A market can be busy, profitable and completely balanced, and still be getting things wrong. That is the whole idea behind market failure: supply meets demand, everyone trades happily, and yet society ends up with too much of one thing and not enough of another. This page builds the four curves and the one condition that every diagram in this sub-topic rests on. Get these right and the rest of the topic is much easier.
📘 What you need to know
In a free market the price mechanism decides how scarce resources get shared out between competing uses.
Most of the time this works well. Market failure is when it does not, and society ends up with a less-than-best allocation of resources.
You will use four curves: MPB, MPC, MSB and MSC. Private is what the buyer or seller counts; social is what everybody counts.
The socially optimum output (Qopt) is where MSB = MSC. That is the point with no market failure.
At Qopt there is allocative efficiency: community surplus (consumer surplus + producer surplus) is as large as it can be.
Three things cause market failure at SL: externalities, public goods and common pool resources.
Market failure is about the quantity produced or consumed, not about whether the price feels high or low.
What a free market is supposed to do
Every economy has the same problem: wants are unlimited, but the factors of production (land, labour, capital and enterprise) are scarce. Something has to decide where those resources go. In a free market that job is done by price.
Price does three jobs at once, and it is worth being able to name them in an essay:
It signals. A rising price tells producers that buyers want more of this.
It gives an incentive. A higher price means higher profit, so firms move resources into that market.
It rations. When something is scarce, price rises until only the people who value it most are still buying.
When those three jobs are done properly, resources end up where people value them most. Economists call that outcome allocative efficiency.
With no external costs or benefits, the demand curve already shows the benefit to society and the supply curve already shows the cost to society, so the free market equilibrium is also the socially best point.
Look at what the two triangles mean. Above the price, buyers were willing to pay more than they had to, and that saving is consumer surplus. Below the price, sellers were willing to accept less than they got, and that is producer surplus. Added together they make community surplus, which is the total gain society gets from this market existing at all.
Why is community surplus biggest exactly at the crossing point? Because every unit to the left of it is worth more to society than it costs to make, so making it adds to the pile. Every unit to the right costs more than it is worth, so making it takes away from the pile. Stop right where the two curves meet and the pile is at its tallest.
The four curves you must be able to name
All four are marginal, which just means “for one more unit”. Do not read them as totals.
Curve
What it measures
Whose costs or benefits are in it
MPB — marginal private benefit
The benefit one more unit gives the person who buys it
The buyer only
MPC — marginal private cost
The cost of making one more unit
The firm only
MSB — marginal social benefit
The benefit one more unit gives everyone
The buyer plus any third parties
MSC — marginal social cost
The cost of one more unit to everyone
The firm plus any third parties
The two links you must remember
MSB = MPB + external benefits
MSC = MPC + external costs
Read those two lines carefully, because they explain the whole topic. If there are no external costs and no external benefits, then MSB is the same line as MPB and MSC is the same line as MPC. One demand curve, one supply curve, no problem. The moment a cost or a benefit lands on somebody outside the deal, the social curve peels away from the private one, and the gap between them is the market failure.
Colour code used on every diagram in this sub-topic: blue lines are the private curves, which is what the market actually reacts to. Green lines are the social curves, which is what society would react to if it could. When a curve is blue and green at the same time, it is labelled with both names, for example S = MPC = MSC.
The socially optimum output
The condition for no market failure
MSB = MSC at Qopt
This is the level of output society would pick if it could count every cost and every benefit, including the ones that land on people who were never part of the transaction. It is not “the level with no pollution” and it is not “as much as possible”. It is simply the point where the last unit made is worth exactly what it costs everyone.
Whenever the free market equilibrium (PeQe) is at a different quantity from Qopt, you have market failure. The size of the mistake is the distance between the two quantities, and the cost of the mistake is the shaded triangle you will meet on the next few pages.
The three things that break a market
Externalities are the cause you will draw most often, and they come in four versions, because the spillover can be a cost or a benefit and it can happen on the production side or the consumption side.
