Why do shared resources so often get ruined?

Shared resources get ruined because what's rational for each user is disastrous for the group. When a resource is shared, and using more of it benefits you while the cost is spread over everyone, it makes sense for each person to overuse it, even though together they drain it for all. Economists call this the tragedy of the commons. Aristotle had already noticed that what is common to the greatest number gets the least care.

The classic picture comes from the English economist William Forster Lloyd in 1833: herders sharing a pasture. Each extra cow brings its owner a benefit, while the damage to the grass is shared by the whole group. If every herder makes that sensible choice, the pasture can be destroyed. The ecologist Garrett Hardin made the idea famous in 1968, and it now describes problems from forests and water to fish and fossil fuels.

The real-life version can be brutal. Off Newfoundland, the cod catch peaked at 810,000 tons in 1968. By 1992 the Northern cod had fallen to 1% of historic levels, Canada closed the fishery, and about 37,000 people lost their jobs.

But the tragedy is not destiny. The political scientist Elinor Ostrom found that local communities often invent their own rules to protect a commons, and in 2009 she became the first woman to win the Nobel Memorial Prize in Economic Sciences.