Tragedy of the Commons
What is Tragedy of the Commons?
The tragedy of the commons is the overuse and depletion of a shared resource that is rival but non-excludable and owned by no one.
Because each user bears only part of the cost of their use, common resources like fisheries and grazing land get overexploited. It reflects a negative externality imposed on other users. Solutions include property rights, quotas, or regulation.
Tragedy of the Commons: a worked example
Herders share an open pasture and anyone may add a cow. Each cow yields milk worth $800 minus $20 for every cow grazing, because extra animals thin the grass, and raising a cow costs $200. Village surplus peaks at 15 cows: milk is worth 800 - 20 × 15 = $500 per cow, so revenue is 15 × $500 = $7,500 against costs of 15 × $200 = $3,000, leaving $4,500. Acting alone, a herder keeps adding while a cow's own milk beats its $200 cost, and that holds until 30 graze, where milk is 800 - 20 × 30 = $200 per cow. Revenue is then 30 × $200 = $6,000 and costs are $6,000, so the whole $4,500 surplus has been grazed away. The 20th cow shows the mechanism: it brings in $400 of milk for a $200 cost, a $200 private gain, while cutting $20 from each of the other 19 cows, a $380 external cost the herder never pays.
The mistake students make with tragedy of the commons
Students label a commons a public good, since both are non-excludable, and then argue the fix is to fund the resource with tax dollars. Rivalry is what separates the two. A tuna one boat lands cannot be landed by another boat, so use by one user subtracts from what is left, which is precisely why the stock collapses. Public goods such as national defense are non-rival and suffer the opposite failure, too little provision rather than too much use. Check rivalry before classifying, then propose quotas, licenses, or property rights.
Tragedy of the Commons questions
Why does the tragedy of the commons happen?
Overuse happens because each user keeps the full benefit of taking one more unit while the cost of depletion is spread across everyone sharing the resource. A fisher who lands one more ton keeps all the revenue and bears only a sliver of the resulting decline in the stock, so private marginal benefit stays above private marginal cost long past the point where society's costs exceed its benefits. Nobody owns the resource, so nobody has a reason to conserve it for later.
What is the difference between a common resource and a public good?
Common resources and public goods are both non-excludable, meaning users cannot be kept out, but common resources are rival and public goods are not. A ton of fish taken from open ocean is gone for everyone else, so heavy use destroys the resource itself. A broadcast signal or a lit street can serve one more person without taking anything from current users, so the failure there is free riding and underprovision rather than depletion.
How can the tragedy of the commons be solved?
Solutions all work by attaching the shared cost to the individual user. Assigning property rights gives an owner a stake in future value. Tradable catch quotas cap total use and make each unit of access scarce enough to carry a price. A fee per unit extracted raises private marginal cost toward social marginal cost. Community rules with monitoring and penalties achieve the same result without formal ownership when users know each other and can enforce their agreements.
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