A monopoly is not just a big firm. It is the firm — the only seller, with no close substitute and a wall around the industry keeping everyone else out. That wall is what lets its profit survive year after year, and it is what makes monopoly a source of market failure.
📘 What you need to know
A monopoly is a market with a single seller and no close substitutes, protected by very high barriers to entry.
The firm is the industry, so it faces the whole market’s downward sloping demand curve: D = AR.
Because it must lower price to sell more, MR lies below AR and falls twice as steeply.
It still maximises profit at MC = MR, then charges the price on the AR curve above that output.
Abnormal profit can last into the long run, because barriers stop new firms competing it away.
A monopoly is usually allocatively inefficient (AR > MC) and productively inefficient (AC above its minimum), creating a welfare loss.
A natural monopoly exists where average costs fall over the whole range of demand, so one firm can supply the market more cheaply than two.
Where the barriers come from
No barriers, no monopoly. These are the four to know.
Legal barriers — patents, licences, or a law giving one firm the sole right to supply.
Natural (cost) barriers — huge fixed costs mean a new entrant would have far higher average costs than the firm already there.
Technical barriers — control of a key input, a network, or technology nobody else has.
Strategic barriers — the firm buys up potential rivals, ties up suppliers, or threatens to cut prices below cost if anyone enters.
If an exam question ever asks “why can this firm keep making abnormal profit?”, the answer is never “because it is big”. It is always “because barriers to entry stop new firms competing the profit away”.
Why MR sits below AR
The monopolist is the only seller, so to sell one more unit it must lower the price — and not just on the new unit. It has to lower the price on every unit it sells. So the revenue gained on the extra unit is partly cancelled by the revenue lost on all the others. That is why MR falls faster than price.
For a straight-line demand curve
MR has the same intercept as AR but twice the slope
Compare this with the perfect competition long-run diagram: the price here sits above both MC and the lowest point of AC, and output is smaller. That gap is the cost of market power.
Diagram analysis you can reuse
Output is Q1, where MC = MR.
Price is P1, read up to the AR curve.
Cost per unit is C1, read from AC at Q1.
Abnormal profit = (P1 – C1) × Q1, the shaded box.
Not allocatively efficient: AR > MC, so output is below the socially best level and a welfare loss exists.
Not productively efficient: AC at Q1 is above the minimum of AC.
Possibly dynamically efficient: the profit can be reinvested in research and new products.
🤔 Why “high price, low output” is the heart of the criticism
Consumers value the next unit at P1, and it would only cost MC to make. Since P1 is above MC, there are units that would make both buyer and seller better off, and the monopolist chooses not to make them — because making them would force the price down on everything else. Society loses that benefit. Economists call the lost gain a welfare loss or deadweight loss.
Is monopoly always bad?
No, and saying so is where evaluation marks live.
Group
Possible benefits
Possible costs
Consumers
Profits can fund innovation and better products; economies of scale can lower prices
Higher prices, less choice, weaker service, no pressure to improve
The firm
Secure profit, economies of scale, strength in global markets
Little pressure to cut waste, so costs drift upwards
Workers
Higher wages are affordable out of abnormal profit
Only one large employer in the industry to work for
Suppliers
Large, reliable orders
The monopolist can dictate the price it pays them
Use a real example. A firm can become dominant simply by being better than everyone else, and consumers may enjoy lower prices and more choice as a result. The same size, though, lets it squeeze the suppliers who depend on it. Both sides in one sentence is exactly what an evaluation question wants.
Natural monopoly
Sometimes having one firm really is the cheapest arrangement. A natural monopoly happens when the fixed costs are so huge, and economies of scale so long lasting, that average cost keeps falling across the whole range of market demand.
Think of water pipes, electricity grids or rail track. Laying a second set of pipes down the same street would double the fixed cost and split the customers, so both firms would end up with higher average costs than one firm had on its own.
The point of the diagram is the gap between C1 and C2. Competition here would not lower prices — it would raise costs. That is why natural monopolies are regulated rather than broken up.
Careful with wording: a natural monopoly is allowed to exist because breaking it up would be wasteful. It is then controlled with a maximum price, so it cannot use its position to overcharge.
Worked examples
WORKED EXAMPLE 1
A monopolist produces 40,000 units where MC = MR. At that output the AR curve gives a price of $18 and AC is $11. (a) Calculate abnormal profit. (b) Explain whether this profit will survive in the long run. [4]
(a) Profit = (AR – AC) × Q= (18 – 11) × 40,000 = 7 × 40,000Abnormal profit = $280,000(b) Think about entry
In perfect competition this profit would attract new firms and be competed away.
Here there are very high barriers to entry, so no new firms can join.
The abnormal profit can continue in the long runThe barriers do the explaining. Always name them.
WORKED EXAMPLE 2
Explain why a monopoly is allocatively inefficient. [4]
Step 1: State the condition
Allocative efficiency needs price (AR) = MC.
Step 2: What the monopolist does
It produces where MC = MR, and since MR is below AR, price ends up above MC.
Step 3: The consequence
Some units that consumers value more than they cost to make are never produced.
Step 4: Name the loss
Output is below the socially best level, creating a welfare loss.
P > MC, so resources are under-allocated to this marketMention MR lying below AR. That single fact is what causes the whole result.
💡 Exam tip
Draw MR with the same starting point as AR but reaching the axis at half the distance. Examiners check this.
Always shade and label the profit box, and write the formula (P – C) × Q beside it.
Compare with perfect competition to earn analysis marks: higher price, lower output, welfare loss.
For natural monopoly, the key phrase is “average cost is still falling at the level of market demand”.
If a question asks you to show falling demand, redraw AR and MR further left and show the smaller profit box.
Balance every criticism with a possible benefit: economies of scale, innovation, global competitiveness.
⚠ Common mix-up
Drawing MR above AR. MR is always below AR when demand slopes down.
Taking the price from the MC = MR point. Go up to AR. It is the most punished error in the whole topic.
Saying monopolies always make abnormal profit. They can break even or even make short-run losses if demand is weak.
Confusing natural monopoly with an ordinary monopoly. The natural version is about the shape of the cost curve, not about barriers or behaviour.
Assuming a monopoly can charge any price it likes. It is still limited by the demand curve — charge too much and quantity collapses.
Forgetting dynamic efficiency, which is the main defence of monopoly power.
Up next: Oligopoly, Collusion and Game Theory — what happens when there are just a few big firms and every decision depends on what the others will do.
Want this explained one-to-one?
Book a free session with an experienced IB Economics tutor and get your trickiest topics made simple.
