Which of the following best defines a monopoly?
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Economics · Foundations
Monopoly
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In 30 seconds
A Monopoly A market structure in which a single firm is the only seller of a product that has no close substitutes. Full entry → is a market with a single seller of a product that has no close substitutes, protected by barriers to entry. Because it is the only supplier, the monopolist faces the whole downward-sloping market demand curve and is a Price maker A seller that faces a downward-sloping demand curve and so must choose a point on it, trading a higher price against a lower quantity; contrasted with a price taker. Full entry →: to sell more it must lower the price on every unit, so its Marginal revenue The change in a firm's total revenue from selling one more unit of output. Full entry → falls below the price. It maximizes profit at the quantity where marginal revenue equals Marginal cost The additional cost of producing one more unit of output. Full entry →, then charges the highest price the demand curve allows there, producing less and charging more than a competitive market would.
Why this matters
Market structure decides who holds power over price, and monopoly is the extreme case where one firm sets it. The model explains why a single seller produces less and charges more than a competitive industry, and why economists call part of that gap a Deadweight loss The loss of total surplus that occurs when output is held below the allocatively efficient level, representing gains from trade that go to neither buyers nor sellers. Full entry → rather than a simple transfer from buyers to the firm. The same logic drives real debates: how patents trade higher prices now for more invention later, why electricity and water utilities are regulated instead of broken into competitors, and when antitrust authorities challenge a merger. Knowing the mechanics lets you weigh those arguments on evidence, and it anchors the comparison with the other market structures you will study next.
The college version
Barriers to entry: what makes a market a monopoly
A monopoly is a market served by a single seller of a product that has no close substitutes. The word describes the whole market, not just a big firm: if buyers who dislike the price have nowhere else to turn, the seller has genuine market power. What keeps that position from being competed away is a Barrier to entry A legal, technological, or market force that discourages or prevents potential competitors from entering a market, allowing a monopoly to persist. Full entry →, a legal, technological, or market force that discourages or prevents potential rivals from entering. Without such a barrier, the profits a lone seller earns would attract new firms until the extra competition eroded them, so barriers are what let a monopoly persist.
Barriers come in a few recognizable forms. Legal barriers are created by government: a patent gives an inventor the exclusive right to make, use, or sell an invention for a limited time (20 years in the United States), and trademarks and copyrights work similarly, while some statutes grant an outright legal monopoly, such as the U.S. Postal Service's exclusive right to deliver first-class mail. A Natural monopoly A situation in which economies of scale are so large relative to market demand that a single firm can supply the whole market at a lower average cost than several firms could. Full entry → arises from technology and cost: when Economies of scale The tendency for a firm's average cost per unit to fall as its scale of output rises, often because large fixed costs are spread over more units. Full entry → are large relative to the size of the market, one firm can supply the entire quantity demanded at a lower average cost than two or more firms could, as with the pipes and wires of water and electric utilities. Control of a scarce key resource can also block entry; historically ALCOA controlled the bauxite needed for aluminum, and De Beers long dominated the supply of rough diamonds. Network effects can act as a modern barrier, because a product that becomes more valuable as more people use it is hard for a newcomer to displace. This lesson owns monopoly; oligopoly and monopolistic competition, the in-between structures, are named here and taught in their own lessons.
The demand curve a monopoly faces, and why marginal revenue is below price
The single most important difference between a monopoly and a competitive firm is the demand curve each one faces. A perfectly competitive firm is a price taker: it is so small relative to the market that it can sell all it wants at the going price, so the demand curve for its own output is effectively a flat line at that price. A monopoly is the entire industry, so it faces the whole market demand curve, which slopes downward. That single fact makes the monopolist a price maker: it can choose a higher price and sell less, or a lower price and sell more, but it cannot have both. Every price-quantity pair it might pick is a point on that one demand curve.
Because the curve slopes down, selling one more unit requires cutting the price, and for a single-price seller that cut applies to every unit, not just the last one. So the marginal revenue from an extra unit, the change in total revenue, has two parts: the firm gains the price of the new unit, but it loses a little on each earlier unit that now sells for less. The loss on the earlier units drags marginal revenue below the price. Marginal revenue therefore lies beneath the demand curve at every quantity past the first. This is not a minor technicality; it is the hinge of the whole model, because the firm's output decision is driven by marginal revenue, while the price it can charge is read from the higher demand curve. Deriving the underlying cost curves is the job of the costs lesson; here we take marginal cost as given and focus on how revenue behaves.
