There is growing interest within public management in using governance tools to influence citizens’ behavior, including changing “choice architecture” by manipulating defaults. This article reports a survey experiment with 4,005 British adults which examined the impact of different defaults on people's propensity to visit, and register on, the organ donor register. There were significant effects of the different defaults on visits to the registration page but not on actual registrations. A default where people were automatically assumed to be donors but could opt out, and a neutral default where people had to answer either “yes” or “no,” both yielded significantly more organ donor register visits than a default where people were not assumed to be donors but could opt in. Attitudinal data collected suggested a preference for a neutral default. The results indicate that changing to a neutral default for organ donation would be socially acceptable and could potentially generate more donors.