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

Saskia Sassen, co-chair on the Committee on Global Thought at Columbia University and a leading expert on smart cities, draws parallels with the office buildings of the sixties, which she describes as "low-ceiling places now standing sad and empty as advanced technologies render them useless".

BBC: How will our future cities look?