Citizens will need to change their behaviour in pro-social ways if governments are to address current challenges in public policy. But how best to promote behaviour change? The argument in this article is that a decentralised rather than a top-down approach is particularly suitable, which can encourage innovation in the public sector and involve citizens in the implementation of policies. As a method of evaluating interventions, randomised controlled trials have a unique role to play in improving the design of policies, particularly if carried out responsively and adaptively. A culture of experimentation would value curiosity, feedback and the continual testing of interventions. The article includes three examples of experimentation: young people's political attitudes, voter turnout and donations for charity. As the popularity of ‘nudge’ interventions grows—shown by the success of the UK government's Behavioural Insights Team—the practice of experimentation in the public sector could become the norm.