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Abstract
Economic adjustment to trade and policy shocks is hampered by the fact that some sectors tend to cluster, so are hard to initiate in new places. This can give rise to persistent spatial disparities between cities within a country. The paper sets out a two-sector model in which cities divide into those producing tradable goods or services subject to agglomeration economies, and those only producing non-tradables for the national market. If import competition destroys some established tradable sectors, then affected cities fail to attract new tradable activities and switch to just produce non-tradables. Full employment is maintained (we assume perfect markets and price flexibility) but disparities between the two types of cities are increased. All non-tradable cities experience real income loss, while remaining tradable cities boom. The main beneficiaries are land-owners in remaining tradable cities, but there may be aggregate loss as the country ends up with too many cities producing non-tradables, and too few with internationally competitive activities. Fiscal policy has opposite effects in the two types of cities, with fiscal contraction causing decline in cities producing non-tradables, increasing activity in cities producing tradable goods, widening spatial disparities, and in the process increasing the share of rent in the economy.

This paper reviews recent work on the economics of fast growing developing country cities, with a focus on Africa. It sets out some of the broad facts about African urbanisation and summarises two recent pieces of research work. The first argues that coordination failure can create multiple equilibria and divergent paths of development, some in which cities are internationally competitive and able to create jobs, others in which cities are stuck in a 'nono-tradables trap'. The second is a dynamic model of city growth, calibrated to changing patterns of land-use in Nairobi; the calibration suggests a very high cost of inefficient land use in the context of urban slums.

This paper argues that cumulative causation processes are fundamental to understanding growth and development. Such processes derive from spatially concentrated increasing returns to scale including thick market effects, knowledge spillovers, sectoral and urban clustering, and self-reinforcing improvements in physical and social infrastructure. These sources of agglomeration have been extensively analyzed in the economic geography literature. They imply that spatial unevenness in economic activity and incomes is an equilibrium outcome. Growth tends to be 'lumpy,' with some sectors in some countries growing fast while other countries lag. The policy challenge is to lift potential new centers of economic activity to the point where they can reap the productivity and investment climate advantages of increasing returns and cumulative causation.