WHY BEHAVIOURAL POLICY NEEDS MECHANISTIC EVIDENCE

Proponents of behavioural policies seek to justify them as evidence-based'. Yet they typically fail to show through which mechanisms these policies operate. This paper shows - at the hand of examples from economics and psychology - that without sufficient mechanistic evidence, one often cannot determine whether a given policy in its target environment will be effective, robust, persistent or welfare-improving. Because these properties are important for justification, policies that lack sufficient support from mechanistic evidence should not be called evidence-based'.