The Great Rat Tail Trick
This is the cleanest real example of a perverse incentive: a reward that makes people do the opposite of what it was designed for. Once kids see it, they spot it everywhere, from star charts to school points.
- • Historian Michael Vann found the rat-catchers' daily kill lists in the French national archives; that paper trail is why he says the Hanoi rats can be proven while the famous Delhi cobra story cannot.
- • The 'cobra effect' was named by German economist Horst Siebert in 2001, but a 2025 investigation by the Friends of Snakes Society found no records from the time and traced the tale to an 1873 newspaper article that only said 'it was alleged'.
- • It still happens: in 2007-2008 the US Army base Fort Benning paid hunters $40 per wild pig tail, and the wild pig population in the area went up anyway.
Would you rather be paid for catching rats, or for growing them?
Hanoi, 1902. The French ruled the city, in today's Vietnam, and had built shiny new sewers. Rats loved them. They popped up through flush toilets into fancy villas, and rat fleas could spread plague.
First, the French hired rat-catchers to climb into the sewage. On one day in June they killed about 20,000 rats. No dent. A single pair of rats can have 35 to 70 babies a year.
Plan B: pay anyone 1 cent per rat. Nobody wanted piles of bodies, so you just handed in the tail.
Now think like a rat-catcher. Step one: a dead rat pays 1 cent, once. Step two: a live rat keeps breeding, and every baby comes with a brand-new tail. Step three: so you snip off the tail and let the rat go.
Soon tail-less rats were hobbling around. A health official even found a rat farm just outside town. The French scrapped the bounty. It had made the rat problem worse.
Economists call this a perverse incentive: reward the wrong thing and people game it. There is a famous version about cobras in Delhi, but it may be a legend. The Hanoi rats are in the archives.
Where in your life are you paid for tails, not rats?