In 1902, French colonial authorities in Hanoi introduced a rat bounty during an outbreak of bubonic plague, offering one cent for every rat tail submitted as proof of a kill. Within a year, more than 55 million rat tails had been collected. But the enormous number of tails did not mean that millions of rats had actually been eliminated. Officials eventually found tailless rats still moving around the city, revealing how people had learned to claim the reward without killing the animals. The episode, known as the Great Hanoi Rat Massacre, is highlighted in a 2026 review published in npj Biodiversity as an example of how financial incentives designed to control unwanted species can produce unintended results. This remarkable historical episode serves as a timeless cautionary lesson, demonstrating how well-intentioned financial incentives can inadvertently perpetuate the exact problems they were designed to eliminate.
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