Both Marshall and Jacobs were Right!

This article adds to the empirical evidence on the impact of agglomeration externalities on regional growth along three main dimensions. On the basis of data on 259 Europe NUTS2 (Nomenclature of Territorial Units for Statistics) regions and 15 NACE (Nomenclature statistique des Activites economiques dans la Communaute Europeenne) 1.1 2-digit industries for the period 1990-2007, we show that agglomeration externalities are stronger in technology-intensive industries, also after controlling for sorting; that specialization externalities are stronger for low density regions, while diversity matters more for denser urban areas; and, finally, that Jacobs externalities comprise a pure diversification effect (related variety) and a portfolio effect (unrelated variety), although evidence of positive effects on regional growth is only found for the latter. An additional contribution of this article is to extend the analysis on the basis of a full geographical coverage of European NUTS2 regions, with the aim to generalize the empirical identification of the impacts of specialization and diversification externalities with respect to the existing literature. Our results are robust to a rich set of consistency checks, including the use of spatial autoregressive models with autoregressive disturbances, used to assess to what extent the effects of agglomeration externalities are localized.