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. Later, Nobel Prize-winning economist Robert Lucas dubbed these forces "human capital externalities," and explained why they seem to override "the usual economic forces," like prices: "What can people be paying Manhattan or downtown Chicago rents for, if not to be around other people?"
There is, however, a tipping point. The forces of price and congestion begin pushing people away from the center. But make no mistake, this has nothing to do with the "decentralization of work," as many have argued. The huge economic advantages of clustering still guide the process, which is why second cities emerge near big cities or in the corridors between them, not in the middle of nowhere.
The biggest Mega in economic terms is the original, the Boston-to-Washington corridor. In 1961 it was home to about 32 million people; today its population has risen to 55 million, more than 17 percent of all Americans. The region generates $2.5 trillion in economic activity, making it the world's fourth largest economy, bigger than France or the United Kingdom. Next in line is Chi-Pitts, the great Midwestern Mega running from Chicago to Detroit, Cleveland and Pittsburgh, with $2.3 trillion in economic activity. Three of the power centers of the U.S. economy even stretch beyond American borders: So-Cal runs from Los Angeles to San Diego across the Mexican border to Tijuana; Tor-Buff-Chester sprawls from Toronto to Rochester, and Cascadia from Portland, Oregon, to Vancouver.