Behavioural economics has made many gains in recent years, but much uncertainty persists about the effectiveness of different behavioural interventions. This column uses data from a large-scale experiment to find the relative effectiveness of multiple treatments within one setting, and to gauge the accuracy of academic experts’ forecasts of responses. It finds monetary incentives are strong motivators, non-monetary psychological inducements are moderately effective, and results using behavioural factors are generally consistent with models of social and time preferences. Further, the interviewed experts correctly anticipate several results, including the effectiveness of psychological inducements, but fail to predict other important features.