IB Economics SLTopic 2 — MicroeconomicsPaper 1 & 2Real world example~9 min read
Common Pool Resources and the Tragedy of the Commons
Some of the most valuable things on the planet have no owner at all. Ocean fish, groundwater, the atmosphere, a shared grazing field. Anyone can help themselves, and every single one of them can be used up. That combination is unusual, and it produces a very specific kind of market failure that no amount of ordinary competition will fix.
📘 What you need to know
Common pool resources are non-excludable but rivalrous in consumption.
Non-excludable means nobody can be stopped from using them, usually because nobody owns them.
Rivalrous means they are finite. What one person takes, the next person cannot have.
The tragedy of the commons is what happens when these resources are used unsustainably: they get depleted, sometimes permanently.
Overuse creates negative externalities of production, so you can draw the same MSC above MPC diagram you already know.
The external costs include pollution, habitat damage and resource depletion, and depletion falls on future generations who cannot bid in today’s market.
Solutions include property rights, quotas and licences, collective self-governance and international agreements.
Two questions sort every good
Economists classify goods with two yes-or-no questions, and it is worth learning them because a lot of exam confusion disappears once you can place a good in the right box.
Is it excludable? Can the seller keep out anyone who does not pay?
Is it rivalrous? Does one person using it leave less for the next person?
A private good answers yes twice, and markets handle those perfectly well. Common pool resources answer no to the first question and yes to the second, and that mismatch is exactly where the trouble starts.
Typical common pool resources are ocean fisheries in international waters, communal grazing land, rivers and aquifers, and natural forests. Notice that they are all things people take out of nature, and all things that can run out.
Why the incentives point the wrong way
Here is the maths that makes the tragedy happen, and it is simpler than it looks.
Suppose twenty boats fish the same bay. One skipper decides to add an extra boat. She gets the whole of the extra catch, so all of the benefit lands on her. The cost of that extra boat is that the stock falls a little, which makes fishing slightly harder for everyone. So she carries roughly one twentieth of the cost.
All of the gain, a twentieth of the cost. Of course she adds the boat. So does every other skipper, for exactly the same reason. Nobody is being greedy or stupid, and nobody is breaking a rule. They are each doing the sensible thing, and the sensible thing adds up to a disaster.
This is why “just tell them to stop” is not an answer. Any skipper who holds back is punished twice: they catch less, and the fish they left behind is taken by someone else within the week. Restraint only pays if everyone else has to show it too, which is why enforceable rules matter more than good intentions here.
The stock does not fall because the resource is being used. It falls because it is being used faster than it can renew. With an agreed limit, the same boats can fish the same water indefinitely.
Where this fits with externalities
Overusing a common pool resource is not a new type of market failure. It is a negative externality of production, so the diagram is one you already know: MSC sits above MPC, the market produces too much, and there is a welfare loss.
What makes it worth its own heading is the shape of the external cost:
Pollution and habitat damage that hit the people living nearby now.
Resource depletion, which hits people who are not born yet. They cannot vote, cannot pay and cannot bid, so the market gives their interests a weight of exactly zero.
Irreversibility. An over-fished stock can collapse below the point where it recovers. A logged rainforest does not grow back the same. Once the resource is gone, no future price rise brings it back.
The link that scores marks: common pool resources are a market failure because there are no property rights. Where a resource does have an owner, that owner has every reason to protect its long-run value. Where it does not, everyone has a reason to take their share first.
A real world example: North Atlantic cod
The Grand Banks off Newfoundland were one of the richest fishing grounds on earth for roughly four hundred years. Through the twentieth century the technology got better and better: bigger factory trawlers, sonar to find the shoals, nets that could be dragged along the sea floor.
Each boat was doing the rational thing. Fishing harder was profitable, and any fish you left behind was simply caught by a competitor. Catches stayed high right up until they did not. In 1992 Canada shut the northern cod fishery altogether, and tens of thousands of people lost their jobs almost overnight. Decades later the stock has still not returned to anything like its old level.
That is the tragedy in one sentence: a resource that had supported people for centuries was exhausted in a few decades, not by villains, but by ordinary competition with nobody in charge of the total.
What can be done
Solution
How it works
Where it struggles
Property rights Give ownership of the resource to a person, a firm or a community
An owner who keeps the resource for the long run has a direct incentive to avoid wrecking it, and can legally stop others taking it
Hard to do for the open ocean or the atmosphere, and it can shut out people who have used the resource for generations
Quotas and licences Government sets a total catch or extraction limit and shares it out
Caps the total directly rather than relying on price, and can be adjusted as the stock recovers
Needs accurate science on what the resource can take, plus real monitoring and real penalties, both of which cost money
Collective self-governance The community that uses the resource agrees its own rules
Local users know the resource best, watch each other closely, and rules they wrote themselves get obeyed
Falls apart if outsiders can still come in, and disagreements about fair shares can turn ugly
International agreements Countries agree binding limits together
The only realistic option when a resource crosses borders, such as high seas fisheries or the atmosphere
Slow to negotiate, and there is rarely any real penalty for a country that signs and then does not comply
Taxes on extraction Charge firms per tonne taken or per hectare cleared
Makes firms pay for part of the external cost, which raises MPC towards MSC
Very hard to price the true external cost, and it can push extraction into countries with weaker rules
WORKED EXAMPLE
Explain why a common pool resource is likely to be overused, while a private good with the same demand is not. [4]
Step 1: name the two properties
A common pool resource is non-excludable and rivalrous.
Step 2: explain what non-excludability does
Nobody can be kept out, so no user has to pay for access and no owner can limit the total taken.
Step 3: explain what rivalry doesEach unit taken really does reduce what is left, so there is a genuine cost, it just does not fall on the person taking it.Step 4: put them together
Each user gets the full private benefit but bears only a small share of the cost, so MPC is below MSC and the resource is used past the sustainable level.
A private good has an owner who can exclude non-payers, so the cost of using it is fully paid by the userFour marks, four moves. Property, property, consequence, contrast.
💡 Exam tip
Use the two words in the definition. Non-excludable and rivalrous. Both, in that order, in every definition answer.
Draw the negative externality of production diagram. You do not need a new diagram for this section, and examiners expect the familiar one.
Bring in future generations. Depletion is an external cost falling on people with no say in today’s market, and that lifts an answer from good to strong.
Have one real example ready. Cod, Amazon logging, groundwater in northern India, plastic in the ocean. One you can describe in three sentences beats five you can only name.
Evaluate by asking who enforces it. Every solution here works on paper. The question is always who checks, and what happens when someone cheats.
Watch out for the development angle. Poorer countries often depend on these resources for income today, which makes conservation genuinely costly for them.
⚠️ Common mix-up
Calling a common pool resource a public good. Both are non-excludable, but a public good never runs out and a common pool resource does. That single difference changes everything.
Saying “the tragedy of the commons” without explaining it. The phrase earns nothing on its own. The incentive story is what earns marks.
Blaming greed. The whole point is that the outcome happens even when everyone is behaving reasonably.
Thinking a higher price will fix it. Without ownership, a higher price makes taking the resource more attractive, not less.
Treating it as a consumption externality. The harm happens during extraction, so it belongs on the production side.
Assuming government is the only answer. Communities have managed shared resources successfully for centuries, and that is a fair evaluation point.
Up next: Government Responses to Externalities, where you take the taxes, subsidies, laws and campaigns and show exactly what each one does to your diagram.
Want this explained one-to-one?
Book a free session with an experienced IB Economics tutor and get your trickiest topics made simple.
