What is the tragedy of the commons? Why shared resources run out

The tragedy of the commons is what happens when a resource is shared, open to all and able to run out: each user keeps the whole gain from taking a little more, while the damage is spread across everyone, so the resource is overused until it is ruined. The biologist Garrett Hardin made the idea famous in 1968 with a shared pasture. The political economist Elinor Ostrom later showed that many communities avoid the tragedy by setting rules of their own.

In short

• A commons is a resource no one can easily be kept out of but that gets used up: fish in the sea, grazing land, groundwater, the air's room for pollution.
• The trap: adding one more cow, boat or well pays its user in full and costs each other user only a little, so everyone keeps adding.
• The tragedy isn't inevitable. Private rights, government limits and, as Ostrom showed, rules made by the users themselves can all stop it.

What did Hardin actually argue?

Garrett Hardin, a biologist at the University of California, Santa Barbara, published "The Tragedy of the Commons" in the journal Science on 13 December 1968. He borrowed the picture from a pamphlet of 1833 by the English economist William Forster Lloyd: a pasture open to all, where every herder grazes as many cattle as they like.

Each herder, Hardin wrote in the essay, asks what adding one more animal is worth. The gain is "nearly +1", because the herder sells that animal. The cost, the extra grazing, is shared by everyone, so for that herder it is "only a fraction of -1". Every herder reaches the same answer, and each adds another animal, and another. "Freedom in a commons brings ruin to all."

The word "tragedy" was meant in the old dramatic sense: not a sad accident but an ending the characters walk into with open eyes, because each step makes sense to them. No one has to be greedy or foolish for the commons to be ruined; each user only has to act sensibly for themselves.

How does the trap work, in numbers?

Take a pasture shared by 10 herders that can feed 100 cows well. At 100 cows, each cow yields $100 of milk a year. Every cow beyond 100 eats grass the others need, and lowers the value of every cow on the pasture by $1.

1. The starting point. Each herder has 10 cows: 10 × $100 = $1,000 a year each. The whole pasture yields 100 × $100 = $10,000, the most it can.
2. One herder adds a cow. Now there are 101 cows, each worth $99. That herder earns 11 × $99 = $1,089, a gain of $89. Each of the other 9 herders loses $10, a total of $90. The pasture as a whole is $1 worse off, but the herder who added the cow is $89 better off.
3. Everyone does the same. Every herder faces the same sums, so each adds a cow. Now 110 cows are worth $90 each, and every herder earns 11 × $90 = $990, less than the $1,000 they started with.
4. It doesn't stop there. At 110 cows, a herder who adds a twelfth still gains: 12 × $89 = $1,068 against $990. The herd keeps growing until about 180 cows, 18 each, where a cow is worth only $20 and one more barely pays even for the herder who adds it.
5. The end. Each herder earns 18 × $20 = $360, and the pasture yields $3,600 instead of $10,000. Every herder would gladly go back to 10 cows, but none will do it alone.

Step 2 is Hardin's +1 and fraction of −1, worked out. The shape is the prisoner's dilemma with ten players: each does better by taking more whatever the others do, and all end up worse off. It is also an externality: each herder's extra cow imposes a cost on others that the herder doesn't pay. A real pasture would degrade faster than this straight-line model, since overgrazed grass can stop growing back.

What counts as a commons?

Economists call the resource a common-pool resource: non-excludable, because it's hard to keep users out, and rival, because what one user takes another can't. That pairing is what separates it from a public good, which no one can be kept from but which doesn't get used up. A lighthouse's beam can't be overgrazed; a fishing ground can.

The clearest real case is the cod off Newfoundland. Cod there had been fished for centuries, but factory trawlers in the 1960s raised the catch sharply: it peaked at about 810,000 tons in 1968. By 1992 the northern cod's spawning stock had fallen to around 1% of its earlier level, and Canada closed the fishery. About 35,000 to 37,000 fishers and fish-plant workers lost their jobs, according to the history of the collapse. The stock recovered only slowly; the first commercial fishery for northern cod since 1992 reopened in 2024.

Other commons work the same way. Farmers who share an aquifer each gain from pumping more, while the falling water table is shared by all. Drivers on a free road at rush hour each gain from taking the car, while the congestion they add slows everyone. On the largest scale, the atmosphere is a commons for carbon dioxide, which is why climate change is so hard to tackle: every country gains in full from cheap energy and bears only part of the warming.

How do communities avoid the tragedy?

Hardin saw two ways out: private property, so each owner bears the cost of overuse, or limits enforced by government, which he called "mutual coercion, mutually agreed upon". Both work in some places. Fishing quotas that owners can trade, and permits for pumping groundwater, are versions of each.

Elinor Ostrom, a political scientist at Indiana University, spent decades studying a third way, and in 2009 became the first woman to win the Nobel prize in economics, shared with Oliver Williamson. In Governing the Commons (1990) she studied cases around the world, from irrigation systems in Spain and the Philippines to groundwater basins in Southern California and alpine pastures in Switzerland, where users had run shared resources for decades or centuries without ruin. The Swiss alpine pastures she described had been grazed under village rules for centuries. Her conclusion was that the tragedy is a risk, not a law: communities often write their own rules and keep a commons healthy.

From what the lasting cases had in common, she drew eight design principles, as listed in the summary of her work:

Principle	What it means on the pasture
Clear boundaries	Everyone knows who may graze and where the pasture ends
Rules fit local conditions	Limits match how much grass this land grows
Users help make the rules	The herders themselves agree the number of cows
Monitoring	Someone the herders trust counts the cows
Graduated sanctions	A first offense brings a small fine; repeat offenses, bigger ones
Cheap conflict resolution	Disputes are settled quickly, locally, without lawyers
Right to organize	Outside authorities let the herders run their own system
Nested layers	For a large resource, local groups sit inside wider ones

Ostrom's cases tend to be resources used by a few hundred or a few thousand people who know each other and expect to stay. Commons on the scale of an ocean or the atmosphere, shared by billions of strangers, are much harder to govern this way.

Still argued

Hardin's essay was mainly about human population. He argued that "freedom to breed will bring ruin to all" and that appeals to conscience would fail, and in later writing he argued for restricting immigration. Those views have been widely criticized, including as coercive and, by the Southern Poverty Law Center, as racist. Historians have also disputed his pasture: Susan Jane Buck Cox and others argue that England's real commons were regulated for centuries by custom and manor courts, and declined for other reasons. Hardin himself later said, in his encyclopedia entry on the idea, that the trouble lies with an "unmanaged commons". Economists still argue over how often private rights, government limits or community rules work best.

Where does the idea lead next?

The tragedy of the commons is a case of scarcity without rationing: a resource that is limited but has no price or rule deciding who gets it. Its logic belongs to game theory, the study of choices that depend on what others choose, and its cures are all ways of changing incentives so that what pays each user also protects the whole.

Check yourself

• Without looking back, explain why adding one more cow pays the herder even when it makes the pasture as a whole worse off.
• What makes a fishing ground a commons, but a lighthouse's beam a public good?
• Name three of Ostrom's design principles, and say why each helps.

Sources

• The Tragedy of the Commons — Garrett Hardin, Science, 1968, via the Garrett Hardin Society
• Tragedy of the Commons — Garrett Hardin, The Concise Encyclopedia of Economics, Econlib, 2008
• Elinor Ostrom — Wikipedia
• Collapse of the Atlantic northwest cod fishery — Wikipedia
• Tragedy of the commons — Wikipedia
• The Prize in Economic Sciences 2009, popular information — Royal Swedish Academy of Sciences, 2009