Goodhart's Law
When a measure becomes a target, it ceases to be a good measure.
Origin & History
Goodhart's Law was formulated by British economist Charles Goodhart in a 1975 paper on monetary policy. He observed that once the Bank of England began targeting specific monetary aggregates as policy instruments, the statistical relationship between those aggregates and economic outcomes broke down — because economic actors began optimizing for the measure rather than the underlying economic reality it had previously tracked. Anthropologist Marilyn Strathern generalized the principle to social science in 1997.
Real-World Examples
A hospital measured doctor quality by discharge speed. Discharge rates improved. So did readmission rates within 30 days — because speed became the goal, and patient recovery was subordinated to it.
When teacher evaluations were tied to standardized test scores, scores improved. Students learned less — because teachers optimized for the test rather than for the underlying skill the test was meant to measure.
Platforms optimized for engagement metrics (clicks, reactions, time on page) discovered that outrage and anxiety are highly engaging — producing a platform that maximized engagement by degrading user wellbeing.
Why It Matters
Goodhart's Law is inevitable in any system where measurement and incentives are coupled. The solution is not to stop measuring — it is to use multiple, varied metrics that are harder to simultaneously game; to rotate metrics regularly so they don't become permanent targets; and to build qualitative checks alongside quantitative ones. The fundamental question: 'If people optimize this metric perfectly and ignore everything else, do we get what we actually want?' If not, the metric is vulnerable.
Related Laws
Can You Spot Goodhart's Law in the Wild?
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When a measure becomes a target, it ceases to be a good measure — because people optimize for the metric rather than the underlying reality it was meant to represent.
Charles Goodhart, a British economist, who observed the phenomenon in monetary policy in 1975.
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