The maps make it clear that the global economy takes shape around perhaps 20 great Megas--half in the United States and the rest scattered throughout the world. These regions are home to just 10 percent of total world population, 660 million people, but produce half of all economic activity, two thirds of world-class scientific activity and three quarters of global innovations. The great urbanologist Jane Jacobs was the first to describe why megalopolises grow. When people cluster in one place, they all become more productive. And the place itself becomes much more productive, because collective creativity grows exponentially. Ideas flow more freely, are honed more sharply and can be put into practice more quickly.