Goodhart’s Law
Goodhart’s Law is a principle named after economist Charles Goodhart, which states that “when a measure becomes a target, it ceases to be a good measure.” In other words, when people start focusing too much on achieving a particular metric, they may start manipulating or distorting the metric in ways that make it lose its original meaning and usefulness.
When Hanoi, Vietnam was under French rule, the colonial regime created a bounty program that paid a reward for each rat killed. To obtain the bounty, people would provide the severed rat tail. Colonial officials, however, began noticing rats in Hanoi with no tails. The Vietnamese rat catchers would capture rats, lop off their tails, and then release them back into the sewers so that they could procreate and produce more rats, thereby increasing the rat catchers’ revenue.
The government implemented a metric of “# of rat’s tails”. Once this became policy the goal became “create rat tails” not “fix rat overpopulation”.