🎣

A shared fishery feeds a village. Each boat gains the full value of one more fish, while the cost of a thinning stock is spread across everyone. So everyone takes more, the stock crashes, and all end up worse. Hardin called it the tragedy of the commons. Ostrom found it is not inevitable.

The commons

A common-pool resource, such as a fishery or pasture, is rival in use but hard to exclude people from. Private incentives can produce overuse, but collapse is not inevitable. Ostrom documented communities governing shared resources with locally fitted rules, monitoring, and graduated sanctions.

From earlier lessons

Two quick ones before we start.

In a commons, the gain from extracting more is private while the depletion cost is...

Ostrom showed self-governing communities can sustain a commons using monitoring and...

Predict, then play

Set your catch in a renewable fishery and play several rounds. The stock grows between rounds, but only up to its biological limit. One other fisher starts above the agreed catch limit. Then switch on monitoring and graduated sanctions and watch harvest, regeneration, and the stock separately.

Why did that happen?

Take a guess, then reveal the reason

Each user receives the full catch but bears only part of the future depletion cost. When total harvest exceeds biological growth, the stock falls. Continued overharvest can then reduce future growth as well as future catch. Monitoring and sanctions may change extraction incentives, but they do not automatically guarantee restraint. The rule, detection probability, penalty, and users’ continuation incentives all matter.

The real evidence

Definition, Ostrom's design principles

Elinor Ostrom’s Governing the Commons (1990) examined long-lasting common-pool resources, including pastures, irrigation systems, forests, and fisheries, and described eight recurring design principles. Two are monitoring and graduated sanctions: first offences draw relatively small penalties, while repeat offences draw larger ones. These institutions can reduce the payoff from overuse, but the equilibrium still depends on detection, sanctions, resource dynamics, and expectations about future play.

The game now uses a stock $S_t$ with carrying capacity $K$ and logistic growth rate $r$ :

$S_{t+1}=\max\!\left\{0,\min\!\left[K,S_t+rS_t\left(1-\frac{S_t}{K}\right)-H_t\right]\right\},$

where $H_t$ is total harvest. Growth is largest at $S_t=K/2$ , where the maximum sustainable yield is $rK/4$ . A catch below current growth raises the stock; a catch above current growth lowers it. A graduated sanction adds a penalty that rises with the amount over the rule and with repeated violations. Whether that deters overharvest depends on expected detection and future consequences.

Work it out

From collapse to sustainable yield

With a stock of $60$ , $K=100$ , and $r=0.8$ , biological growth is $0.8(60)(1-60/100)=19.2$ . If total catch is $20$ , the next stock is $59.2$ . If total catch is $16$ , it rises to $63.2$ . The sustainable catch is therefore state-dependent, not a fixed contribution to a pot.

Now add a catch limit of $4$ , monitoring, and a penalty that rises after each repeated violation. In the demonstration, the high extractor responds by reducing catch. That response is an assumption you can see, not a theorem. Ostrom’s evidence shows that monitoring and graduated sanctions often appear in durable self-governed commons. It does not establish that one penalty schedule always beats every alternative.

Key idea

A commons is a stock-flow problem: today’s private harvest changes tomorrow’s shared resource. Monitoring and graduated sanctions can support restraint, but their effect depends on the biological dynamics and the incentives they create.

Check yourself

Check yourselfapply

Why does an unmanaged common fishery tend to collapse?

Check yourselfanalyse

What did Ostrom show about saving a commons, against Hardin's conclusion?

Check yourselfbrings back: public goods

What is one reason to make sanctions graduated rather than imposing the maximum penalty after every violation?

Sources

Ostrom, E. (1990). Governing the Commons. Cambridge University Press.
Hardin, G. (1968). The tragedy of the commons. Science, 162, 1243-1248.