By Ed Crooks
Goodhart’s Law holds that “any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes”. Originally coined by the economist Charles Goodhart as a critique of the use of money supply measures to guide monetary policy, it has been adopted as a useful concept in many other fields. The general principle is that when any measure is used as a target for policy, it becomes unreliable. It is an observable phenomenon in healthcare, in financial regulation and, it seems, in energy efficiency standards.
When governments set efficiency regulations such as the US Corporate Average Fuel Economy standards for vehicles, they are often what is called “attribute-based”, meaning that the rules take other characteristics into consideration when determining compliance. The Cafe standards, for example, vary according to the “footprint” of the vehicle: the area enclosed by its wheels. In Japan, fuel economy standards are weight-based. Like all regulations, fuel economy standards create incentives to game the system, and where attributes are important, that can mean finding ways to exploit the variations in requirements. There have long been suspicions that the footprint-based Cafe standards would encourage manufacturers to make larger cars for the US market, but a paper this week from Koichiro Ito of the University of Chicago and James Sallee of the University of California Berkeley provided the strongest evidence yet that those fears are likely to be justified.