Once you can show that a market is producing the wrong quantity, the obvious next question is what to do about it. Every policy in this section is doing the same job: pushing the market from Qe towards Qopt by changing what buyers or sellers actually face. What separates a good answer from an average one is not listing the policies, it is knowing what each one does to your diagram and where each one runs into trouble.
📘 What you need to know
An indirect (Pigouvian) tax raises costs, shifting supply left, and is used where a good is over-provided or over-consumed.
A carbon tax is a specific tax per tonne of emissions, aimed at pollution from production.
A subsidy lowers costs, shifting supply right, and is used where a good is under-provided or under-consumed.
Legislation and regulation ban or limit an activity directly, and can be aimed at either side of the market.
Education campaigns change the demand curve by changing what people believe the good is worth to them.
The aim is the polluter pays principle: whoever creates the external cost should be the one who carries it.
No policy here removes market failure completely. They reduce the welfare loss, and reducing it is enough to be worthwhile.
Indirect taxes
A Pigouvian tax, named after the economist Arthur Pigou, is a tax placed on a good that causes harm, set so that the producer or consumer starts paying for that harm.
The mechanics are simple. A tax is a cost, so it shifts the supply curve up and to the left, from S to S1. Price rises, quantity falls, and the market moves towards Qopt.
The tax does not make the MSC curve move. MSC was always there. What the tax does is drag the curve firms actually respond to closer to it, which shrinks the welfare loss from the pale triangle to the dark one.
Reading the tax diagram
Before the tax the market sits at PeQe and there is over-provision of Qe − Qopt.
The tax shifts supply from S to S1, and the vertical distance between them is the tax per unit.
The new equilibrium is P1Q1: a higher price and a lower quantity.
Q1 is closer to Qopt, so external costs fall and the welfare loss shrinks.
If the tax were set exactly equal to the external cost per unit, S1 would land on top of MSC and the welfare loss would disappear entirely.
That last point is the one to remember for a 10-mark question. In theory the perfect tax exists. In practice, putting a dollar figure on a tonne of smoke, or on one person’s asthma, is nearly impossible, so governments guess. Getting the size wrong is the single biggest reason a tax under-performs.
Indirect taxes: advantages
Indirect taxes: disadvantages
The people causing the external cost are the ones who pay it, which most people accept as fair
If demand is price inelastic, which is normal for addictive goods, a big price rise cuts quantity only slightly
Output falls towards Qopt, so resources are allocated more efficiently
Measuring the external cost accurately is very hard, so the tax is rarely the right size
Raises revenue that can be spent on treating the harm or on other programmes
Can push activity into illegal markets, where there is no tax and no safety standard at all
Firms keep the freedom to choose how to cut emissions, rather than being told how
Indirect taxes are regressive, so they take a bigger share of income from poorer households
Carbon taxes
A carbon tax is an indirect tax charged per tonne of carbon dioxide emitted. It raises the cost of production for firms that burn fossil fuels, shifts supply left, and gives every firm a running reason to cut emissions.
The clever part is what it does over time. If the tax per tonne is higher than the cost of installing cleaner technology, the firm installs the technology, because that is now the cheaper option. The tax does not just cut output today, it changes what firms invest in.
The elasticity problem, again. Electricity and fuel have very inelastic demand in the short run, because people still have to heat their homes and get to work. So a carbon tax can raise prices a lot while cutting emissions only a little, at least until households and firms have had time to switch. That is a strong evaluation point, not a reason to dismiss the policy.
Subsidies
A subsidy is a payment from government to producers per unit made. It cuts the cost of production, shifts supply right, lowers the price and raises the quantity. It is the tool for under-provided goods: vaccines, insulation, public transport, electric vehicles, education.
The subsidy works on the supply side even though the market failure sits on the demand side. Consumers pay a lower price, buy more, and more of the external benefit actually gets enjoyed.
Reading the subsidy diagram
Before the subsidy the market rests at PeQe, with under-consumption of Qopt − Qe.
The subsidy shifts supply from S to S1, down by the amount of the subsidy per unit.
The new equilibrium is P1Q1: a lower price and a higher quantity.
Q1 is closer to Qopt, so more of the external benefit is captured and the welfare gap shrinks.
The cost of the subsidy is the amount per unit multiplied by the new quantity, and that money comes from taxpayers.
Subsidies: advantages
Subsidies: disadvantages
Can be targeted precisely at the good, region or group you want to help
There is an opportunity cost: that money could have gone on hospitals, schools or debt repayment
Lower prices improve access, which matters most for lower income households
They distort markets, and can leave firms producing more than anyone actually wants
Over time they change habits, which is how electric cars moved from niche to normal
Once given, they are politically very hard to remove, and lobbying keeps them alive
Help domestic firms build up scale in new industries
Firms that are guaranteed support may stop working on becoming efficient
Legislation and regulation
Sometimes the government does not use price at all. It just writes a rule, sets up an agency to enforce it, and punishes people who break it.
