Why shared fish, fields and water get used up, and how some places don't

When something is shared and nobody controls it, taking a bit more is always worth it for you. You keep the whole gain; the damage is split among everyone. When every user thinks this way, the resource runs out, even if nobody is greedy.

That's the tragedy of the commons, and it is how the Grand Banks cod fishery, once one of the richest in the world, collapsed in 1992 and put about 30,000 people out of work.

But it isn't inevitable. Elinor Ostrom won a Nobel prize for showing how villages and fishing crews have shared resources for centuries without ruining them.