Common Resources and Overuse

A common resource, like an open ocean fishery or a shared pasture, is rival: one more fish caught or one more cow grazing leaves less for everyone else. But it is non-excludable: nobody can stop another boat or herder from using it. Put those two together and you get the tragedy of the commons. Each user only weighs their own private cost and benefit, ignoring the cost they impose on everyone else who now has fewer fish or less grass. Since nobody pays the full social cost, individuals keep using the resource past the point where marginal social benefit equals marginal social cost, the efficient quantity, and the resource gets depleted or degraded.

Walk through a fishery boat by boat. Boat 1 catches fish and earns a profit, barely denting the stock. But as more boats enter, each new boat still earns a positive private return even as it pushes the fish population down, because the boat captures its own catch but doesn't pay for the stock depletion it causes to future catches. Boats keep entering until private profit hits zero, long after the point where social benefit and social cost were equal. The fishery can collapse even though every single boat behaved rationally.

Assigning catch limits, licenses, or tradable permits restores excludability: now someone owns the right to a slice of the resource and bears the cost of overusing it. This is the same fix in spirit as taxing a negative externality. Both problems have the same shape, a gap between private and social cost, so both get solved by making someone internalize a cost they were previously dumping on others.