A fishing ground shared by a dozen villages. A pasture open to any herder. A groundwater aquifer beneath a dry region. These are common resources: goods that anyone can use, but where one person's use leaves less for everyone else. They are rivalrous but non-excludable, which is a fancy way of saying they get depleted.

The problem is built into the structure. Each user gains the full benefit of taking one more unit, while the cost of depletion is spread across everyone. So people take more than they should. The result is overfishing, overgrazing, and drained aquifers. Ecologist Garrett Hardin called this the tragedy of the commons in a famous 1968 essay, though he overstated how inevitable it was.

Elinor Ostrom later showed that communities often govern shared resources successfully through rules, monitoring, and sanctions. Swiss alpine villages have managed common pastures for centuries. Japanese fishing cooperatives set catch limits. The lesson is not that commons always fail, but that they fail when no one has the authority or incentive to set limits.

Common examples

• Ocean fisheries
• Grazing land
• Groundwater basins
• Atmospheric air as a carbon sink
• Public roads at peak hours