Microeconomics guide, Unit 6, step 24 of 28Previous step Next step

Step 24: Government Policy and Market Structures

By the EconTutorials team. 10-minute read with 6 practice questions.

SyllabusAP Micro 6.4 · Cambridge 8.1 · IB 2.11 (HL)

In 30 seconds

A lump-sum tax on a monopoly is a fixed cost: it lowers profit but leaves MC, price and output unchanged. A per-unit tax raises MC, so price rises and output falls.
A natural monopoly can be regulated at the socially optimal price $ P = MC $ (efficient, but usually needs a subsidy) or the fair-return price $ P = ATC $ (normal profit, some inefficiency remains).
A price ceiling set between the monopoly price and the $ P = MC $ point can raise a monopoly’s output and cut deadweight loss.
Competition (antitrust) policy blocks harmful mergers, breaks up monopolies and bans collusion.

Video lesson coming soonA short animated lesson for this step will appear here.

Why market structure changes the effect of a policy

Imagine the government taxes two businesses: a stall in a busy food market with dozens of rivals, and the only bus company in a town. The stall is a price taker; the bus company chooses its own price. The same tax can have quite different effects.

Always ask: does the policy change marginal cost (MC) or marginal revenue (MR)? A firm produces where $ MR = MC $, so if neither curve shifts, only profit changes.

Key termLump-sum tax: a fixed tax that does not depend on how much the firm produces, such as an annual licence fee. It is a fixed cost.

Key termPer-unit (specific) tax: a tax of a fixed amount on each unit sold. It raises marginal cost and average total cost.

Lump-sum tax vs per-unit tax on a monopoly

Worked example

For example, a monopoly faces demand $ P = 100 – Q $, so $ MR = 100 – 2Q $. MC is a constant $20 and fixed cost is $400.

Step 1No tax: $ 100 – 2Q = 20 \Rightarrow Q = 40,\ P = 60 $. Profit $ = (60 – 20)(40) – 400 = 1{,}200 $.

Step 2Lump-sum tax of $500: MC is unchanged, so still $ Q = 40,\ P = 60 $. Profit $ = 1{,}200 – 500 = 700 $.

Step 3Per-unit tax of $10: MC becomes $30. $ 100 – 2Q = 30 \Rightarrow Q = 35,\ P = 65 $.

Step 4Profit $ = (65 – 30)(35) – 400 = 1{,}225 – 400 = 825 $. Tax revenue $ = 10 \times 35 = 350 $.

The lump-sum tax only takes profit: consumers see no change. The per-unit tax raises the price by $5 and cuts output by 5 units, so consumers pay more, buy less, and deadweight loss grows.

DiagramDraw a monopoly graph: price on the vertical axis, quantity on the horizontal; downward-sloping D and steeper MR; MC and a U-shaped ATC. Lump-sum tax: shift only ATC up. The MR = MC point does not move, so $ Q_m $ and $ P_m $ stay the same; the profit rectangle gets shorter. Per-unit tax: shift both MC and ATC up by the tax. The new MC crosses MR at a lower quantity; go up to the demand curve to find a higher price.

Common mistakeShifting MC for a lump-sum tax, or a licence fee. A fixed charge never moves MC, so it never changes the profit-maximising quantity or price in the short run. Only ATC moves.

Subsidies work the same way in reverse. A per-unit subsidy lowers MC, so the monopolist raises output and cuts its price. A lump-sum subsidy raises profit only. In perfect competition, a per-unit tax shifts the market supply curve up by the tax; price rises and quantity falls, and how the burden is shared depends on the price elasticities of supply and demand.

Regulating a natural monopoly

A natural monopoly, such as a water network, has huge fixed costs, so ATC keeps falling over the whole range of market demand. One firm can supply the market more cheaply than two. So governments regulate its price rather than break it up.

\[ \text{Socially optimal price: } P = MC \qquad \text{Fair-return price: } P = ATC \]

Socially optimal (allocatively efficient) price, $ P = MC $. Output is where demand crosses MC, so there is no deadweight loss. But because ATC is falling, MC is below ATC, so the firm makes a loss and would shut down without a subsidy.
Fair-return price, $ P = ATC $. Output is where demand crosses ATC. The firm earns normal profit and needs no subsidy, and output is higher than the unregulated monopoly level. But $ P > MC $, so some deadweight loss remains.

