In 1833 an Oxford economist, William Forster Lloyd, described a village pasture open to everyone's cattle. Each herder asks a simple question: should I add one more cow? If he does, he gets all of the extra milk and meat. The cost, a little less grass per animal, is shared by every herder in the village. With twenty herders he pays only a twentieth of the damage he causes.
So adding the cow is the sensible choice for him, and for every other herder too. Each one acts reasonably, and together they strip the pasture bare. In 1968 the ecologist Garrett Hardin made the idea famous in the journal Science as "the tragedy of the commons", and summed it up in one line: "Freedom in a commons brings ruin to all."
The trap has a precise shape. The gain from taking more is private and immediate. The cost is shared and comes later. Nobody needs to be greedy or stupid for it to happen. They only need to be sharing something that no one controls.
The Grand Banks off Newfoundland once held one of the richest cod stocks in the world. For centuries boats caught it with lines and small nets. From the 1950s, factory trawlers arrived with sonar and nets that could haul up tonnes of fish at a time. Catches of northern cod peaked at around 810,000 tonnes in 1968.
A fish stock grows fastest when it is about half its natural maximum: there are plenty of fish to breed and plenty of food for the young. Take less than that growth each year and the stock holds steady. Take more and it shrinks, and a smaller stock grows back more slowly, so the same catch now takes a bigger bite. The decline speeds itself up.
That is what happened. The spawning stock of northern cod fell by roughly 93% in 30 years, from about 1.6 million tonnes in 1962 to somewhere between 72,000 and 110,000 tonnes in 1992. Warning signs were argued over and quotas stayed too high. In July 1992 Canada's fisheries minister, John Crosbie, closed the fishery. About 30,000 people in Newfoundland and Labrador lost their jobs almost overnight. The ban was expected to last two years. Commercial fishing for northern cod only reopened in 2024.
Here is a small fishery. The fish grow the way real stocks roughly do, fastest at half their natural size, with some good and bad years thrown in. Each year every boat tries to land its catch. In everyone for themselves mode each boat takes as much as it can. In agreed quota mode the fleet shares a limit of 20% of the current stock, and fishing stops if the stock falls below a fifth of its original size.
Things to try:
A quota changes the question each fisher is asking. Without one, a fish you leave in the sea is likely to be caught by someone else, so there is no point leaving it. With a fixed limit, taking more yourself doesn't help, and a healthier stock means a bigger limit next year for everyone.
Some countries go further and give each fisher a guaranteed share of the annual limit, often called a catch share or individual transferable quota. Iceland and New Zealand adopted them widely in the 1980s and 1990s. The share works a bit like owning part of the stock: if the fish recover, your share is worth more. A 2008 study in Science looked at more than 11,000 fisheries worldwide and found that those managed with catch shares were about half as likely to collapse. Catch shares have critics too. They can concentrate fishing rights in a few large companies and push small boats out.
Hardin's essay was often read as a choice between two options: let the government run a shared resource, or split it into private property. The political scientist Elinor Ostrom spent decades showing that there is a third option, and that people use it all the time.
She studied groups that had shared a resource for a long time without destroying it. In the Swiss village of Törbel, farmers have shared alpine meadows for centuries. A rule from 1517 said nobody could send more cows up to the summer pasture than they could feed through the winter. In Alanya, on Turkey's coast, local fishers were fighting over the best spots in the 1970s. They agreed to list the fishing spots, draw lots each September for starting positions, and move one spot along each day, so everyone got a fair turn at the good water.
From cases like these, in her 1990 book Governing the Commons, she drew out a set of design principles that successful groups tend to share. Among them:
In 2009 Ostrom became the first woman to win the Nobel prize in economics, shared with Oliver Williamson, "for her analysis of economic governance, especially the commons". Hardin himself later conceded the point, saying his essay should have been called the tragedy of the unmanaged commons.
A commons becomes a tragedy only when nobody manages it. Fisheries, groundwater, grazing land, even the atmosphere all have the same shape: private gain, shared cost. What saves them is rarely goodwill alone. It's a limit everyone can see, a way to check who takes what, and consequences that everyone knows about. The cod fishers of Newfoundland learned it the hard way. The farmers of Törbel had worked it out five centuries earlier.