Encyclopedic article (Wikipedia, English) on Goodhart's law: stated by British economist Charles Goodhart in 1975 regarding monetary policy — "any observed statistical regularity tends to collapse once pressure is placed upon it for control purposes" — then generalized by anthropologist Marilyn Strathern (1997) into the canonical aphorism "when a measure becomes a target, it ceases to be a good measure." The subject connects economics, incentive theory, public policy evaluation and, by extension, metric optimization in AI systems.
By Wikipedia contributors// Source en.wikipedia.org ↗/Reading 2 min/.md// Auto-verified translation
Goodhart's law is a social-science adage describing how a measure loses its reliability as soon as it is turned into a steering objective. It owes its name to British economist Charles Goodhart, who formulated its core in a 1975 article devoted to monetary policy in the United Kingdom: "any observed statistical regularity tends to collapse once pressure is placed upon it for control purposes". The intuition arose from analysis of British monetary-management difficulties: the stable correlations exploited by central banks as policy levers ceased to hold once instrumentalized.
The most widespread formulation, however, is not Goodhart's but that of anthropologist Marilyn Strathern, who in 1997, in a text on accountability in the university system, proposed the generalized and memorable version: "when a measure becomes a target, it ceases to be a good measure". This reformulation emphasizes the loss of diagnostic value a metric suffers when individuals optimize toward the measure itself rather than toward the underlying objective it is meant to represent.
The idea belongs to a constellation of related principles. Campbell's law (Donald T. Campbell, 1976) addresses the corruption of quantitative social indicators used for decision-making. The Lucas critique (1976) offers its macroeconomic equivalent: the effects of a policy cannot be predicted from historical relationships, because agents adapt to them. To these are added the cobra effect (an incentive that inadvertently rewards counterproductive behavior) and the McNamara fallacy (dismissing the qualitative because it escapes quantification). Several authors have enriched the corpus: Jerome Ravetz (1971), Keith Hoskin (1996), and Jon Danielsson for financial risk modeling.
Illustrations span numerous fields: in healthcare, making length of stay a target causes premature discharges and readmissions; in research, the h-index erodes as a measure of reputation as it becomes an evaluation criterion; in conservation, IUCN extinction classifications have been tightened after being used to lift protections; in education, No Child Left Behind encouraged grade advancement without mastery; during the pandemic, UK COVID testing targets conflated capacity with diagnostic usefulness. The principle ultimately reflects how rational actors optimize within measured systems — a legacy of accountability practices born in the 19th century. Today, it directly illuminates reward hacking and the fragility of optimization metrics in AI systems.
Key takeaways
Original statement (Charles Goodhart, 1975)."Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes" — formulated in an article on monetary management in the United Kingdom. The economic regularity exploited as a policy lever ceases to hold once it is used as such.
Canonical reformulation (Marilyn Strathern, 1997)."When a measure becomes a target, it ceases to be a good measure" — the generalized and popularized version, from a text on audit in the British university system. It is the most frequently cited formula today, often wrongly attributed to Goodhart himself.
Core mechanism. rational actors optimize toward the measure rather than toward the real objective it is supposed to capture → the link between indicator and reality degrades. The metric loses its diagnostic value by becoming an explicit objective.
Campbell's law (Donald T. Campbell, 1976/1979). a variant centered on quantitative social indicators — the more an indicator is used for social decision-making, the more it is subject to corruption and the more it distorts the processes it was meant to monitor.
Lucas critique (Robert Lucas, 1976). a macroeconomic relative — the effects of a policy cannot be predicted from observed historical relationships, because agents adapt their behavior to the policy itself.
Related concepts. the cobra effect (an incentive inadvertently rewards counterproductive behavior), the McNamara fallacy (dismissing what cannot be quantified as irrelevant).
Intellectual contributors cited. Jerome Ravetz (1971, manipulation of systems with complex objectives), Keith Hoskin (1996, linkage to accountability frameworks), Jon Danielsson (application to financial risk modeling).
Sectoral examples. healthcare (reducing length of stay as a target → premature discharges and readmissions); research (the h-index loses value once it becomes a target); conservation (IUCN extinction classifications tightened after being used to lift protections); education (No Child Left Behind → grade advancement without genuine mastery); pandemic (UK COVID testing targets conflating capacity with diagnostic usefulness).
Historical root. a consequence of the modernist accountability practices born in 19th-century Britain.
AI relevance (link to draw). Goodhart's law is the direct conceptual ancestor of reward hacking and of the critique of gameable coverage gates / metrics "at machine speed" — cf. Chris Williams' ADLC series (mutation testing preferred over percentage coverage precisely because it is Goodhart-able).
Attributed claims
toute régularité statistique observée s'effondre dès qu'on exerce une pression sur elle à des fins de contrôle
— Charles Goodhart
quand une mesure devient une cible, elle cesse d'être une bonne mesure
— Marilyn Strathern
a rational actor optimizes toward the measure and degrades the link between indicator and reality
— loi de Goodhart
The knowledge graph extracted from this fiche — 10 entities, 15 relations.
In this graph :Charles Goodhart · Marilyn Strathern · loi de Goodhart · loi de Campbell · critique de Lucas · effet cobra · sophisme de McNamara · h-index · Donald T. Campbell · Robert Lucas