Natural Monopoly & Two Regulated Prices

You’ll be able to

Explain why large economies of scale can make one firm the efficient producer
Distinguish socially optimal pricing from fair-return pricing
Explain why socially optimal pricing may require a subsidy

When one firm is genuinely cheaper

A natural monopoly arises when economies of scale are so extensive that average total cost keeps falling across the whole relevant range of demand. Water distribution, electricity grids and rail track are the standard examples: the fixed cost of the network is enormous and the marginal cost of another user is tiny. Splitting the market among several firms would mean duplicating the network, raising average cost for everyone. So the efficient number of firms is one — and the policy problem becomes how to constrain it rather than how to create competition.

The two regulated prices

socially optimal (marginal-cost) pricing: P = MC — allocatively efficient, but the firm loses money · fair-return (average-cost) pricing: P = ATC — firm breaks even, but P > MC so some deadweight loss remains

Because ATC is falling, MC lies BELOW ATC throughout. So P = MC necessarily means P < ATC and a loss.

Why marginal-cost pricing loses money

When average cost is falling, marginal cost lies below average cost — that is what pulls the average down. So setting P = MC sets price below ATC and the firm makes a loss on every unit. The regulator must then either subsidize the firm or accept that it will not operate. This is a genuine dilemma, not a technicality: the allocatively efficient price is not financially viable without public money.

Why fair-return pricing is the common compromise

P = ATC lets the firm earn zero economic profit — a normal return — with no subsidy. Since ATC is above MC, price still exceeds marginal cost, so some deadweight loss remains. Regulators mostly choose this because it is self-financing and requires less information. Understanding why both prices exist, and what each sacrifices, is what the exam actually tests.

Worked example

A regulated water utility faces a demand curve crossing MC at 900 units where MC is $2, and crossing ATC at 700 units where ATC is $5. Compare the two regulated outcomes.

1.Socially optimal pricing: P = $2, Q = 900 units. Allocatively efficient, since P = MC.
2.But ATC at 900 units exceeds $2, so the firm makes a loss and needs a subsidy to survive.
3.Fair-return pricing: P = $5, Q = 700 units. The firm breaks even with zero economic profit.
4.Price $5 exceeds MC, so 200 units that consumers value above marginal cost go unproduced — residual deadweight loss.

Answer: Socially optimal pricing gives 900 units at $2 with no deadweight loss but a loss requiring subsidy. Fair-return pricing gives 700 units at $5 with the firm self-financing but 200 units of forgone efficient output. The choice is between public money and residual inefficiency.

Watch out

For a natural monopoly, MC is below ATC everywhere in the relevant range — the falling average requires it. So the socially optimal price is always the lower of the two and always implies a loss. An answer claiming P = MC lets the firm break even has the geometry backward.

Checkpoint

A natural monopoly exists when:

Checkpoint

Regulating a natural monopoly at the socially optimal price P = MC results in:

Checkpoint

Fair-return pricing sets P = ATC. The consequence is that:

On the exam

Mark both regulated prices and both quantities on the graph before writing. The comparison is the answer, and having both points visible prevents describing one outcome's efficiency with the other's profitability.

Answer the 3 checkpoints as you read.