common-pool resource
Picture a village pasture open to all herders. Each one gains fully by adding one more animal to graze, but the wear on the grass is spread across everybody. So every herder keeps adding animals — and the pasture is stripped bare, ruining it for all. This grim logic, named the 'tragedy of the commons' by Garrett Hardin in 1968, is the signature failure of a common-pool resource.
A common-pool resource is rival but non-excludable — an awkward mix. It is rival because one person's use does subtract from what's left for others (a fish caught is a fish gone), yet non-excludable because it is hard or costly to keep anyone out (you can't fence the open ocean). This is the opposite corner from a public good, which is non-rival. The trouble is that each user reaps the full benefit of grabbing more, while the cost of depletion is shared by all — so individually rational over-use leads to collective ruin: collapsed fisheries, drained aquifers, deforestation, overgrazing.
Common-pool resources matter enormously for the real world — most environmental crises are commons problems. The classic fixes are privatization (assign property rights, as Coase suggests) or government regulation (quotas, licenses, protected areas). But economist Elinor Ostrom won a Nobel Prize for showing a crucial third path the textbook tragedy ignores: real communities, from Swiss alpine pastures to Maine lobster fisheries, have for centuries governed shared resources sustainably through their own rules, monitoring, and sanctions — proving the 'tragedy' is a tendency, not an iron law.
An open ocean fishery has no owner. Each boat that catches more makes money now, but the dwindling fish stock is everyone's loss. Without quotas or shared rules, the fleet keeps fishing until the stock collapses — and then no one can fish at all.
Rival but non-excludable — shared use, individual gain, collective ruin.
The 'tragedy of the commons' is a tendency, not destiny. Ostrom's Nobel work showed communities often govern shared resources sustainably through local rules — so neither pure privatization nor top-down control is the only answer.