Choosing output and price: MR = MC, then read the price off demand
A monopolist maximizes profit the same way any firm does, by weighing the revenue and the cost of one more unit. As long as an extra unit adds more to revenue than to cost, that is, marginal revenue exceeds marginal cost, producing it raises profit; once marginal cost would exceed marginal revenue, the firm stops. Profit is largest at the quantity where marginal revenue equals marginal cost: MR = MC. Notice that this rule picks a quantity, not a price.
To find the price, the monopolist takes that profit-maximizing quantity up to the demand curve and charges the most buyers are willing to pay for it. That price sits on the demand curve, which lies above marginal revenue, and marginal revenue was set equal to marginal cost, so the price ends up above marginal cost. This is the defining signature of monopoly, and the one-line contrast with perfect competition captures it: a competitive market is pushed to price equal to marginal cost (P = MC), while a monopoly settles at price greater than marginal cost (P > MC). The monopolist is not charging "as much as possible"; a price too high would sell too little to be profitable. It is charging exactly the price that the MR = MC quantity commands on the demand curve. A monopoly earning profits can keep them precisely because barriers to entry keep rivals out; the same profits in a competitive market would invite entry that competed the price down.
Why monopoly costs society: deadweight loss, natural monopoly, price discrimination
Because a monopoly stops where MR = MC rather than where price equals marginal cost, it produces less than the allocatively efficient quantity. Allocative efficiency The output level at which the price buyers place on the last unit equals its marginal cost, so all mutually beneficial units are produced. Full entry → is the output at which the price buyers place on the last unit just equals the marginal cost of making it; every unit whose value to buyers exceeds its cost of production is worth making. A monopoly leaves some of those mutually beneficial units unproduced, and the value society loses on them, gains that go to neither the buyer nor the seller, is the deadweight loss. It is important to separate this from the higher price on the units that are sold: that higher price transfers surplus from consumers to the firm, but a transfer is not a loss to society, whereas the deadweight loss is surplus that simply vanishes.
The natural-monopoly case is the important exception to the usual policy instinct. When one firm can serve the whole market more cheaply than several could, forcing competition would raise average costs, so governments typically allow a single regulated provider, as with electricity and water, and control its prices rather than break it up. Another wrinkle is Price discrimination Charging different buyers different prices for the same product, based on differences in their willingness to pay. Full entry →, charging different buyers different prices for the same good (student and senior discounts are everyday examples). Price discrimination can expand the quantity sold and shrink the deadweight loss, because some buyers who would not pay the single monopoly price are now served, though it tends to shift surplus toward the seller. The broader takeaway is that market structure, not the seller's character, drives these outcomes: change the barriers to entry and you change the price, the quantity, and the efficiency of the whole market.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine you are the only lemonade stand allowed on the whole street, and a fence keeps anyone else from opening one. You get to pick the price, but there is a catch: the whole street shares one demand list, so to sell more cups you have to drop the price for everybody, not just the next customer. That is why earning an extra sale is worth less than the sticker price to you, because you give up a few coins on all the cups you could have charged more for. So you do not sell as many cups as you could. You pick the amount where the money from one more cup just covers what it costs to make, then charge the highest price that many cups will sell for. The result is fewer cups sold at a higher price than if lots of stands were competing. Some thirsty people who would happily pay more than a cup costs to make never get one, and that missed win is pure waste, not money moved around, just gone.
Picture it like this
A monopoly is like the only bridge across a river. The bridge owner can set any toll, but a higher toll means fewer cars cross, so there is always a trade-off between the toll and the traffic. The owner picks the toll that makes the most money, and it leaves the bridge emptier than a free-flowing road would be, with some travelers who would gladly pay a fair toll turned away.
Where the picture stops working
The bridge makes the single-seller idea vivid, but a real monopolist usually charges one price to everyone at once rather than negotiating car by car, and the analogy hides where the barrier comes from, a patent, a resource, or huge fixed costs, which is exactly what determines whether the monopoly can last.
Worked example
Suppose a firm is the only seller of a patented gadget, and the market demand schedule is P = 11 - Q (in dollars). Its marginal cost is a constant $4 per unit. Build the revenue table: at Q = 1 the price is $10 and total revenue is $10; at Q = 2, price $9, revenue $18, so marginal revenue is $8; at Q = 3, price $8, revenue $24, MR $6; at Q = 4, price $7, revenue $28, MR $4; at Q = 5, price $6, revenue $30, MR $2. Notice marginal revenue is below the price at every step, exactly as the model predicts. The profit-maximizing rule is MR = MC, and MR equals the $4 marginal cost at Q = 4. So the monopolist makes 4 units and reads the price off the demand curve: P = 11 - 4 = $7. Price ($7) exceeds marginal cost ($4), the monopoly signature. An efficient market would keep producing until price met marginal cost, at P = $4, Q = 7. The three units between Q = 4 and Q = 7 are worth more to buyers than they cost to make, yet go unproduced; that forgone surplus is the deadweight loss.
