The commons usually refers to a resource that is simultaneously made available to multiple people. As each person claims their share of the commons, they regard the commons as being available for their own purposes. From a systems perspective, the Tragedy of the Commons provides unique insights into how the behavior of an individual can have a profound effect on the overall, long-term performance of the system.
The figure below represents the system archetype for the Tragedy of the Commons. The behavior that is shown can be summarized as rational actions by individuals to improve individual performance at the expense of a common resource which results in destroying the ability of the whole system to perform and eventually the ability of the individual to perform.
In the diagram, an increase of activity of either A or B initially results in an increase in gain for A or B, respectively. However, as the total activity pushes the resource limit, the gain for either A or B eventually decreases with increasing activity.
There are numerous examples of the Tragedy of the Commons. Overfishing, for example, depletes the fish stock and the ability of fish to reproduce. This does not stop individual fishermen from overfishing, however, since scarcity of fish initially results in increased prices leading to even more fishing. Eventually the fish stock is so severely depleted that all fishermen suffer.
