now on bbc news, it's time for hardtalk. welcome to hardtalk. i'm stephen sackur. 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? well, my guest today is richard thaler the world—renowned economist and 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?