MP David Willetts conceded that it is impossible to make policy solely on the basis of scientific evidence. Yet despite this, politicians today rarely make policy statements without citing ‘the evidence’. Whether it’s behavioural economics and social psychology, or neuroscience guiding government intervention, politicians seem keener than ever to cite experts’ findings. Is democracy well-served by an ‘evidence-based’ approach to decision making? Frequently ‘facts’ and ‘evidence’ are deployed to trump ethics or indeed politics. Might this reflect a lack of political conviction or moral authority? Can science and politics collaborate without damaging both? Experts hammer out the issues in this debate filmed at the Battle of Ideas.