Concern for the environment goes back hundreds of years, but mistreatment of natural resources became a big cause midway through the 20th century. Huge industrial complexes spilled their effluence into the environment as worldwide population exploded and consumer demands skyrocketed. A deadly smog killed thousands of Londoners in 1952; Rachel Carson’s 1962 book Silent Spring catalogued the fatal chemicals being released into the air and water; Cleveland’s Cuyahoga River, bright yellow with pollution, caught fire in 1969. Something had to be done.

In 1968, Garrett Hardin gave a lecture, later published as an article in the magazine Science, asserting that the main cause of environmental danger comes from the burgeoning size of the human population. His essay, “The Tragedy of the Commons,” pointed out that a single person can kill an animal or dump garbage onto a wilderness, and that act won’t have much effect, but when populations increase and millions of people do so, the ecosystems involved become stressed to the breaking point.

Hardin’s 1968 lecture borrows a concept first presented in 1833 by mathematician and economic theorist William Forster Lloyd, who explained that an unregulated resource opened for common use, such as a grazing area, would quickly be overrun as more and more people take advantage of the resource.