Chapter 15: Monopoly

When one firm becomes the market — and what it costs everyone else

Section 1

Why Monopolies Arise

A monopoly is a firm that is the sole seller of a product without close substitutes. Unlike a competitive firm — which takes the market price as given — a monopolist is a price maker. It can raise price without losing all its customers, because there is nowhere else for them to go.

The root cause of monopoly is always the same: barriers to entry. If another firm could easily enter and copy the product, any profits would be competed away. Mankiw identifies three sources.

1. Resource

Key Resource Ownership

A single firm owns the essential input. DeBeers once controlled most of the world's diamond mines — no mines, no diamonds, no competition.

2. Government

Government-Created

Patents and copyrights give exclusive rights to produce a good for a fixed period. New drugs, books, music — the government creates the monopoly on purpose to reward innovation.

3. Natural

Natural Monopoly

A single firm can supply the entire market at lower cost than two or more firms could. This happens when average total cost keeps declining over the relevant range of output — usually because of huge fixed costs relative to marginal cost (water pipes, power grids, subway systems).

Key distinction: A competitive firm faces a horizontal demand curve at the market price — it can sell all it wants at P, but nothing above. A monopolist faces the entire market demand curve, which slopes down. To sell more, the monopolist must cut price.

Section 2

How a Monopolist's Revenue Works

For a competitive firm, marginal revenue equals price: selling one more unit just adds P to total revenue. But for a monopolist, MR < P. Why? Because to sell one more unit, the monopolist must lower the price — and that price cut applies to every unit it sells, not just the new one.

Output effect +P × 1 unit revenue from the new unit sold
Price effect −ΔP × Q lost revenue on all existing units
Marginal revenue MR = P + Q (ΔP/ΔQ) sum of both effects, < P

The "Twice the Slope" Rule

For any linear demand curve P = a − bQ, marginal revenue is also linear, with the same intercept but twice the slope:

If P = a − bQ → MR = a − 2bQ

So MR starts at the same point as demand on the price axis, but falls twice as fast. This is the single most useful fact about monopoly math — every profit-max problem in the worksheet uses it.

MR shares the demand curve's intercept on the price axis but is twice as steep. It crosses the quantity axis at exactly half the demand intercept.

Section 3

Profit Maximization

The profit-maximizing rule is the same as for any firm: produce where MR = MC. But then there's one extra step that distinguishes monopoly from perfect competition.

Step 1 — Find Q Set MR = MC, solve for Qm
Step 2 — Find P Plug Qm into the demand curve
Step 3 — Profit π = (Pm − ATC) × Qm
The most common mistake: using MR to find the price. MR gives you the profit-maximizing quantity. But consumers pay the price on the demand curve at that quantity. MR is below P, so reading P off MR understates the price and undershoots profit.

Qm is where MR = MC. Read Pm straight up from Qm on the demand curve (not MR). The shaded rectangle is profit = (Pm − ATC) × Qm.

Section 4

The Welfare Cost of Monopoly

In a competitive market, producers make the quantity where price equals marginal cost. That's the socially efficient level — every unit whose benefit to consumers (P) exceeds its cost (MC) gets produced. A monopoly stops short of that point, producing where MR = MC instead.

The result: some units that consumers would gladly buy for more than their production cost never get made. That lost gain is the deadweight loss of monopoly — a welfare triangle that nobody captures.

DWL is the triangle between the demand curve and MC, from Qm out to Qe. Those units are worth producing (P > MC) but the monopolist refuses because selling them would require lowering the price on everything.

Monopoly profit is not DWL. Profit is a transfer from consumers to the producer — not a loss to society. The DWL is the trade that never happens: units between Qm and Qe that would benefit both sides but don't get made.

Section 5

Price Discrimination

Price discrimination means charging different prices to different customers for the same good. It requires two things: the firm must have some market power (a monopolist does, a competitive firm doesn't), and the firm must be able to separate buyers by willingness to pay and prevent resale between them.

Perfect Price Discrimination

Imagine a monopolist charges every customer exactly the maximum they would pay — reading each buyer's value off the demand curve and charging exactly that. This extreme case is called perfect price discrimination.

Quantity produced Qe (the efficient level) same as perfect competition
Deadweight loss 0 every beneficial unit gets sold
Consumer surplus 0 monopolist captures all of it

The surprising lesson: perfect price discrimination produces the efficient quantity. It just moves all the surplus from consumers to the producer. There's no DWL because no trade that would benefit both sides gets skipped.

In the Real World

Firms can't read your mind — perfect price discrimination is a thought experiment. Instead they use observable proxies for willingness to pay:

Examples: movie theater student discounts, airline tickets (weekend-stay vs. business travelers), coupons (only price-sensitive shoppers clip them), hardcover vs. paperback books, brand-name vs. generic drugs, quantity discounts.

Each scheme increases both profits and total output — price discrimination raises efficiency relative to uniform-price monopoly because more units get sold.

Section 6

Public Policy Toward Monopolies

Because monopolies produce less than the efficient quantity, there's a case for government intervention. Mankiw lists four policy responses.

Policy What it does
Antitrust laws Break up large firms, block mergers that would concentrate power (Sherman Act 1890, Clayton Act 1914).
Regulation Cap prices — often for natural monopolies like utilities. Price set at marginal cost leads to losses; average cost pricing is a compromise.
Public ownership Government runs the firm directly. Post office, subways, some water systems. Avoids private profit motive but creates its own inefficiencies.
Doing nothing If intervention is likely to cause more harm than good — imperfect regulation, capture, innovation loss from patent revocation — sometimes the best policy is none.

Natural Monopoly: A Special Case

A natural monopoly exists where ATC is still declining at the market's output level. Because scale lowers cost, a single firm serves the market more cheaply than several competing firms would. Breaking it up would raise prices, not lower them.

In a natural monopoly, ATC keeps falling at the market's demand level. MC lies below ATC (as it must when ATC is declining). Competition would mean two firms each paying the huge fixed cost — more expensive than one firm serving everyone.

Section 7

Key Takeaways

  • Monopoly = one seller + barriers to entry. Three sources: resource control, government grant (patents), or natural monopoly (declining ATC).
  • MR < P for a monopolist — cutting price to sell one more unit also reduces revenue on all existing units. For linear demand P = a − bQ, then MR = a − 2bQ.
  • Profit max: set MR = MC to find Qm, then read Pm off demand. Never read the price off MR.
  • Profit = (Pm − ATC) × Qm, the rectangle between the monopoly price and the ATC curve, Q units wide.
  • Monopoly creates DWL — the triangle between D and MC from Qm to Qe. Profit is not DWL; it's a transfer. DWL is the trades that never happen.
  • Perfect price discrimination eliminates DWL but captures all consumer surplus. Real-world discrimination (discounts, coupons) both raises profits and raises total output.
  • Policy options: antitrust, regulation, public ownership, or doing nothing. No tool is perfect — each has costs.