Past economic data can mislead you about how people will react to a new policy

Governments used to assume relationships seen in historical data — like unemployment falling when inflation rises — would hold after a policy change. Robert Lucas argued this is naive: people adjust their expectations and behaviour once they understand the new policy, breaking the old pattern. It reshaped how central banks and governments model policy decisions.

— Robert Lucas Jr., Econometric Policy Evaluation: A Critique — Carnegie-Rochester Conference Series on Public Policy, 1976