If you want to save the world, you have only so much money, time and attention. So the question is - what effort should you throw yourself into that provides the biggest bang for the buck? The Copenhagen Consensus is controversial because these thinkers assess, for example, cutting the emission of greenhouse gases to be a high-cost effort with questionable impact. They point instead to relatively low-cost, high-impact plans like educating women and making sure every child on the planet has enough vitamins and minerals. Their latest list of what to do is fascinating not least because so much of it involves bottom-up, innovative group behavior. In a networked age, that probably has greater odds of success than waiting around for some top-down government to act.