Imagine a village with a green pasture, held in common, on which all the villagers can graze their sheep. The common land has a peak carrying capacity – fewer sheep than the optimal number means the village isn’t taking full advantage of the meadow, but more than that level will eat too much of the grass, leaving the land barren.

This was the problem that demographer Garrett Hardin posed in a classic 1968 paper in Science magazine called “The Tragedy of the Commons,” posing a depressing conundrum for the millions of college undergraduates who have found it on their reading lists ever since, the way I did.