Worked example

For example, a water company faces demand $ P = 100 – Q $, with constant MC of $20 and fixed cost of $1,200, so $ ATC = 20 + \frac{1200}{Q} $.

Step 1Unregulated: $ MR = 100 – 2Q = 20 \Rightarrow Q = 40,\ P = 60 $. Profit $ = (60 – 20)(40) – 1{,}200 = 400 $.

Step 2Socially optimal: $ 100 – Q = 20 \Rightarrow Q = 80,\ P = 20 $. ATC $ = 20 + \frac{1200}{80} = 35 $. Loss $ = (35 – 20)(80) = 1{,}200 $, so a subsidy of $1,200 is needed.

Step 3Fair return: $ 100 – Q = 20 + \frac{1200}{Q} \Rightarrow Q^2 – 80Q + 1200 = 0 \Rightarrow (Q – 60)(Q – 20) = 0 $. Take the larger root, $ Q = 60 $, so $ P = 40 $.

Step 4Check: ATC at 60 $ = 20 + \frac{1200}{60} = 40 = P $. Profit is zero (normal profit).

Fair return cuts the price from $60 to $40 and raises output from 40 to 60. The socially optimal price goes further, to $20 and 80 units, but needs a $1,200 subsidy (equal to the fixed cost, since price only covers MC).

DiagramDraw a downward-sloping D and MR, a horizontal (or gently falling) MC, and an ATC curve that slopes down over the whole graph, staying above MC. Mark three points on the demand curve: the unregulated monopoly ($ Q_m $ where MR = MC, highest price), the fair-return point (where D crosses ATC) and the socially optimal point (where D crosses MC, lowest price). At the socially optimal output, shade the loss rectangle between ATC and price.

Price ceilings on a monopoly

On a monopoly, a price ceiling can actually increase output. With a ceiling at $ P_c $, the firm cannot charge more than $ P_c $, so up to the quantity where $ P_c $ meets demand, each extra unit sells for exactly $ P_c $: MR becomes a flat line at $ P_c $. The monopolist no longer has to cut its price to sell more, so it expands output.

A ceiling between the monopoly price and the $ P = MC $ point: output rises to where the ceiling meets demand, and deadweight loss falls.
A ceiling at the $ P = MC $ point: output is the allocatively efficient quantity with no deadweight loss.
A ceiling below that point: the firm supplies only up to where MC equals the ceiling (or nothing, if the ceiling is below ATC in the long run), so a shortage appears.

Using the first example ($ P = 100 – Q $, $ MC = 20 $), a ceiling of $40 makes MR equal $40 up to 60 units. Since $40 > MC of $20, the firm produces 60 units at $40 instead of 40 units at $60.

Exam tipAP Micro FRQs on government and monopoly reward precise labelling: show which curve shifts (MC and ATC for a per-unit tax; ATC only for a lump-sum tax), mark the new quantity where MR = MC, and read price off the demand curve, not the MR curve. For natural monopoly, label the socially optimal (P = MC) and fair-return (P = ATC) points and say a subsidy is needed at P = MC. Cambridge 9708 and IB HL essays also need evaluation.

Competition policy, mergers and evaluation

Competition policy (called antitrust policy in the USA) aims to protect consumers from abuse of market power. Its main tools are:

Blocking or conditioning mergers that would greatly reduce competition. A horizontal merger joins two firms at the same stage of production; a vertical merger joins firms at different stages (a car maker buying a tyre firm); a conglomerate merger joins unrelated businesses.
Breaking up monopolies, as in the 1911 break-up of Standard Oil in the USA.
Banning collusion and cartels, such as agreements to fix prices or share out markets, and fining firms that abuse a dominant position.

When judging a merger, authorities weigh economies of scale and lower average costs against greater market power, which can mean higher prices, lower output and a bigger deadweight loss.

Evaluation of intervention in general: regulators often lack accurate cost data; regulated firms may inflate costs to justify higher prices; firms may lobby or “capture” the regulator; and subsidies must be funded by taxes. Government failure is possible.

Practise: 6 questions

Try each one on paper first, then check the worked answer.

EasyQ1. A monopoly must pay a new $50,000 annual licence fee. What happens to its price, output and profit in the short run? Show worked answer

The fee is a lump-sum tax, a fixed cost. MC and MR do not change, so price and output stay the same. ATC rises, so profit falls by $50,000.

