Who's right, Marshall or Jacobs? The localization versus urbanization debate

A large amount of literature provides empirical evidence in support of Marshall or Jacobs theories regarding the specialization or diversity effects on the economic performance of regions. This paper surveys these scholarly contributions and summarizes their results according to their similarities and differences. The reviewed empirical work presents a diverse picture of possible conditions and circumstances under which each kind of externalities could be at work. The wide breadth of findings is generally not explained by differences in the strength of agglomeration forces across industries, countries or time periods, but by measurement and methodological issues. The levels of industrial and geographical aggregation together with the choice of performance measures, specialization and diversity indicators are the main causes for the lack of resolution in the debate. The 3-digit industrial classification seems to be the level at which MAR and Jacobs effects are undistinguishable from one another, and this is often exacerbated by a high level of geographical aggregation.