The Necessity of Construct and External Validity for Generalized Causal Claims
The credibility revolution has facilitated tremendous progress in the social sciences by advancing design-based strategies that rely on internal validity to deductively identify causal effects. We demonstrate that prioritizing internal validity while neglecting construct and external validity undermines causal generalization and misleadingly converts a deductive claim of causality into a claim based on speculation and exploration – undermining the very goals of the credibility revolution. We develop a formal framework of causal specification to demonstrate that internal, external and construct validity are jointly necessary for generalized claims regarding a causal effect. If one lacks construct validity, one cannot assign meaningful labels to the cause or to the outcome. If one lacks external validity, one cannot make statements about the conditions required for the cause to occur. Re-balancing considerations of internal, construct and external validity via causal specification preserves and advances the intent of the credibility revolution to understand causal effects.