EasyQ2. Define the socially optimal price and the fair-return price for a regulated natural monopoly. Show worked answer

Socially optimal price: $ P = MC $, where demand crosses MC; allocatively efficient, but the firm makes a loss and needs a subsidy.
Fair-return price: $ P = ATC $, where demand crosses ATC; the firm earns normal profit, but $ P > MC $.

MediumQ3. A monopoly faces $ P = 80 – Q $ and constant MC of $10. A per-unit tax of $20 is imposed. Find price and output before and after, and the tax revenue. Show worked answer

Before: $ MR = 80 – 2Q = 10 \Rightarrow Q = 35,\ P = 45 $.
After: MC = $30. $ 80 – 2Q = 30 \Rightarrow Q = 25,\ P = 55 $.
Price rises by $10 (half the tax); output falls by 10. Tax revenue $ = 20 \times 25 = \$500 $.

MediumQ4. A natural monopoly faces $ P = 60 – Q $, MC = $10 and fixed cost $400. Find (a) the unregulated outcome, (b) the socially optimal outcome and subsidy, (c) the fair-return outcome. Show worked answer

(a) $ 60 – 2Q = 10 \Rightarrow Q = 25,\ P = 35 $. Profit $ = (35 – 10)(25) – 400 = 625 – 400 = 225 $.
(b) $ 60 – Q = 10 \Rightarrow Q = 50,\ P = 10 $. ATC $ = 10 + \frac{400}{50} = 18 $. Loss $ = (18 – 10)(50) = 400 $: subsidy $400.
(c) $ 60 – Q = 10 + \frac{400}{Q} \Rightarrow Q^2 – 50Q + 400 = 0 \Rightarrow (Q – 40)(Q – 10) = 0 $. $ Q = 40,\ P = 20 $. Check: ATC $ = 10 + 10 = 20 $.

MediumQ5. A monopoly faces $ P = 100 – Q $ and MC = $20. A price ceiling of $50 is set. Find the new output and the change in deadweight loss. Show worked answer

Unregulated: $ Q = 40,\ P = 60 $. Efficient output is 80. DWL $ = \tfrac{1}{2}(60 – 20)(80 – 40) = 800 $.
With the ceiling, MR = $50 up to $ Q = 50 $. Since $50 > MC, the firm produces 50 units at $50 (beyond 50, MR on the demand curve is $ 100 – 100 = 0 < 20 $).
New DWL $ = \tfrac{1}{2}(50 – 20)(80 – 50) = 450 $. DWL falls by 350.

ExamQ6. Evaluate whether a government should regulate a natural monopoly by setting price equal to marginal cost or price equal to average total cost. [8 marks] Show worked answer

Indicative mark scheme
Knowledge [2]: natural monopoly has falling ATC over the market’s demand; define P = MC and P = ATC.
Analysis [3]: diagram with D, MR, MC and falling ATC; P = MC gives the efficient output and no deadweight loss but a loss (ATC > P) needing a subsidy; P = ATC gives normal profit and no subsidy, more output than unregulated, but P > MC so some deadweight loss remains.
Evaluation [3]: subsidies are funded by taxes that create distortions; regulators may not know true costs; with cost-based pricing the firm has little incentive to cut costs; P = ATC is simpler and self-financing. Reach a justified judgement.

Check yourself

1. A per-unit tax on a monopoly will:

It raises MC, so MR = MC at a lower quantity and a higher price on the demand curve.

2. At the socially optimal price for a natural monopoly, the firm:

With ATC falling, MC is below ATC, so P = MC means P < ATC.

3. A ceiling set between the monopoly price and the P = MC point:

MR becomes flat at the ceiling, so the firm expands output to where the ceiling meets demand.

Your score: 0 out of 3

The short version

Ask whether a policy changes MC or MR: if not, only profit changes.
Lump-sum tax: ATC up, profit down, P and Q unchanged. Per-unit tax: MC up, P up, Q down.
Natural monopoly: $ P = MC $ is efficient but needs a subsidy; $ P = ATC $ gives normal profit with some deadweight loss.
A well-placed price ceiling can raise a monopoly’s output; set too low, it causes a shortage.
Competition policy controls mergers, breaks up monopolies and bans cartels, but government failure is possible.

Step 24: Government Policy and Market Structures

Why market structure changes the effect of a policy

Lump-sum tax vs per-unit tax on a monopoly

Regulating a natural monopoly

Price ceilings on a monopoly

Competition policy, mergers and evaluation

Practise: 6 questions

Check yourself

The short version

Further reading on EconTutorials

Contact Info

Quick Links

Success!