What Is the Tragedy of the Commons?

Ten families in a village share one meadow. Each family keeps ten cows there, and the grass is enough for all of them. One day a herder buys an eleventh cow. It earns him money, and nobody notices much. Soon the others do the same, and a few years later the meadow is bare mud. Nobody wanted that result, and nobody acted foolishly. So how did it happen?

Why do sensible people ruin a resource they all need?

This pattern is called the tragedy of the commons. A commons is a resource that many people use together, such as a pasture, a fishing ground, a forest or the air. The tragedy happens when anyone may take from the commons without limit. Each user gets the whole gain from taking a bit more. The cost of the damage, however, is shared by everybody. So for each person, taking more always pays, even when it ruins the resource for the group.

We can follow this with a simple model of the meadow. Assume that each cow gives 20 litres of milk a day, minus 0.1 litres for every cow on the meadow, because each cow eats grass the others need.

1. The start. There are 100 cows, ten per family. Each cow gives 20 minus 10, so 10 litres. Each family gets 100 litres a day, and the village gets 1,000.
2. One extra cow. Anna buys an eleventh cow. Now there are 101 cows, and each gives 9.9 litres. Anna’s eleven cows give 108.9 litres, so she gains 8.9 litres a day. Each of the nine other families loses 1 litre. The village as a whole loses a little, but Anna is better off.
3. Everyone copies her. Each family sees the same sum and buys an eleventh cow. Now 110 cows give 9 litres each. Each family gets 99 litres. Every family now has more cows and less milk than at the start.
4. It still pays to add more. At 110 cows, a twelfth cow still brings its owner almost 8 more litres. So the herders keep adding cows.
5. The end. Adding a cow stops paying for its owner only at about 180 cows. Then each cow gives about 2 litres, and each family gets about 36 litres a day. That is about a third of what they had before.

The key step is the second one. Anna receives all the milk from her new cow, but she bears only a small part of the damage. Most of it falls on her neighbours. The American biologist Garrett Hardin described this in 1968. For the herder, he wrote, the gain from one more animal is “nearly +1”, while the loss is “only a fraction of -1”.

Notice that nobody in the model is greedy or stupid. A family that refuses to add cows does not save the meadow. It only gets less milk while the others still add theirs. This is why the problem is so hard to solve. Being good on your own does not help.

Freedom in a commons brings ruin to all.

Hardin used the word “tragedy” in the old sense of Greek drama. In that sense a tragedy is a disaster that follows step by step from the situation, and the people involved can see it coming but cannot escape it.

What the tragedy of the commons teaches you

The main lesson is a question you can ask about any shared thing that is getting worse: who gets the gain from using more, and who pays the cost? When the gain goes to one person and the cost is spread over many, expect overuse, even among decent people.

• Fishing. The sea was long open to every boat. Each crew gained from one more catch, and the loss of young fish was shared. Off Newfoundland, cod stocks collapsed after decades of heavy fishing, and in 1992 Canada closed the fishery. About 30,000 people in the province lost their work.
• Traffic. Each driver who takes the car into the city at rush hour gains a faster trip for themselves. Each car also slows down everybody else a little. The road is the commons, and the traffic jam is the bare meadow.
• The climate. The atmosphere is a commons for waste. A country or a company that burns more fuel gets the full benefit, while the warming is shared by the whole world. Hardin himself pointed out that pollution is the same pattern in reverse. People put something into the commons instead of taking something out.
• Your own shared spaces. A shared kitchen, a group project or a household works the same way. One person who leaves dishes saves ten minutes. Everybody pays with a dirty kitchen. Asking people to be considerate rarely solves this for long. Clear, agreed rules usually do.

The same logic appears in economics, where the cost that falls on others is called an externality. It also appears in game theory as a game in which each player’s best move leads to a result that is bad for all. Economists, ecologists and lawyers use it to design fishing quotas, road tolls and emission permits.

Is the tragedy really unavoidable?

Hardin thought it was, unless the commons was divided into private property or the state took control. His essay ends in strong words. He calls for “mutual coercion, mutually agreed upon by the majority of the people affected”. Many readers took this as proof that shared land always fails. That conclusion is the most common misunderstanding of the idea.

The political scientist Elinor Ostrom spent decades studying real commons. She found many communities that had shared pastures, forests, fishing grounds and water for centuries without ruin. A famous case is Törbel, a mountain village in the Swiss canton of Valais. Its farmers signed an agreement on the use of water and pasture in 1483. Their rules allowed each family to send to the summer pasture only as many cows as it could feed through the winter.

Ostrom’s main point is a distinction Hardin had missed. Hardin’s pasture has open access: anyone may use it, and there are no rules. A real village commons usually has owners and rules. From her cases, Ostrom drew eight design principles that successful commons share. The first five are these.

1. Clear boundaries. Everyone knows who may use the resource and what belongs to it.
2. Rules made by the users. The people affected can take part in changing the rules.
3. Monitoring. Someone who answers to the users checks the resource and how people use it.
4. Graduated sanctions. A first offence brings a small penalty, and repeated offences bring larger ones.
5. Cheap ways to settle conflicts. Disputes can be solved quickly and locally.

The others are rules that fit local conditions, the right of users to organise themselves without the government stopping them, and layers of organisation for large resources. A review in 2010 tested the principles against 91 studies and found them well supported.

Hardin accepted part of this. In 1998 he wrote that, judging from his critics, his “weightiest mistake” had been to leave out one word: “unmanaged”. The tragedy belongs to the unmanaged commons.

The idea still has real limits. Ostrom’s successes are mostly small, local groups whose members know each other and meet often. The climate or the open sea has billions of users and no village meeting. For those, it is much harder to set boundaries, watch everyone and punish cheaters. There, Hardin’s warning remains a serious description of the problem.

Historians add a second objection. The English village commons that Hardin had in mind were usually not open to all. Villages limited the number of animals each household could graze, a practice called stinting. So the parable is better read as a model of open access than as a piece of history.

Learn more

The idea is older than its name. In lectures at Oxford in 1832, published in 1833, the English economist William Forster Lloyd compared cattle on a shared common with cattle on a private field. Garrett Hardin, a biologist at the University of California, Santa Barbara, gave the pattern its name in Science in 1968. The parable fills only a few paragraphs of his essay. Most of it argues for limits on population growth, under headings such as “Freedom to Breed Is Intolerable”.

Elinor Ostrom answered with Governing the Commons (1990). In 2009 she became the first woman to receive the Nobel Prize in economics, “for her analysis of economic governance, especially the commons”.

The tragedy of the commons is a case where many separate choices add up to a bad result. For a case where they add up to a good one, read What Is the Wisdom of the Crowd?. Both pieces show that the result depends on the rules that connect the choices. For an economist who designs such rules for real markets, read “Who Gets What and Why” by Alvin E. Roth: Summary and Review. The best book on the solutions is Ostrom’s own Governing the Commons (Affiliate Link). It is academic, but its case studies of Swiss alps, Japanese villages and Spanish irrigation systems are easy to follow.