The Effects of Government Intervention in Different Market Structures: Study Guide (Microeconomics 6.4)

One question for every policy

In Topic 2.8 you analysed taxes, subsidies and price controls in a perfectly competitive market. This topic uses the same tools in every market structure. Every firm, whatever the market, produces where marginal revenue equals marginal cost (or, in a labour market, where the marginal revenue product equals the marginal resource cost). So for any policy, ask:

Does the policy change marginal cost, or the price the firm can charge? Or does it change only fixed cost?

• If it changes MC or the firm’s MR, the profit-maximising quantity changes, so price and output change.
• If it changes only fixed cost, the profit-maximising quantity stays the same. Only profit changes.

Per-unit taxes and subsidies

A per-unit tax of $t adds $t to the cost of every unit, so it shifts MC (and supply) up by $t. A per-unit subsidy of $s shifts MC down by $s.

In perfect competition, as in Topic 2.8, a per-unit tax raises the price consumers pay, lowers the net price firms keep, cuts the equilibrium quantity, reduces consumer and producer surplus, raises government revenue (tax × quantity) and creates deadweight loss. A subsidy does the opposite, but costs the government subsidy × quantity and also creates deadweight loss (the extra units cost more than they are worth). Elasticity decides who bears the tax: the side of the market that is less elastic bears more of it. The more elastic demand and supply are, the more quantity falls, and the bigger the deadweight loss.

In monopoly, the tax shifts MC up, so the new MR = MC point is at a lower quantity. The firm sets a higher price from the demand curve. A monopoly already produces too little, so the tax pushes output further below the efficient quantity and deadweight loss grows. With a straight-line demand curve and constant MC, the price rises by half the tax; with rising MC it rises by less than half. A per-unit subsidy does the reverse: MC shifts down and output rises towards the efficient level. In principle, a large enough subsidy could bring a monopoly to the efficient quantity, but it is expensive.

Lump-sum taxes and subsidies

A lump-sum tax is a fixed amount the firm pays whatever its output (for example, a $108 thousand licence fee per month). A lump-sum subsidy is a fixed payment to the firm. Neither changes marginal cost or marginal benefit; only fixed cost changes, and so does ATC.

• In the short run, every firm, in any market structure, keeps the same quantity and price. Profit falls by the tax (or rises by the subsidy).
• A monopoly that is earning economic profit can pay a lump-sum tax without changing its output, so the tax creates no extra deadweight loss.
• If a lump-sum tax turns profit into a long-run loss, firms will eventually leave the market. Only then do quantity and price change.

Price controls in different market structures

A binding price ceiling (a maximum price below the current price) and a binding price floor (a minimum price above it) have different effects in different market structures.

Perfect competition. A binding ceiling cuts the quantity supplied, causing a shortage; a binding floor cuts the quantity demanded, causing a surplus. Either way, the quantity traded falls and deadweight loss appears. The more elastic supply and demand are, the larger the shortage or surplus.

Monopoly. A monopoly normally holds back output because selling one more unit forces it to cut the price on all units, so MR is below price. A ceiling changes that. Up to the quantity where the ceiling meets demand, each extra unit sells at the ceiling, so MR equals the ceiling price (the MR curve becomes flat there). The firm now produces where the ceiling meets MC, or where it meets demand, whichever comes first.

• A ceiling where demand meets MC gives the allocatively efficient quantity: output rises, price falls, deadweight loss disappears.
• A ceiling between the monopoly’s MC (at its unregulated output) and its unregulated price still raises output, but if it is below the price where demand meets MC it also causes a shortage.
• A ceiling below MC at the unregulated output makes the firm produce less than before.

A binding price floor above the monopoly price works the other way: buyers take fewer units at the higher price, so output falls further below the efficient quantity and deadweight loss grows, just as in perfect competition.

Monopolistic competition. Each firm also faces a downward-sloping demand curve, so the same logic applies: a ceiling a little below its price can raise its output in the short run.

Monopsony. A price floor in a labour market is a minimum wage. A monopsony hires where MRC = MRP and pays the lower wage from the supply curve. A minimum wage set above that wage makes MRC equal the minimum wage over the range where workers are willing to work. Up to the wage where supply meets MRP, a minimum wage raises both the wage and employment. In a perfectly competitive labour market, a binding minimum wage always reduces employment.

Regulating a natural monopoly

A natural monopoly has ATC falling across the whole of market demand, so MC is below ATC (Topic 4.2). A regulator can choose between three outcomes:

Option	Rule	Result
Unregulated	MR = MC, price from demand	low output, high price, economic profit, large deadweight loss
Fair-return price	P = ATC	zero economic profit, more output, smaller deadweight loss
Socially optimal price	P = MC	allocatively efficient output, no deadweight loss, but P < ATC, so the firm makes a loss

At P = MC the firm would shut down in the long run, so the government must pay it a lump-sum subsidy equal to the loss. It must be lump-sum: a per-unit subsidy would shift MC and change the output the regulator wanted.

Antitrust policy

Antitrust policy (also called competition policy) tries to make markets more competitive. Governments can block mergers that would create a dominant firm, break up existing monopolies, and outlaw collusion such as cartel price-fixing. More competition pushes price towards MC and raises output. Antitrust suits markets where many firms could survive; for a natural monopoly, splitting the firm would raise ATC, so regulation is usually better. You will not be asked to draw a graph of collusion and the policy against it.

