The reversal of opinion about fast-growing cities, previously considered bad news, began with The Challenge of Slums, a 2003 UN-Habitat report. The book’s optimism derived from its groundbreaking fieldwork: 37 case studies in slums worldwide. Instead of just compiling numbers and filtering them through theory, researchers hung out in the slums and talked to people. They came back with an unexpected observation: “Cities are so much more successful in promoting new forms of income generation, and it is so much cheaper to provide services in urban areas, that some experts have actually suggested that the only realistic poverty reduction strategy is to get as many people as possible to move to the city.

While slums clearly are places where living standards often are way below what we think is adequate for humans, and there are huge challenges to informal urban living to be faced, they might be a positive phenomenon.