Mechanistic Evidence and the Challenge of Mechanism Individuation

Randomized controlled trials (RCTs) are widely employed to inform policy decisions in the social and behavioral sciences. Yet both philosophers and scientists have questioned the reliability of effect-size evidence derived from RCTs for policy extrapolation, arguing that RCT findings should be complemented by evidence about the mechanism through which a policy works. We agree that mechanisms are important but argue that existing accounts face the challenge of mechanism individuation . In this article, we explain what the challenge consists in, focusing on common practices of policy scaling-up, transferring policy to new domains, and aggregating policy-relevant evidence.