This is the first collective study of the thinking behind the effective altruism movement. This movement comprises a growing global community of people who organise significant parts of their lives around the two key concepts represented in its name. Altruism is the idea that if we use a significant portion of the resources in our possession-whether money, time, or talents-with a view to helping others then we can improve the world considerably. When we do put such resources to altruistic use, it is crucial to focus on how much good this or that intervention is reasonably expected to do per...