26,000 gallons! My brother moved into his new home in 2014, at the height of a recent Southern California drought. His water bill showed that he was pouring 26,000 gallons onto his lawn every month. He was not alone. Every morning, water flowed from lawns along his street down driveways and into storm drains. I asked him what he was going to do about it. He replied, sensibly, “Fix it. I am responsible for my decisions.” He replaced his lawn with drought-resistant native plants. His neighbors were annoyed, and he was briefly ostracized. However, over the next few years most of his neighbors followed suit.

The Prisoner’s Dilemma is a widely studied example in game theory. Two men are charged with a serious crime, for which the sentence is five years. The police do not have enough evidence...

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