In the early years of the last century, Hanoi had a rat problem. To solve it, the French colonial government placed a one-cent bounty on the rodents, which could be claimed by anyone who delivered a rat’s tail. Thousands of tails were tendered, but Hanoi’s rat population didn’t shrink. Instead, tailless rats were running through streets, and rat farms were discovered. To make money selling rats’ tails, you need lots of rats breeding more rats. The moral of the story: be careful which behaviors you reward.