the editors of the article, as well as three anonymous reviewers for their thorough comments and suggestions on earlier versions of the article. Elliot Rasmuson, Keith Garrett, Egle Pugaciauskaite, Le Duy Binh and his team, Hanane Lahnaoui, Ding Xu, and Benjamin Stewart provided invaluable support in assembling the different data sets. Tom Farole and Douglas Zeng gave generous advice throughout the project. We also would like to thank all participants of the World Bank Brown Bag Lunch discussion in Washington, DC-in particular Thomas Farole and Leonardo Iacovone, who acted as peer reviewers-and in seminars and conferences in Stavanger, Tokyo, Vancouver and Vienna. The research was conducted under the auspices of the Competitive Industries and Innovation Program of the World Bank.

The paper investigates (1) the evolution of urban concentration from 1985 to 2010 in 68 countries around the world and (2) the extent to which the degree of urban concentration affects national economic growth. It aims to overcome the limitations of existing empirical literature by building a new urban population dataset that allows the construction of a set of Herfindahl-Hirschman-Indices which capture a country's urban structure in a more nuanced way than the indicators used hitherto. We find that, contrary to the general perception, urban concentration levels have on average decreased or remained stable (depending on indicator). However, these averages camouflage diverging trends across countries. The results of the econometric analysis suggest that there is no uniform relationship between urban concentration and economic growth. Urban concentration is beneficial for economic growth in high-income countries, while this effect does not hold for developing countries. The results differ from previous analyses that generally underscore the benefits of urban concentration at low levels of economic development. The results are robust to accounting for reverse causality through IV analysis, using exogenous geographic factors as instruments.

Policy makers and academics frequently emphasize a positive link between city size and economic growth. The empirical literature on the relationship, however, is scarce and uses rough indicators for the size of a country's cities, while ignoring factors that are increasingly considered to shape this relationship. In this paper, we employ a panel of 113 countries between 1980 and 2010 to explore whether 1) there are certain city sizes that are growth enhancing and 2) how additional factors highlighted in the literature impact the city size/growth relationship. The results suggest a nonlinear relationship which is dependent on the country's size. In contrast to the prevailing view that large cities are growth-inducing, for a majority of countries relatively small cities of up to 3 million inhabitants are more conducive to economic growth. A large share of the urban population in cities of more than 10 million inhabitants is only growth promoting in countries with an urban population of 28.5 million and more. In addition, the relationship is highly context-dependent: a high share of industries that benefit from agglomeration economies, a well-developed urban infrastructure, and an adequate level of governance effectiveness allow countries to take advantage of agglomeration benefits from larger cities.

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This paper examines the link between average city size and aggregate economic growth in a total of 114 countries for the period between 1960 and 2010. The analysis-which includes pooled two-stage least-squares (2SLS), panel data analysis, system generalized method-ofmoments (GMM) estimator, and an instrumental variable (IV) approach-finds that, in contrast to the prevailing view, there is no universal positive relationship between average city size and economic growth and that the results vary between high-income and developing countries. In high-income countries, there is consistent evidence of a positive albeit decreasing link between city size and economic growth. In contrast, the relationship does not hold for developing countries, for which most of the coefficients display insignificant results or point towards a negative connection between both factors.