World Bank

Economic development is both a cause and a consequence of urbanization. The sectoral effects through which the association arises, well explicated by Kelley and Williamson (1984a,b) among others, mainly concern how exogenously occurring income growth (acting via Engel's Law) and technological change encourage urbanization. But until now the literature has lacked an accessible account of the other direction of causation, by which urbanization propels economic growth. World Development Report 2009 provides that account in an elegant and compelling manner. Its authors believe that the core ideas have not yet been given a hearing, and it is perhaps understandable that in striving to ensure that these ideas are fully appreciated, they occasionally give in to over-statement and allow theory to outrun evidence. This is a volume that is meant to start a conversation, even to provoke, and because its thesis runs counter to the conventional wisdom, it will probably succeed in doing so where a more cautious academic treatment might have failed.