Our environmental and social problems are pressing and, in many countries, money is short and politics deadlocked. Wouldn’t it be wonderful if there turned out to be a new way of making real progress cheaply and without getting enmeshed in party politics? About two decades ago, along with many of our colleagues in the behavioural sciences, we thought there might be. The idea was elegant: social problems often arise from people making the “wrong” choices, either for themselves (eating unhealthily, smoking, gambling – the list goes on!) or for others (by, for example, damaging the environment by dropping litter). The old-fashioned approach to bad choices is to tax them or ban them. But the new strategy aimed for a gentler, more psychologically subtle approach: to redesign the way options are presented so the “right” choice becomes easy, natural and appealing. The bad choices are still available, but the clever policy ensures they are picked less often. Such “nudges” seemed to offer the hope of addressing big social problems through small changes to reshape individual behaviour. …