The article by Beaudry and Schiffauerova (2009) reviews the literature on the debate between Marshall and Jacobs theories regarding the impact of specialization and diversity on regional economic performance. The authors survey empirical studies that provide evidence for either theory and summarize their findings, highlighting both similarities and differences. The review reveals a diverse picture of conditions under which each type of externality operates, suggesting that the lack of resolution in the debate is primarily due to measurement and methodological issues rather than differences in the strength of agglomeration forces across industries, countries, or time periods. The levels of industrial and geographical aggregation, as well as the choice of performance measures and indicators, are identified as key factors contributing to the inconclusive results. The three-digit industrial classification is noted as a threshold where Marshall and Jacobs effects become indistinguishable, often exacerbated by high geographical aggregation. The paper also discusses the role of competition externalities, which are associated with both Jacobs and Porter's theories, and suggests that the debate remains unresolved due to methodological issues and the choice of independent variables. Overall, the study aims to provide a census of regression-based studies on the MAR-Jacobs dichotomy and identify the conditions under which each theory becomes dominant.The article by Beaudry and Schiffauerova (2009) reviews the literature on the debate between Marshall and Jacobs theories regarding the impact of specialization and diversity on regional economic performance. The authors survey empirical studies that provide evidence for either theory and summarize their findings, highlighting both similarities and differences. The review reveals a diverse picture of conditions under which each type of externality operates, suggesting that the lack of resolution in the debate is primarily due to measurement and methodological issues rather than differences in the strength of agglomeration forces across industries, countries, or time periods. The levels of industrial and geographical aggregation, as well as the choice of performance measures and indicators, are identified as key factors contributing to the inconclusive results. The three-digit industrial classification is noted as a threshold where Marshall and Jacobs effects become indistinguishable, often exacerbated by high geographical aggregation. The paper also discusses the role of competition externalities, which are associated with both Jacobs and Porter's theories, and suggests that the debate remains unresolved due to methodological issues and the choice of independent variables. Overall, the study aims to provide a census of regression-based studies on the MAR-Jacobs dichotomy and identify the conditions under which each theory becomes dominant.