Oftentimes many individual acts lead to a significantly (dis-)valuable outcome though the performance of each act makes no valuative difference to that outcome. Such cases give rise to a dilemma. For it seemingly doesn’t matter whether one performs an act (or not) if it doesn’t make a difference. Yet it matters a great deal when many of these acts are performed, provided they bring about a significant outcome. One might think, therefore, that at least some reason favours the performance of (...) such acts. But in the absence of a valuative difference, it is difficult to say what that reason is, exactly. The above dilemma arises in both intrapersonal and interpersonal cases. For instance, in the intrapersonal case, it seemingly makes little difference to my goal of running the marathon whether I skip a single workout. Yet it makes a significant difference to my goal of running the marathon if I skip all of my workouts. And, in the interpersonal case, it seemingly makes no difference to the bad effects of climate change if I choose to be a vegan or not. Yet it makes a significant difference to the bad effects of climate change if we all choose to be vegans. This dilemma has generated a substantial body of literature in both ethics and rational choice. Surprisingly, however, there has been little crossover between the two fields of study. The motivation for the special issue was to offer an avenue to explore new solutions or perspectives on the dilemma through the lens of one, or both of these fields. (shrink)