Nonexcludability
It is hard, costly, or politically difficult to stop people from using the resource. Access is not perfectly controlled.
Tragedy of the Commons
Common Pool Resource
A common pool resource is shared by many users, difficult to keep people from using, and limited enough that one person's use leaves less available for everyone else.
Building the Intuition
Why common pool resources are vulnerable
The tragedy does not happen just because people share something. It happens when access is hard to control, use subtracts from the resource, and each person feels only part of the cost of using more.
If a resource is easy to protect or charge for, users can be made to account for the damage they cause. If one person's use does not reduce anyone else's use, there is no depletion problem. Common pool resources sit in the difficult middle: they are open enough to invite many users, but limited enough that total use matters.
This creates a gap between the private decision and the shared outcome. A user may think, "One more unit of use helps me, and the harm is spread across everyone." That logic can be individually reasonable while still producing a bad collective result.
Subtractability
Each person's use subtracts from what remains. The resource can recover, but only if use stays within its limits.
Shared Costs
The individual user receives a direct benefit from using more, but the damage from overuse is spread across the whole group.
The Problem
Why overuse can be rational for each user
The trouble starts when a person asks, "Should I use a little more?" If the benefit mostly goes to them, and the cost is shared by everyone, using more can feel rational from their point of view.
But common pool resources add up individual choices. When many people make the same reasonable-seeming decision, total use can pass the resource's recovery limit. That is how a commons can become overused even when nobody set out to ruin it.
The Example
Grassland for grazing
A shared grassland fits the definition well. It can be hard to exclude herders from an open field, especially if the land has long been treated as a community resource. It is also subtractable: every cow eats grass that other cows cannot eat.
Each herder gets the direct benefit of bringing another cow, while the cost of thinner grass is shared by everyone using the pasture. The field can recover when grazing stays moderate. But when too many cows eat the grass faster than it regrows, the pasture becomes less productive for the whole group.
Solutions
How rules can make users face the shared cost
Solutions work by changing the decision each herder faces. The goal is to make the private choice line up better with the health of the shared pasture.
Open access
This is the baseline problem. No one pays for the extra pressure their cows place on the field, so the cost of overgrazing remains external to each herder's decision.
Property rights and entry fees
If someone controls access and charges a fee, using the pasture is no longer free at the margin. The fee can make herders internalize some of the cost their cows impose on others.
Cow caps
A cap limits total grazing directly. Instead of relying on each herder to voluntarily stop, the rule keeps use below the level the grassland can recover from.
Toy Model
Change the rules, then watch the pasture respond
Use the controls to compare open access, an entry fee, and a cow cap. The same grazing demand plays out under different rules.
Pasture health
72%
Cows grazing
18
Private payoff
High
Shared outcome
Strained
What this teaches
Open access can reward overuse
With no limits, each herder has a reason to add cows. The grass absorbs the shared cost until it cannot recover fast enough.