Saskia Sassen is the Robert S. Lynd Professor of Sociology and co-chair of the Committee on Global Thought, Columbia University ( www.saskiasassen.com).

FORBES: The Global City And The Global Slum

Prof Sassen gives three main reasons.

BBC: Megacities face delicate balancing act

The ones that have done best are those that have plugged into global industries and been able to capture the headquarters or lesser corporate centres of globalised companies, especially banks and other financial firms, argues Saskia Sassen, of the University of Chicago.

ECONOMIST: The reinvention test