Rules can be aimed at either side of the market, and knowing which side tells you which curve to shift:
Aimed at consumers. A minimum age for buying alcohol, a ban on smoking indoors, a limit on gambling advertising. Fewer people are legally allowed to buy, so demand shifts left.
Aimed at producers. Emission limits, a closed season on fishing, a ban on single-use plastics, planning restrictions. Producing becomes harder or costlier, so supply shifts left.
Legislation: advantages
Legislation: disadvantages
Direct and certain. A ban does not depend on how consumers respond to a price
Enforcement costs money: inspectors, courts, agencies, all funded by the taxpayer
Fines and prison sentences are a strong deterrent, especially for firms
Proving a breach can be slow and difficult, so weak enforcement makes the rule symbolic
Can be aimed at exactly the harmful activity rather than the whole market
Bans can create black markets, where the product is unregulated and often more dangerous
Sends a signal that shifts what people think is normal, which lasts beyond the law itself
Unpopular with voters and with powerful firms, so governments often water them down
Education and awareness
Education is the slowest tool and, in the long run, often the strongest. It works differently from everything else on this page, and that difference is worth a mark or two.
A tax leaves people wanting the good just as much, and simply prices some of them out. Education changes how much they want it in the first place. It moves the MPB curve itself: towards MSB for a merit good, and down towards MSB for a demerit good. When it works, you do not need to keep paying for it, because the preference has genuinely changed.
For merit goods: campaigns on the benefits of vaccination, exercise, or home insulation shift demand right.
For demerit goods: anti-smoking campaigns, drink-driving adverts and warnings on packaging shift demand left.
Best in combination. Education plus a tax beats either on its own. The tax makes the good expensive today, the education makes people want less of it tomorrow, and the tax revenue can pay for the campaign. Saying this in an evaluation paragraph is one of the easiest ways to show you can weigh policies against each other.
WORKED EXAMPLE
In a market for cement, MPC = 10 + Q, MSC = 25 + Q and D = MPB = MSB = 70 − Q, with Q in thousands of tonnes. Find the tax per tonne that would remove the market failure, and the revenue it would raise. [4]
Step 1: find the external cost per unitMSC − MPC = (25 + Q) − (10 + Q) = 15The correct tax is $15 per tonneA tax equal to the external cost lifts MPC exactly onto MSC.Step 2: find the quantity after the tax70 − Q = 25 + Q, so 45 = 2Q and Q = 22.5Step 3: revenue is tax times quantity sold15 × 22.5 = 337.5Tax revenue = $337,500Check it makes sense: the free market quantity was 70 − Q = 10 + Q, so Qe = 30. Output falls from 30 to 22.5, which is the correction we wanted.
WORKED EXAMPLE
Explain why an indirect tax on cigarettes may do less to cut consumption than the government hopes. [4]
Step 1: name the mechanism
The tax raises costs, shifts supply left and raises price, so quantity demanded falls.
Step 2: bring in elasticityCigarettes are addictive and have few close substitutes, so demand is price inelastic.Step 3: apply it
A large percentage rise in price causes a much smaller percentage fall in quantity demanded, so consumption barely moves and Q stays well above Qopt.
Step 4: add a second limitationHigher prices also make smuggling and illegal sales profitable, which puts some consumption beyond the reach of the tax entirely.The welfare loss falls, but far less than the size of the tax suggestsNotice that revenue stays high for the same reason consumption stays high, which is why governments keep using the tax anyway.
💡 Exam tip
Shift the curve the policy actually touches. Taxes and subsidies move supply. Education and age limits move demand. Never move MSB or MSC, because they were never the market’s curves.
Label the shift on the diagram. S to S1 with the arrow, plus the new equilibrium, gets the marks that a bare diagram misses.
Use three angles when you evaluate: how hard the externality is to measure, how effective the policy is, and who it hits hardest.
Always mention opportunity cost for subsidies and provision. It is the single most reliable evaluation point in this sub-topic.
Say who the stakeholders are by name. Poorer consumers, workers in the industry, firms, taxpayers, future generations.
Finish with a judgement. “A tax combined with education is likely to work better than either alone, because…” is what a top band answer looks like.
⚠️ Common mix-up
Drawing the tax as a shift in MSC. MSC does not move. Only the curve the firms respond to moves.
Saying a tax “solves” market failure. It reduces the welfare loss. Only a perfectly measured tax would remove it, and nobody measures that well.
Using a subsidy on a demerit good. Check the direction first: over-consumed means discourage, under-consumed means encourage.
Forgetting who pays for the subsidy. Taxpayers do, and that money had other uses.
Treating elasticity as an afterthought. It decides whether a price-based policy works at all, so put it in the analysis, not the last line.
Listing policies without weighing them. A list gets you into the middle bands. Comparing them gets you out.
Up next: International Cooperation on Sustainability, for the externalities that spill straight over national borders, where no single government can fix the problem on its own.
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