Monopoly

The Only Seller

Economists in the late 19th century believed monopolies were temporary — that high profits would inevitably attract competitors who'd break the stranglehold. Standard Oil proved them spectacularly wrong. John D. Rockefeller controlled over 90% of U.S. oil refining by 1880 and maintained that grip for decades through railroad rebates, predatory pricing, and horizontal integration. The competitors never showed up, because Rockefeller made sure the door was welded shut behind him.

A monopolist is the sole seller in a market with no close substitutes. Unlike a wheat farmer competing against thousands of identical operations, the monopolist faces no competition. One seller, full control over price. That makes the monopolist a price maker.

Pfizer holding a patent on a blockbuster cancer drug. De Beers controlling 85% of the world's rough diamond supply through the 1990s. Your local electric utility. Buyers either pay up or go without.

What keeps competitors out? A barrier to entry:
- Patents grant pharmaceutical companies 20 years of exclusivity — Humira earned AbbVie over $200 billion before its patent expired in 2023
- Exclusive resource control, like De Beers locking up diamond mines across southern Africa
- Massive startup costs — nobody casually builds a second electrical grid serving the same city
- Government franchise, where your local water utility holds a legal monopoly granted by the municipality

Remove the barrier and monopoly power evaporates. That point matters far more than most AP students realize when answering free-response questions about long-run monopoly behavior.

Why MR Is Less Than Price

Say you sell 10 units at $50 each. You want to sell an 11th. But the demand curve slopes down, so the price must drop to $49 for all units. You gain $49 from the new buyer. You also lose $1 on each of the 10 units you were already selling, because those buyers now pay $49 instead of $50. Net gain: $49 minus $10 = $39. That $39 is your marginal revenue, and it sits well below the $49 price.

For a monopolist, MR is always less than price. Always.

A competitive firm never faces this problem because it sells at the market price without nudging it. The monopolist *is* the market. Every output decision moves the price for everyone. On the graph, the MR curve sits below the demand curve. For a linear demand curve, both start at the same vertical intercept, but MR falls twice as fast. That growing wedge between price and MR drives everything else in monopoly analysis — profit maximization, the deadweight loss triangle, all of it.

Profit Maximization: MR = MC

A monopolist follows the same profit rule as any firm: keep producing as long as the next unit brings in more revenue than it costs. Stop when MR = MC.

The mistake that kills exam scores is confusing the quantity decision with the price decision. If a free-response question asks you to identify the monopoly price and you point to where MR crosses MC on the vertical axis, that's wrong. The process has two distinct steps:

1. Find the quantity where MR = MC. Call it Qm.
2. Go *up* to the demand curve at Qm to read the maximum price buyers will pay. That's the monopoly price, Pm.

Pfizer doesn't look at the MR = MC intersection and charge that dollar amount. Pfizer finds the output level where MR = MC, then checks the demand curve to see what patients or insurance companies will actually pay for that quantity. The price lands above MC. Sometimes far above it.

The result: less output and a higher price than a competitive market would deliver. Competition pushes production out to where P = MC. The monopolist stops well short, leaving willing buyers unserved and transactions unmade.

Deadweight Loss

A patient is willing to pay $200 for a drug that costs $30 to produce. Under competition, that trade happens and both sides gain. Under monopoly, the firm prices the drug at $250 and the patient walks away empty-handed. The $170 of potential surplus vanishes entirely. Nobody gets it.

Multiply that across every unit between the monopoly quantity (Qm) and the competitive quantity (Qc), and you get the deadweight loss (DWL) triangle. On the graph, it sits between those two quantities — representing trades where buyers valued the good above the production cost but got priced out anyway.

DWL is *not* the monopolist's profit. This trips people up constantly. Profit is a transfer — surplus moves from consumers to the producer, but it still exists somewhere in the economy. Deadweight loss is surplus that nobody captures. Destroyed.

The U.S. Department of Justice pursues antitrust regulation precisely because of this. The bigger the gap between Qm and Qc, the larger that triangle, and the more society loses. The 1998 antitrust case against Microsoft, the 2020 suit against Google — the underlying economic argument is always about this deadweight loss and the consumer harm it represents.

Worked Example

Demand: P = 100 - Q. Cost: MC = 10 + 0.5Q. Work through the monopoly outcome, then compare it to competition.

Derive MR. For any linear demand P = a - bQ, double the slope: MR = a - 2bQ.

MR = 100 - 2Q

Set MR = MC.

100 - 2Q = 10 + 0.5Q
90 = 2.5Q
Qm = 36 units

Read the price off the demand curve (not MR):

Pm = 100 - 36 = $64

Compare to competition. A competitive market sets P = MC directly:

100 - Q = 10 + 0.5Q
90 = 1.5Q
Qc = 60 units, Pc = $40

The monopolist produces 24 fewer units (36 vs. 60) and charges $24 more ($64 vs. $40). Every unit between Q = 36 and Q = 60 would have generated surplus, since buyers valued those units above production cost. That lost triangle is the deadweight loss.