Behavioural insights for the design of social interventions

Behavioural insights have transformed social policy design, yet their application has too often been reduced to a single instrument: the nudge. This chapter argues that nudges capture only a fraction of what social psychology and behavioural economics reveal about human decision-making, and that durable behaviour change requires understanding why people are not already behaving as an intervention intends. It describes four theoretical frameworks as a diagnostic toolkit for intervention design. The Theory of Planned Behaviour explains how attitudes, social norms, and perceived control combine to form intentions, and why intentions alone do not guarantee action. Social Cognitive Theory examines self-efficacy and its sources, distinguishing the vicarious learning and enactive mastery that build confidence to act. Social Identity Processes show how group membership shapes behaviour through identity rather than incentives. Ecological Models situate behaviour within nested environmental layers, revealing when single-level interventions fall short. Each framework is illustrated with field-experimental evidence — from civic participation in Tunisia, education and early marriage in Nigeria, female genital cutting in Somalia, and extreme poverty across six countries — demonstrating that identifying the binding constraint is the difference between interventions that work and those that miss.