The tragedy of the commons is a metaphor used to illustrate the conflict for resources between individual interests and the common good. The term was coined and popularized by Garrett Hardin in his 1968 Science article "The Tragedy of the Commons". While the original article was primarily concerned with human population growth, Hardin also focused on the use of resources such as the atmosphere and oceans.
Hardin used a hypothetical example of English Commons, shared plots of grassland used in the past by all livestock farmers in a village. Each farmer keeps adding more livestock to graze on the Commons, because it costs him nothing to do so. In a few years, the soil is depleted by overgrazing, the Commons becomes unusable, and the village perishes. (Hardin actually misunderstood how commons were managed in England and elsewhere — see below for details — but this does not detract from the significance of his argument, which pertains to truly open-access commons such as the sea and the atmosphere.)
The cause of any tragedy of the commons is that when individuals use a public good, they do not bear the entire cost of their actions. If each seeks to maximize individual utility, he ignores the costs borne by others. This is an example of an externality. The best (non-cooperative) short-term strategy for an individual is to try to exploit more than his share of public resources. Assuming a majority of individuals follow this strategy, the theory goes, the public resource gets overexploited.
The tragedy of the commons is a source of intense controversy, precisely because it is unclear whether individuals will or will not always follow the overexploitation strategy in any given situation, and especially because Hardin had a very poor understanding of how traditional commons were managed. Experiments have indicated that individuals do tend to behave in this way, when the common is unregulated, but historically, most commons have been regulated by communities, and the more use-pressure a common is under, the more heavily regulated its use would be. Hardin's misunderstanding of the traditions of commonland and resource management, however, has been widely influential and has caused a great deal of trouble for those who wish to advocate regulated communal land use, as opposed to enclosure and privatisation (which has historically been associated with the alienation of resources from poorer people).
Hardin begins his essay with a discussion of the concept of a "commons", which he draws largely from the history of common land use in England, and other parts of North Western Europe. However, these historical commons to which he refers were not public land and most were not open to the access of all — the public at large had very limited rights (e.g. passing drovers could lease grazing for "thistle rent"). Only those locals who were also "commoners" had access to a bundle of rights; each commoner then had an interest in his own rights, but the common itself was not property.
Under many modern understandings of property, e.g. in the USA, the bundles of rights would not have been "property" either, since they could not be traded or otherwise disposed of. However these commoner's rights applied in a medieval culture which did recognise inalienable property (e.g. entailed inheritances), so under this system the bundles of rights were considered property.
In a traditional English village these rights provided commoners with rights of grazing, gathering fuel wood non-destructively "by hook or by crook", etc. (the form "commons" is plural, and refers to the whole group of commons subject to these effects).
Consider an area used for grazing (among other purposes — it could be "Lammas Land", used for private crops in season) that can support 50 cattle indefinitely, a population of 25 peasant householders who keep cattle among a range of subsistence activities, and that each peasant can advantageously graze and profit from 2 cattle indefinitely. By grazing one extra cow, a peasant can make roughly 1/2 extra "profit" at a "cost" of only 1/50. Thus each peasant is logically tempted to keep adding cattle beyond the capacity of the common to sustain them all optimally. Where the grazing area could sustain 50 cattle indefinitely, this increased grazing load could diminish or even destroy the ability of the land to sustain any cattle, at least until it has recovered. (A tragedy of the commons can occur even without complete and permanent destruction of a resource; such activites as overfishing can indeed lead to this.)
Though this metaphor is not an accurate description of how the system worked during most of its history, it serves here for purposes of illustration.
Historically, most English commons were reserved for their own commoners, whose use was restricted in various ways according to local custom. In reponse to overgrazing, for example, a common would be "stinted", that is, a limit would be put on the number of animals each commoner was allowed to graze. This stint might be related to the ownership of a commonable cottage, or to the amount of land owned in the open fields. These regulations were responsive to demographic and economic pressure; rather than let the commons be degraded, access was usually restricted even further. By the time of parliamentary enclosure, in many manors in southern England few labourers and poorer people held common grazing rights; enclosure, however, did have an impact on smaller landholders who supported their farming through use of common grazing and other resources. While historians continue to debate the significance and impact of enclosure on small landholders and labouring people in England, they agree that there is no evidence that commonland use was itself unsustainable.
Modern Equivalents - The contribution of each person is minute but summed over all people these actions degrade the resource. Modern equivalents include:
Uncontrolled population growth
Burning of fossil fuels
Pollution of waterways and the atmosphere
Logging of forests.
Overfishing of the oceans
Private vehicles jamming public roadways.
Tossing of trash out of automobile windows (littering)
Possible solutions to the 'tragedy' - The tragedy of the commons can be seen as a collective prisoner's dilemma. Individuals within a group have two options: cooperate with the group or defect from the group. Cooperation happens when individuals agree to protect a common resource to avoid the tragedy. By cooperating, every individual agrees not to seek more than his share. Defection happens when an individual decides to use more than his share of a public resource.
Game theory shows that individuals benefit from defecting in the prisoner's dilemma (even though both would be better off if both cooperated than if both defected), unless there is some individual cost to defecting. In the iterated prisoner's dilemma, retaliation for past defection can make cooperation the best choice even for a selfish individual. Similarly, far-sighted groups that impose some sort of sanction on members that overexploit a resouce can make over-exploitation unprofitable. This is trickier for larger groups.
Articulating solutions to the tragedy of the commons is therefore one of the main problems of political philosophy. Many such solutions involve enforcement of conservation measures by an authority, which may be an outside agency or selected by the resource users themselves, who agree to cooperate to conserve the resource. Another frequently-proposed solution is to convert each common into private property, giving the owner of each an incentive to enforce its sustainability. Effectively, this is what took place in the English "Enclosure of the Commons"; this case highlights the effects of hidden wealth transfer in privatization, if no or inadequate matching compensation occurs. Moreover, as demonstrated by recent conflicts over logging, snow cover and water resources in the Upper Rio Grande watershed, there are questions about whether individual ownership does provide an incentive to enforce its sustainability, particularly if the property is not looked on as a long term investment. Increasingly, many agrarian studies scholars advocate studying traditional commons management systems, to understand how common resources can be protected without alienating those whose livelyhoods depend upon them.
A popular solution to the problem is also the "Coasian" one, where the individuals using the commons make payments to one another in exchange for not overusing the resource.
In Hardin's essay, he proposed that the solution to the problem of overpopulation must be based on "mutual coercion, mutually agreed upon" and result in "relinquishing the freedom to breed." Hardin discussed this topic further in a 1979 book, Managing the Commons, co-written with John A. Baden.