Tragedy of the Commons is a concept that intends to explain the unintended destruction of a shared renewable resource by those that use it.  The concept of the Tragedy of the Commons is applicable to many circumstances, some of them surprising.

The essence of the Tragedy of the Commons is that those sharing the use of an unmanaged resource (a common) will act more in self-interest than for the sustained good of the resource. 

In the context of the Tragedy of the Commons, a common is an intrinsically renewed resource that is used in an unregulated way; it is not owned by anyone or any group, and multiple users have access to use it.  An intrinsically renewed resource renews itself by regrowing from within itself.  The Tragedy if the Commons only applies to intrinsically renewed resources.

A common can be harvested either sustainably or unsustainably for its productive output.  If it is harvested at the correct rate, the use is sustainable and it will provide its products indefinitely.  If it is harvested too fast, the use is unsustainable and it will be depleted, and eventually destroyed.

A common has many users who share what the common can sustainably produce. If one user takes more than their share, while the other users continue to take what was their share, the common will be over-used beyond a sustainable level, and will be depleted. Once the common is depleted the harvest that is shared by all of the users will be reduced.. 

The individual user who took more than their share will get all of the gains of the extra that they took, while the depletion of the common caused by the unsustainable use will be shared equally amongst all of its users.  Because that user only gets a share of the depletion, their gain must always be greater than their loss, so they will be better off than they were and better off than any of the other users.

The diagram on the left shows a healthy common being shared sustainably; the blue arrows are the product being taken by the users. The diagram on the right show a degraded common being shared unequally – one of the users is taking a lot more than their sustainable share. The brown arrows are the consequent degradation being shared equally by all of the users, including the user who has taken more than their share. 

For the long-term, it is better if the users of the resource ensure that they harvest the common sustainably, so that its productive output can be taken indefinitely.  However, in the short-term it is advantageous for the individual user to take more.

In practice the common may be so big, diffuse, and unfathomable that none of the users will know if they are using is sustainably or unsustainably, and are therefore likely to use it unsustainably.  One of the best examples of these circumstances is the collapse of the fisheries of the Grand Banks, near Newfoundland, Canada.

Three elements are necessary for a tragedy of the commons to occur:

a common: an intrinsically renewed resource that is available to be used in an unregulated way

a product: which is harvested from the common

multiple users: who harvest the product.

This is the rule of the Tragedy of the Commons:

A Tragedy of the Commons occurs because an individual user of a common gains all of the advantage of their use of it, while the disadvantage that their use incurs is shared amongst all of the users. This means that an individual always gets more advantage than disadvantage from increasing their use, so they will always increase their use.

This rule will be true regardless of how much product is taken from the resource, even as the sustainable rate of harvest is exceeded to the point where the common is degraded, and eventually destroyed. 

The Tragedy of the Commons is hard to understand without an example.  The classic example is that of common land used for pasture. You can see such an example here.

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underpinnings – supporting concepts

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