<p>Cultural values, social norms, and institutional trust shape human decision-making, influencing how individuals respond to behavioral interventions. While nudge experiments have been widely tested in Western societies, their effectiveness often fails to extend beyond these contexts. Replications in non-Western settings yield divergent results, highlighting the cultural contingency of nudges and their limited external validity. This paper draws on 4E cognition and meshed-architecture theory to argue that culture is not a background “moderator” but a constitutive element of the cognitive system that any intervention must traverse. A comparative review of empirical evidence on organ-donation defaults, in-group cooperation, and charitable giving shows that identical choice-architecture tweaks can elicit acquiescence, indifference, or reactance depending on cultural considerations. These findings underscore a design imperative for behavioral public policy: external validity must be treated as a <i>design</i> problem—requiring iterative cultural calibration—rather than as a simple ad hoc experimentation or sampling concern. By reframing culture as a cognitive filter instead of an exogenous variable, the paper provides a theoretical basis and practical guidelines for crafting experiments and behavioral policies that are contextually intelligible and effective.</p>