Hanoi’s 'rat bounty' scheme backfired: They paid for 55 million tails, but the plague-ridden infestation only grew.

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.The rat campaign took place in Hanoi, then part of French Indochina, during the Third Plague Pandemic. Historical research by historian Michael G. Vann has documented the French colonial administration's efforts to control the city's rat population in 1902. The campaign was closely connected to the growing concern over plague in Hanoi and the discovery that rats were moving through the city's sewer system.The 2026 review, titled Society's struggle with unwanted species: what centuries of history reveal about species bounty programs, describes the Hanoi programme as a public bounty scheme. Instead of requiring hunters to present an entire dead rat, authorities accepted a severed tail as proof and paid one cent for each tail. The arrangement created a simple incentive: the more tails a person submitted, the more money they could receive. But the system also created a loophole because the proof required by the authorities did not necessarily demonstrate that the rat itself had been killed.According to the 2026 npj Biodiversity review, more than 55 million severed rat tails were collected over the course of a year. On paper, the figure suggested an enormous campaign against Hanoi's rats. The results on the streets told a different story. Government officials began noticing rats that were alive but had no tails. The review explains that rat catchers had discovered they could cut off a rat's tail, collect the bounty and then release the animal.That meant the same rat could remain alive after generating a payment. More importantly, the surviving animal could reproduce, creating the possibility of future rats that could also become a source of bounty payments. The episode illustrates what the researchers call a form of fraudulent behaviour in species bounty programmes: not killing the target species. Rather than achieving the intended reduction in the population, the financial incentive could encourage participants to keep the target animals alive.The Hanoi episode is significant because it demonstrates a broader problem with bounty schemes. The 2026 review found that financial rewards can change how people interact with the species being targeted. The researchers examined historical and contemporary bounty programmes involving at least 283 species across 449 programmes in 60 countries. Their review covers roughly eight centuries, with examples involving mammals, birds, fish, reptiles, plants, insects and other groups.The authors identify several ways such schemes can go wrong. Participants may submit fraudulent proof, claim rewards for non-target species, harvest animals outside the permitted area or period, or manipulate the programme in ways that allow the target species to survive or even increase. The Hanoi case falls into the last category. The researchers specifically use it as an example of participants claiming a reward while leaving the target animal alive.The review identifies other historical cases in which bounty programmes produced unintended consequences. In Britain, for example, a grey squirrel bounty was discontinued in 1958 after authorities concluded that the scheme had not successfully controlled the squirrel population. A contemporary statement in the UK Parliament said the Forestry Commissioners had decided to end the programme because it had not proved successful in controlling grey squirrels. The review also describes cases in which hunters released adult female wolves from traps so that the animals could continue breeding and future bounty payments could be generated.Another famous example is the so-called "cobra effect", a story about a British colonial bounty for cobras in India. According to the widely repeated account, people began breeding cobras to collect the reward and later released them when the programme was cancelled. However, that story should not be presented as an established historical fact. Recent historical examination has questioned whether the Delhi cobra-breeding episode actually occurred as traditionally described. The 2026 npj Biodiversity review itself presents it as an example of the phenomenon, but the underlying historical evidence is considerably less secure than the documented Hanoi case.The researchers behind the 2026 review do not argue that every bounty programme is ineffective. Their assessment is broader: bounty schemes vary considerably in their design, implementation and outcomes. Some are carefully planned, while others have been introduced with limited investigation of whether they can actually achieve their intended goal. Financial rewards can increase participation, but they can also encourage behaviour that works against the programme's objective.The authors identified bounty programmes motivated by economic concerns, ecological damage and human health, among other reasons. They conclude that such programmes can remain one tool for managing unwanted species, but their design and monitoring matter. That makes the Hanoi episode more than an unusual story about 55 million rat tails. The extraordinary number of tails collected did not automatically translate into an equivalent number of dead rats. Instead, the reward system created a new incentive for participants to exploit the difference between the evidence required for payment and the outcome the government actually wanted. More than a century later, the case remains one of the clearest historical examples of how a policy designed to remove a problem can create incentives that allow the problem to persist.