Each cause produces one of three outcomes, and examiners like you to name the outcome, not just the cause:
Over-provision or over-consumption. Too many resources go into this good. Cigarettes and coal power are the usual examples.
Under-provision or under-consumption. Too few resources go into it. Vaccinations, education and flood defences are the usual examples.
Overuse of something finite. A resource nobody owns gets stripped out, which is what happens with fish stocks and rainforests.
In all three cases the phrase to use is the same: there is a lack of allocative efficiency from society’s point of view. Community surplus is smaller than it could be, and the amount it falls short by is called the welfare loss.
🧩 How to read any market failure diagram
Find the free market point. It is where the two private curves cross, so where MPB meets MPC. Label it PeQe.
Find the social optimum. It is where MSB meets MSC. Label it PoptQopt.
Compare the two quantities. Qe bigger than Qopt means over-provision. Qe smaller means under-provision.
Find the triangle. Its point sits at Qopt and its flat side sits at Qe. That is the welfare loss.
Say what it means in words. Too many or too few factors of production have been put into this good.
WORKED EXAMPLE
In a market with no externalities, demand is P = 100 − Q and supply is P = 20 + Q, where Q is in thousands of units. Find the equilibrium and calculate community surplus. [3]
Step 1: set demand equal to supply100 − Q = 20 + Q80 = 2Q, so Q = 40 and P = 60No externalities, so D = MSB and S = MSC. This equilibrium is also Qopt.Step 2: consumer surplus is the triangle above the priceCS = ½ × 40 × (100 − 60) = 800Step 3: producer surplus is the triangle below the pricePS = ½ × 40 × (60 − 20) = 800Step 4: add themCommunity surplus = 1600 (in thousands of dollars)Any other quantity gives a smaller total. That is what allocative efficiency means.
WORKED EXAMPLE
Take the same market, but now each unit produced creates an external cost of $20. Find the socially optimum output and say how badly the free market gets it wrong. [3]
Step 1: the free market does not changeFirms still only see their private costs, so Qe is still 40.Step 2: build MSC from MPCMSC = MPC + external cost = (20 + Q) + 20 = 40 + QStep 3: set MSB = MSC100 − Q = 40 + Q60 = 2Q, so Qopt = 30Over-provision of 10 thousand unitsNothing about the market has changed except what we counted. That is the whole point.
💡 Exam tip
Label the vertical axis “costs and benefits”, not “price”. Once MSB and MSC are on the diagram you are measuring value to society, not just the price tag.
Always mark both quantities. A diagram with only Qe on it cannot show market failure, because market failure is the gap between two quantities.
Say which curve moved and why. “MSC lies above MPC because firms do not pay for the pollution” earns marks. Drawing the line alone does not.
Learn the definitions word for word. Definition questions on MSB, MSC and allocative efficiency are free marks in Paper 2.
Talk about resources, not just output. Examiners want to hear about factors of production being over-allocated or under-allocated.
Practise drawing these fast. In a 15-mark essay the diagram is worth marks, but the analysis around it is worth more, so do not spend five minutes on a ruler.
⚠️ Common mix-up
Thinking market failure means the market has stopped working. It has not. It clears perfectly. It just clears at the wrong quantity.
Thinking a high price is market failure. Expensive is not the same as inefficient. The test is always the quantity of resources going in.
Mixing up private and social. Private is the person in the deal. Social is everybody. If you cannot say who the third party is, you have not found an externality.
Putting Qopt where MSB crosses MPC. The optimum is always MSB = MSC. Both social curves, every time.
Forgetting that MSB and MPB can be the same line. If the externality is on the production side, the demand curve is labelled D = MPB = MSB, and that is correct, not a mistake.
Saying “market failure means the government must step in”. Government action might help, but it has costs of its own, and that is exactly what evaluation questions want you to discuss.
Up next: Negative Externalities and Demerit Goods, where MSC sits above MPC and the market makes far too much of something.
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