A common can take many forms, such as wild growing fruit or animals, a supply of water, a peat bog, or a clay pan, but it is frequently understood to be an area of land.

Most villages in the United Kingdom have an area of land called "the common". This is usually a public space used for recreation. Many English commons are now wooded areas; however, in the past many were pasture; that is, agricultural land that is used to feed grass-eating animals. These commons have a long history, and make a great example for describing the concept of the Tragedy of the Commons.

Many of the people of a village had the right to run animals such as cattle, sheep, or horses on the common land. Mostly there was some control on how this common land was used, but in our example we'll assume that the common land was unmanaged, as many other types of commons are now.

Like any land, the common land has a carrying capacity: the number of animals that can sustainably live on it. Although the land may be able to support more grass eating animals than its carrying capacity for a while, this is not sustainable because the grass will be over-eaten, will be unable to regrow, and may die out altogether.

The people who have a right to use the common land (the commoners) will gain something from the animals that they place onto the common land; it may be meat, milk, or other animal products, or the exchange value of those products in the community. For the commoners to be able to get the greatest collective gain from the common land, they should collectively use it at the carrying capacity, which gives maximum productivity.

As long as the carrying capacity of the common land has not been reached each commoner can get more personal gain from it by adding more animals, without consequence to the other animals or their owners. The extra animals simply eat grass that would otherwise go uneaten.

As the number of animals on the common land increases, it will eventually reach its carrying capacity. Once the common land is stocked at its carrying capacity, adding more animals will reduce the condition of all of the animals sharing the common land. They will begin to struggle to feed themselves adequately, which means that they will grow more slowly, produce less milk or breed less successfully, so that the personal gain that the commoners get from each animal will be reduced.

Because overstocking affects all of the animals on the common land, all of the commoners share a proportion of the consequences of exceeding the carrying capacity, including the commoner who placed the extra animal. However; the commoner who placed the extra animal will receive all of the gains of that extra animal, but only a share of the consequent reduction in the condition of the animals. The loss of condition of his or her herd is more than offset by the gains he or she makes from the extra animal.

As more and more animals are added to the common land in excess of the carrying capacity, the condition of all of the animals will continue to reduce and in time the pasture will be damaged. However, no matter how damaged the pasture becomes, in the short-term each individual commoner will still always gain more that they lose by adding an extra animal. So, in the absence of an implemented agreement more and more animals will be added, and the pasture may be destroyed.

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The Tragedy of the Commons

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