During the spring of 1902, Hanoi, Vietnam, was on the brink of its first bubonic plague outbreak. Years earlier, French colonists built nine miles of sewage piping in a bid to modernize the city with basic amenities like running water and, in particular, flushing toilets. [1]
The problem was, while well-built, the sewer’s warm, moist conditions made it the perfect breeding ground for plague-bearing vermin.Before long, sewer-rats began traversing the streets, riding Hanoi with filth the likes of which the city had never seen.
To curb overbreeding and curtail an impending pandemic, French colonists began paying rat-catchers—and eventually, enterprising civilians—a bounty for every rat tail handed in at the municipal officers.
Tails came pouring in, first, by the hundreds; later, by the thousands. At its peak, 20,112 were killed in a single day. The program was a rousing success. The colonists congratulated themselves, having spurred entrepreneurialism in Vietnam. And for a while, all was well.
Until it wasn’t.
After several reported sightings of rats in Hanoi with no tail, the authorities realized that the rat-catchers were capturing rats, cutting off their tails, and then releasing them. Others were smuggling rats into the city and even breeding themselves to collect higher bounties.
The French government abolished the program, and Hanoi’s rat problem became a famous example of Goodhart’s Law. The adage states that after economist Charles Goodhart, when a measure becomes a target, it ceases to be a good measure. [2]
The colonists aren’t the only ones that unwittingly fell for Goodhart’s Law. We’re all guilty of letting a metric drive our behavior, rather than its purpose. No truer is this than with habits we’re trying to build and maintain.
We go through the motions, learning vocabulary for a language we never plan to practice, in a bid to keep an arbitrary streak alive. We attempt a new personal best (PB) in the gym and forgo proper form, thereby potentially hurting ourselves.
Measuring improvement is important. But it isn’t everything. Oftentimes, it’s worth remembering the reason you’re making a change and the difficult-to-quantify benefits that come from having made a commitment in the first place.
Footnotes
[1] Thanks to Richard Shotton for introducing me to The Great Hanoi Rat Massacre of 1902 in his book, The Choice Factory: 25 Behavioral Biases that Influence What We Buy.
[2] The Great Hanoi Rat Massacre is also an example of the cobra effect depending on how it’s used.
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