The assumption that a shared resource will be tragically exploited for individual gain, despite any evidence to the contrary.

Since its publication in Science in December 1968, “The Tragedy of the Commons” has been anthologized in at least 111 books, making it one of the most-reprinted articles ever to appear in any scientific journal. It is also one of the most-quoted: a recent Google search found “about 302,000” results for the phrase “tragedy of the commons.” – Ian Angus

Hardin assumed that human nature is selfish and unchanging and that society is just an assemblage of self-interested individuals who don't care about the impact of their actions on the community. The same idea, explicitly or implicitly, is a fundamental component of mainstream (i.e., pro-capitalist) economic theory. – Ian Angus

see also