H052
How did paying for rat tails give Hanoi more rats?
In 1902, French colonial officials in Hanoi offered a bounty for every rat tail. The tails poured in. The rats didn't go away.
A reward that grew the problem
The bounty paid for tails, not for fewer rats
When you pay for a measure of a problem, people can produce the measure instead of solving the problem. The city wanted fewer rats; it paid for tails. Cutting off a tail and releasing the rat to breed earned the reward and kept the supply going. Some residents went further and raised rats. The program was dropped, and plague reached Hanoi in 1903.
Economists call this a perverse incentive, and it shows up wherever a reward is tied to something easy to count: lines of code written, tickets closed, arrests made, test scores raised. Before rewarding a number, ask what the cheapest way to raise that number would be.
Try it somewhere else
A company pays programmers a bonus for every bug they find and fix in their own code.
What might happen to the number of bugs they write?
The "cobra effect" story
The better-known version of this idea is the "cobra effect": British officials in colonial Delhi supposedly paid for dead cobras, people bred cobras for the bounty, and when the bounty ended the breeders released them, leaving more cobras than before. It is widely repeated, but historians have found little documentation for it. The Hanoi rat bounty is the documented case, recorded in French colonial archives and studied by historian Michael Vann.
Related exhibits
Sources: Michael G. Vann, "Of Rats, Rice, and Race: The Great Hanoi Rat Massacre, an Episode in French Colonial History," French Colonial History, 2003; Michael G. Vann and Liz Clarke, The Great Hanoi Rat Hunt (Oxford University Press, 2018). Cobra-effect term: Horst Siebert, Der Kobra-Effekt (2001).