Are people always going to over-exploit common resources? Many economists believed in this "tragedy of the commons" for a very long time, but Elinor Ostrom (who was awareded the Nobel Prize in Economics in 2009) proved them wrong.
According to her research findings, "groups are capable of avoiding the tragedy of the commons without requiring top-down regulation, at least if certain conditions are met".
Ostrom's "Eight Principles for Managing a Commons" have been reworded frequently, and some versions are much easier to understand than the original version I posted above, e. g. this one from P2P Foundation:
http://wiki.p2pfoundation.net/Elinor_Ostrom%E2%80%99s_Eight_Commons_Governance_Design_Principles
Ostrom's 8 Principles for Managing a Commons (1992, p. 10):
1 Clearly Defined Boundaries
2 Congruence between Appropriation and Provision Rules and Congruence Between
Provision Rules and Local Conditions
3 Collective-Choice Arrangements
4 Monitoring
5 Graduated Sanctions
6 Conflict Resolution Mechanisms
7 Minimal Recognition of Rights to Organize
8 Nested Enterprises
http://dlc.dlib.indiana.edu/dlc/bitstream/handle/10535/5460/design%20principles%20for%20local%20and%20global%20commons.pdf