Tragedy of the Commons
Plain definition
The tragedy of the commons describes how a shared resource gets overused and depleted when individuals each follow their own self-interest and ignore the cost they impose on others.
In economics
Garrett Hardin's 1968 idea formalizes the common-resource problem: each user captures the full benefit of extra use but shares the depletion cost, so everyone overuses. Fixes include quotas, taxes, property rights, or Ostrom-style community governance.
Example
If every herder adds one more cow to a shared pasture, the pasture is soon overgrazed and all herders lose — even though each addition was individually profitable.
The 'tragedy' is not about bad morals; fully rational people cause it because the incentives reward overuse, which is why changing the incentives (not the people) fixes it.