Ed Glaeser asks: "If the world is so flat, then why are cities growing so quickly, especially in the third world?" He explains:

In the developing world, urbanization has often taken the form of exploding populations in megacities. Mumbai's population increased to 19 million in 2007 from 10.8 million in 1985. Bangalore, the urban symbol of the flat world, has had its population double over two decades, to 6.8 million today from 3.4 million in 1985.

The growth of these cities and the continuing strength of older urban areas -- like New York, London and Paris -- is no accident. Globalization and new technologies attract people to big cities, by increasing the returns to urban proximity ...

Globalization and technological change have increased the returns to being smart; human beings are a social species that get smart by hanging around smart people.

This powerful clustering force - identified by Jane Jacobs and Robert Lucas, among others - is making the world more geographically concentrated everyday.

Figuring out ways to adjust to it - especially how to address the huge costs being borne by people and places being left behind - remains one of the most pressing domestic and international public policy questions of our time.