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Thirty years ago in Science, Garrett Hardin introduced the metaphor "tragedy of the commons" to help explain overpopulation, air pollution, and species extinction. People often overuse resources they own in common because they have no incentive to conserve. Today, Hardin's metaphor is central to debates in economics, law, and science and powerful justification for privatizing commons property. While the metaphor highlights the cost of overuse when governments allow too many people to use a scarce resource, it misses the possibility of underuse when governments give too many people rights to exclude others. Privatization can solve one tragedy, but cause another.