From Covid to climate change, governments around the world face challenges which demand modifications of human behaviour. When it comes to getting people to do things differently, what works best: the carrot of persuasion, or the stick of coercion? Stephen Sackur speaks to Richard Thaler, the world renowned economist and behavioural scientist who believes a nudge often works better than a shove when change is needed. Does that hold good when the problems we face become urgent and existential?