We all know how to be more healthy. And yet we are also remarkably good at NOT doing what we know is good for us. We keep meaning to get fit, but the sofa seems so much more appealing than a run. We know we shouldn’t have another slice of cake, but we do. Behavioural psychologist, Professor Dame Theresa Marteau wants to understand why, despite the best of intentions, so many of us fail to adopt healthier lifestyles. She talks to Jim Al-Khalili about her life and work and why, after studying the evidence she changed her mind about how to change our behaviour. Back in the 90s, it seemed reasonable to assume that telling people they were at high risk of dying would jolt them into eating more healthily and taking more exercise. Now we know better. Thanks in large part to research pioneered by Theresa, we have a much more sophisticated understanding of what drives our behaviour. It turns out that small scale interventions to redesign our environment can exert a big influence on our behaviour by nudging us all into make better decisions, in ways that are beyond our awareness. Spoiler alert - smaller wine glasses really do make you drink less! Responses to Covid-19 show that nations can act rapidly and radically in response to immediate threats to health, even at huge cost. Can we do the same to tackle other threats to global health? Producer: Anna Buckley