Behavioural research suggests that the intensity with which policy instruments indicate a direction of desired behavioural change affects how target populations respond to them. However, comparative research on policy instruments focuses on their calibration, restrictiveness, density and formal intensity, but does not account for the degree to which they specify the particular policy goal. Moving beyond nudging and command and control approaches, this paper adds the dimension of explicitness to existing taxonomies of policy instruments. The explicitness of an instrument results from two questions: first, does the instrument specify a direction of behavioural change? Second, does the instrument attach valence to this behaviour? The paper proposes a stepwise measurement procedure and links explicitness with policy outcomes. A comparative case study of organ donor policy in Switzerland and Spain illustrates how accounting for the explicitness dimension can improve our understanding of policy instruments and their effectiveness.