The Great Hanoi Rat Massacre (Vietnamese: Đại thảm sát chuột ở Hà Nội; chữ Nôm: 大慘殺𤝞於河內; French: Massacre des rats de Hanoï) occurred in 1902, in Hanoi, Tonkin, French Indochina (present day Hanoi, Vietnam), when the French government authorities attempted to control the rat population of the city by hunting them down. As they felt that they were making insufficient progress, and due to labour strikes, they created a bounty programme that paid a reward of 1¢ for each rat killed. To collect the bounty, people would need to provide the severed tail of a rat. Colonial officials, however, began noticing rats in Hanoi with no tails. The Vietnamese rat catchers would capture rats, sever their tails, then release them back into the sewers so that they could produce more rats.
The Great Hanoi Rat Massacre happened in the middle of a global pandemic only a few years after Swiss-French physician and bacteriologist Alexandre Yersin linked the spread of the pandemic to rodents.
Today, the events are often used as an example of a perverse incentive, commonly referred to as the Cobra Effect. The modern discoverer of this event, American historian Michael G. Vann argues that the cobra example from the British Raj cannot be proven, but that the rats in Vietnam case can be proven, so the term should be changed to the Rat Effect.

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