The theory of the Tragedy of the Commons states that when a resource is collectively owned by a group of people, each will exploit the resource, overusing it, and thus ultimately destroying the resource.  In other words, everyone acts as a free rider, ignoring the group's collective interests in favor of their own.

The idea dates to an 1833 essay by William Forster Lloyd.  He noticed that in a common pasture owned by all of the villagers, each villager overgrazed the pasture, ruining it for everyone.

In 1968, Garret Hardin applied the Tragedy of the Commons theory to population growth.  The idea is that having children is beneficial to individuals, who will therefore have many kids, but leads to overpopulation in society, which has numerous negative effects.

If the population gets out of control, many scarce natural resources will be entirely consumed.  Humanity will not have enough water, food, energy, or room to support an increased population.  To combat this problem, Hardin proposes an international agreement meant to restrict population growth.

One other major area of concern is in harvesting animals in international waters.  Applying the theory, one might predict that each country will attempt to catch as many animals as possible.  This has often occurred, and has lead certain species to become extinct.

There are many people who oppose the Theory of the Commons.  Some say that the subjects of Hardin and Lloyd's essays were not genuine community resources.  Others, fighting the theory's implication that private property is superior to public property, have presented a plethora of evidence showing that natural resources on privately owned property are even more quickly consumed.

Some people also say that it is not population growth that threatens the environment, but overuse of resources.  In poorer countries, populations far larger than those of rich countries use far fewer natural resources.