uses Pm, ATC at Qm, and Qm.
- 1 ptDWL shown as the triangle between Qm and Qe.
- 1 ptMR is below demand because lowering price to sell one more unit lowers revenue on all previous units.
- 1 ptDWL is destroyed surplus from trades that never occur; a transfer would still leave the surplus inside the economy.
- 1 ptOutput rises to Qe and deadweight loss falls to zero under perfect price discrimination.
- 1 ptThe firm gains (it captures consumer surplus as profit); buyers as a group lose surplus, though some previously excluded buyers are now served.
A monopolist's deepest cost is not the money it moves from buyers into profit, but the mutually beneficial trades that never happen — yet the same power, turned to sorting buyers by what they will pay, can win those trades back while keeping the gains.
they would not punish bigness as such. They would attack the barriers that let firms trap consumers and restrict output. The target is protected power, not success.
- For a single-price monopolist, MR lies below demand, so output is restricted and price is raised.
- Deadweight loss is surplus destroyed, not transferred.
- Price discrimination can raise output and shrink that loss while capturing surplus as profit.
- When is sorting buyers by willingness to pay fair, and when is it exploitation?
- If price discrimination raises output, should we still mind that the firm keeps the surplus?
- Which market power is earned reward, and which is entrenched rent?
When you're not offered the discount, you may be the inelastic buyer — the one with no easy alternative. Before you pay, find out what your alternatives actually are.
If you have pricing power, sorting customers by willingness to pay can grow the market, not just the margin — but only if you can keep the cheap version from being resold to the buyer who'd have paid full price.
