Trying to fix problems that affect vast numbers of people has an intuitive appeal that politicians and policymakers find irresistible, but several warehouses of research studies show that intuition is often a poor guide to fixing systemic problems. While it seems like common sense to pump money into an economy that is pulling the bedcovers over its head, the problem with most social interventions is that they target not robots and machines but human beings -- who regularly respond to interventions in contrarian, paradoxical and unpredictable ways.