The cobra effect: when rewards backfire
Pay people to kill pests and some of them will start breeding pests. The famous cobra version may be a legend, but the rat version in Hanoi really happened.
The legend
The story goes that colonial officials in Delhi, worried about venomous snakes, offered a reward for every dead cobra. Enterprising locals began breeding cobras for the bounty. When the government cancelled the scheme, the breeders released their now-worthless snakes, and there were more cobras than before.
It is a perfect parable, and there is no solid historical evidence for it. The economist Horst Siebert popularised it in 2001 under the name Der Kobra-Effekt.
The documented version
In 1902, French colonial Hanoi was building modern sewers, which turned out to be ideal for rats, and plague was a threat. The administration paid a bounty for each rat, and to save the officials from handling corpses, people only had to hand in the tail.
The historian Michael Vann found the results in the archives:
- Tailless rats turned up around the city: people cut off the tails and released the rats to breed.
- Rats were smuggled in from outside the city.
- Inspectors found rat farms on the outskirts.
The lesson
Rewards change behaviour, often not in the way intended. Before designing an incentive, ask: what is the cheapest way to earn this reward without solving the problem? Someone will find it.
Bring it up
“In 1902 Hanoi paid people for rat tails, and people started cutting off tails and letting the rats go so they could keep breeding.”
Follow the thread
- Same patternChesterton’s fence: ask why it’s there before you tear it down Both are warnings about changing a system without understanding how it actually works.
- Same patternWhen a measure becomes a target Paying for dead rats turns a count of dead rats into a target, and the count stops meaning what it did.
- Same patternThe Streisand effect Another case of an action producing the opposite of its intended effect.
- RelatedThe city in Kazakhstan that provoked Genghis Khan A short-sighted decision with catastrophic unintended consequences.
- In contrastThe snail in the ginger beer Rules can backfire when they ignore consequences; the neighbour principle asks people to foresee them.
- In contrastThe veil of ignorance The veil asks about fairness of design; the cobra effect warns that even well-meant design can backfire.