Key takeaway
A monopoly is a single seller shielded by barriers to entry; facing the whole market demand curve it is a price maker whose marginal revenue lies below price, so it produces where MR = MC and charges a price above marginal cost, yielding less output at a higher price than competition and a deadweight loss to society.
Quick check
3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.
A city grants a single company the exclusive right to run water pipes because one firm can supply the whole city at a lower average cost than several competing firms could. This barrier to entry is an example of a:
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A single-price monopolist faces demand P = 11 - Q and has a constant marginal cost of $3. Using the rule that it produces where marginal revenue equals marginal cost, what quantity does it produce and what price does it charge?
Choose an answer, then check it.
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related
You’ll learn to
- Define monopoly and identify the barriers to entry, legal, natural (economies of scale), control of a key resource, and network effects, that protect it.
- Explain why a monopolist is a price maker facing the market demand curve and why its marginal revenue lies below the price.
- Apply the MR = MC rule to find a monopolist's profit-maximizing quantity and read the corresponding price off the demand curve.
- Analyze why monopoly yields lower output at a higher price than perfect competition, and why this produces a deadweight loss.
- Distinguish a natural monopoly, explain the basic case for regulating it, and identify price discrimination.
Common mistakes
Thinking a monopolist can charge any price it likes with no limit.
The monopolist still faces the market demand curve. A higher price always means a lower quantity sold, so it is constrained to points on that curve and picks the one that maximizes profit, not the highest number it can imagine.
Setting price where MR = MC.
MR = MC determines the profit-maximizing quantity, not the price. The price is then read upward from that quantity to the demand curve, and it lies above marginal revenue and marginal cost.
Believing marginal revenue equals the price for a monopoly, as it does for a price taker.
Because the demand curve slopes down and one price applies to all units, selling more lowers the price on every unit, so marginal revenue is below the price for a single-price monopolist.
Calling the entire gap between the monopoly price and marginal cost a loss to society.
The higher price on units actually sold is a transfer from consumers to the firm, not a social loss. The deadweight loss is only the value of the beneficial units that go unproduced because output is restricted.
Assuming every monopoly should be broken up into competing firms.
A natural monopoly serves the whole market at lower average cost than several firms could, so splitting it up would raise costs. The usual response is to allow one provider and regulate it, as with electricity and water utilities.
Easily confused
Monopoly vs. Perfect competition
A monopolist is a price maker facing the whole downward-sloping demand curve and settles at price greater than marginal cost (P > MC); a competitive firm is a price taker and the market is driven to price equal to marginal cost (P = MC).
Deadweight loss vs. Transfer from consumers to the firm
The deadweight loss is surplus that vanishes because beneficial units go unproduced; the higher price on units still sold merely moves surplus from buyers to the seller and is not a net loss to society.
Natural monopoly vs. Legal monopoly
A natural monopoly arises from economies of scale that make one firm the cheapest supplier; a legal monopoly is created directly by government through patents, licenses, or statute.
Key vocabulary
- Monopoly
- A market structure in which a single firm is the only seller of a product that has no close substitutes.
- Barrier to entry
- A legal, technological, or market force that discourages or prevents potential competitors from entering a market, allowing a monopoly to persist.
- Natural monopoly
- A situation in which economies of scale are so large relative to market demand that a single firm can supply the whole market at a lower average cost than several firms could.
- Price maker
- A seller that faces a downward-sloping demand curve and so must choose a point on it, trading a higher price against a lower quantity; contrasted with a price taker.
- Marginal revenue
- The change in a firm's total revenue from selling one more unit of output.
- Marginal cost
- The additional cost of producing one more unit of output.
- Allocative efficiency
- The output level at which the price buyers place on the last unit equals its marginal cost, so all mutually beneficial units are produced.
- Deadweight loss
- The loss of total surplus that occurs when output is held below the allocatively efficient level, representing gains from trade that go to neither buyers nor sellers.
- Price discrimination
- Charging different buyers different prices for the same product, based on differences in their willingness to pay.
- Economies of scale
- The tendency for a firm's average cost per unit to fall as its scale of output rises, often because large fixed costs are spread over more units.
Sources & references
- Principles of Economics 3e, 9.1 How Monopolies Form: Barriers to Entry — OpenStax (Rice University)
- Principles of Economics 3e, 9.2 How a Profit-Maximizing Monopoly Chooses Output and Price — OpenStax (Rice University)
- Principles of Microeconomics, 8.2 Fixing Monopoly — University of Victoria / eCampusOntario Pressbooks
EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.
Researched 2026-08-19
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