When intervention increases efficiency

Government intervention in an imperfect market increases efficiency only if it corrects the incentive that caused the inefficiency. A monopoly’s problem is that MR is below price, so it restricts output. A ceiling where demand meets MC fixes that incentive. A per-unit tax on the monopoly makes it worse, and a lump-sum tax leaves it unchanged.

Worked example 1: regulating Marrick Water

Question. Marrick Water (fictional) is a natural monopoly. Demand is P = 60 − 0.5Q, so MR = 60 − Q (Q in thousand households, P in $ per month). MC is constant at $10 and fixed cost is $800 thousand per month, so ATC = 10 + 800 ÷ Q. Find the outcome (a) unregulated, (b) with a socially optimal price, (c) with a fair-return price. Compare the deadweight loss.

(a) MR = MC: 60 − Q = 10, so Q = 50. P = 60 − 0.5 × 50 = $35. ATC = 10 + 800 ÷ 50 = $26. Profit = (35 − 26) × 50 = $450 thousand.

(b) P = MC: 60 − 0.5Q = 10, so Q = 100, P = $10. ATC = 10 + 800 ÷ 100 = $18. Loss = (18 − 10) × 100 = $800 thousand, equal to the fixed cost. A lump-sum subsidy of $800 thousand per month keeps the firm open without changing its MC.

(c) P = ATC: 60 − 0.5Q = 10 + 800 ÷ Q. Q = 80 works: P = 60 − 40 = $20 and ATC = 10 + 10 = $20. So Q = 80, P = $20, economic profit $0. (Q = 20 also solves the equation, but the regulator chooses the larger output.)

Deadweight loss (triangle between demand and MC, from the quantity produced to 100):

• Unregulated: ½ × (100 − 50) × (35 − 10) = $625 thousand.
• Fair return: ½ × (100 − 80) × (20 − 10) = $100 thousand.
• Socially optimal: $0.

Interpretation. Moving from U to F to S raises output and consumer surplus ($625 thousand, then $1,600 thousand, then $2,500 thousand) and shrinks the deadweight loss. The cost of full efficiency is a subsidy paid by taxpayers.

Worked example 2: taxing Ashgrove Biotech

Question. Ashgrove Biotech (fictional) holds a patent on an inhaler. Demand is P = 60 − Q, so MR = 60 − 2Q. MC = Q, and fixed cost is $100 thousand per month (Q in thousand inhalers per month, P in $). Compare (a) no tax, (b) a $6 per-unit tax and (c) a lump-sum tax that raises the same revenue.

(a) MR = MC: 60 − 2Q = Q, so Q = 20, P = $40. The efficient quantity is where P = MC: 60 − Q = Q, so Q = 30. Deadweight loss = ½ × (30 − 20) × (40 − 20) = $100 thousand.

(b) MC becomes Q + 6. MR = MC: 60 − 2Q = Q + 6, so Q = 18, P = 60 − 18 = $42.

1. Consumers pay $2 more; the firm keeps $42 − $6 = $36 per unit.
2. Tax revenue = 6 × 18 = $108 thousand.
3. Deadweight loss = ½ × (30 − 18) × (42 − 18) = $144 thousand, up from $100 thousand.

(c) A lump-sum tax of $108 thousand does not change MC, so Q stays 20 and P stays $40. Profit falls by $108 thousand (from $500 thousand to $392 thousand). Deadweight loss stays $100 thousand.

Interpretation. Both taxes raise $108 thousand, but the per-unit tax adds $44 thousand of deadweight loss and raises the price; the lump-sum tax adds none.

Worked example 3: a price ceiling on Ashgrove

Question. Return to untaxed Ashgrove. Find output and deadweight loss with a ceiling of (a) $30, (b) $24, (c) $16.

The firm produces where MC (= Q) equals the ceiling, as long as buyers want that many.

(a) $30: Q = 30, and buyers want 60 − 30 = 30, so there is no shortage. Deadweight loss = $0 (Figure 1).

(b) $24: Q = 24, but buyers want 36: a shortage of 12 thousand. Output still rises from 20 to 24. Deadweight loss = ½ × (30 − 24) × (36 − 24) = $36 thousand, less than $100 thousand.

(c) $16: Q = 16, below the unregulated 20, with a shortage of 44 − 16 = 28 thousand. Deadweight loss = ½ × (30 − 16) × (44 − 16) = $196 thousand: worse than no ceiling.

Check. A ceiling between $20 (MC at the unregulated output) and $40 (the unregulated price) raises output; $30 is best.

Common misconceptions

• “A lump-sum tax raises a monopoly’s price.” It changes only fixed cost. MR = MC at the same quantity, so price and output stay the same.
• “A price ceiling always reduces output.” That is true in perfect competition, not for a firm with market power.
• “Regulating a natural monopoly at P = MC lets the firm break even.” P = MC is below ATC, so the firm makes a loss and needs a lump-sum subsidy. P = ATC gives zero economic profit.
• “A per-unit subsidy is fine for the natural monopoly loss.” It would shift MC and change output. Covering the loss needs a lump-sum payment.
• “Any government intervention fixes a market failure.” Only policies that correct the underlying incentive raise efficiency. A per-unit tax on a monopoly increases deadweight loss.
• “A minimum wage always reduces employment.” In a monopsony, a minimum wage up to the level where supply meets MRP raises employment.
• “Breaking up any monopoly helps.” For a natural monopoly, two firms would duplicate fixed costs and raise ATC.

Where this leads

Next, Topic 6.5, Inequality, looks at how income and wealth are distributed and how taxes and transfers change that distribution. For the competitive-market versions of these policies, see the Topic 2.8 study guide. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.