IB Economics SL & HLTopic 2.8 — Externalities & Common Pool ResourcesPaper 1 & 2Core idea~11 min read
What Market Failure Means
Market failure does not mean a market has collapsed. It usually looks perfectly healthy — busy shops, happy buyers, profitable firms. The failure is hiding in the people who were never part of the deal, and whose costs and benefits nobody added up.
📚 What you need to know
Market failure is a less-than-best allocation of resources from society’s point of view.
Private costs and benefits are felt by the buyer and seller. External ones spill onto third parties.
The socially optimum output, Qopt, is where MSB = MSC.
At Qopt there is allocative efficiency and community surplus is at its largest.
The free market lands at Qe, where MPB = MPC. If Qe is not Qopt, there is a welfare loss.
Market failure comes in six flavours: negative and positive externalities of production and consumption, public goods, and common pool resources.
Four abbreviations, then everything else follows
Every diagram in this sub-topic is built out of the same four ideas. Learn them properly now and the rest of 2.8 stops being scary.
Term
Plain meaning
Example
MPB — marginal private benefit
The benefit the buyer gets from one more unit
The pleasure of your coffee
MPC — marginal private cost
The cost the firm pays to make one more unit
Beans, milk, wages, rent
MSB — marginal social benefit
MPB plus any benefit spilling onto everyone else
Your flu jab also protects strangers
MSC — marginal social cost
MPC plus any cost dumped on everyone else
The factory’s smoke over the next town
The two equations behind every diagram
MSC = MPC + external cost
MSB = MPB + external benefit
If there is no spillover at all, MSC sits exactly on top of MPC and MSB sits on MPB. That is the healthy market. Market failure is what happens when those lines pull apart.
The best output for society
Society should keep producing a good while the extra benefit to everyone is bigger than the extra cost to everyone. The moment those two are equal, stop. That point is Qopt, and it is where MSB = MSC.
When a market lands exactly there, it is allocatively efficient: the right amount of society’s scarce land, labour and capital has gone into that good, and community surplus (consumer surplus plus producer surplus) is as big as it can get.
Notice the axis label. In market failure diagrams we write “costs and benefits” rather than “price”, because the lines are no longer only about what people pay.
Why the axis label changes. A demand curve shows what buyers will pay. An MSB curve shows what the good is worth to everyone, including people who never bought it. Same shape, wider meaning.
Over-provision and under-provision
Once MSB and MSC pull away from MPB and MPC, the free market misses Qopt in one of two directions.
Over-provision. Qe is bigger than Qopt. Too many resources have gone into this good. Typical of harmful things — cigarettes, polluting factories.
Under-provision. Qe is smaller than Qopt. Too few resources have gone in. Typical of good things — vaccines, education, public transport.
The triangle between the two quantities is the welfare loss: value that society could have had and did not. It always has its point at Qopt and its open end at Qe.
Welfare loss, every single time
welfare loss = ½ × (Qe − Qopt) × (gap between MSC and MSB at Qe)
The six causes
Everything in this sub-topic and the next two fits in this box. Keep coming back to it.
Spot which box a question sits in before you draw anything. The box tells you which curve splits and in which direction.
Worked examples
WORKED EXAMPLE
Is this market failing?
At the free market output, MPC is $12, MPB is $14, and producing each unit imposes an external cost of $5 on nearby residents. Explain whether resources are allocated well. [3]
Step 1: Build MSCMSC = MPC + external cost = 12 + 5 = $17Step 2: Build MSB
No external benefit, so MSB = MPB = $14Step 3: CompareMSC ($17) > MSB ($14)Over-provision: the last unit costs society more than it is worthfewer resources should go into this good, so Qe is above Qopt
WORKED EXAMPLE
Calculating the welfare loss
A market produces 90,000 units. The socially optimum output is 60,000 units. At 90,000 units MSC is $70 and MSB is $40. Calculate the welfare loss. [2]
Step 1: Base of the triangle90,000 − 60,000 = 30,000 unitsStep 2: Height of the triangle70 − 40 = $30Step 3: Area(30,000 × 30) ÷ 2 = 450,000Welfare loss = $450,000the height is always the vertical gap at Qe, never at Qopt (there the gap is zero)
WORKED EXAMPLE
Which cause is it?
Name the type of market failure in each case. [4]
a) A factory dumps waste into a river
Negative externality of production.
b) A neighbour restores an old house and the whole street looks better
Positive externality of consumption.
c) Nobody will pay for a coastal lighthouse
Public good.
d) A shared grazing field is stripped bare
Common pool resource.
production or consumption? ask whether the spillover came from making it or using it
💡 Exam tip
Label the vertical axis costs and benefits, not price. It is a stated marking point.
Write MSB = MSC when you identify Qopt. Writing “where the curves cross” is too vague.
The welfare loss triangle always has its point at Qopt. If your triangle points the other way, you have mislabelled something.
Say “from society’s point of view” at least once. It is the phrase that defines market failure.
Market failure questions are about the quantity of resources used, not really about whether the price is high or low.
Expect 10 and 15 mark essays on this. Practise drawing each diagram in under two minutes.
⚠ Common mix-up
“Market failure means no market exists.” Usually the market is thriving. It is just producing the wrong amount.
Confusing private and social. Private is buyer and seller only. Social is everybody.
Splitting the wrong curve. A production spillover splits the supply side. A consumption spillover splits the demand side.
Measuring the welfare loss gap at Qopt. The gap there is zero, which is the whole point of Qopt.
Thinking positive externalities are not a failure. They are. Society is missing out on benefits it could have had.
Using “deadweight loss” and “welfare loss” as if they are unrelated. They describe the same triangle of lost value.
Up next: Negative Externalities and Demerit Goods — the two diagrams where society ends up with too much of something it never asked for.
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