Geographic Concentration as a Dynamic Process
利用美国制造业数据，发现产业地理集中度在二十年间仅小幅下降，但集聚地点变化较大；新企业选址和增长率差异是集中度下降的主因，而企业关闭则强化了集聚；检验马歇尔三理论后发现劳动力混合对工业区位影响最大。
This degree of geographic concentration of individual manufacturing industries in the U.S. has declined only slightly in the last twenty years. At the same time, new plant births, plant expansions, contractions and closures have shifted large quantities of employment across plants, firms and locations. This paper uses data from the Census Bureau's Longitudinal Research Database to examine how relatively stable levels of geographic concentration emerge from this dynamic process. While industries agglomeration levels tend to remain fairly constant we find that there is a greater variation in the locations of these agglomerations. We then decompose aggregate concentration changes into portions attributable to plant births, expansions, contractions, and closures, and find that the location choices of new firms and differences in growth rates have played the most significant role in reducing levels of geographic concentration, while plant closures have tended to reinforce agglomeration. Finally, we look at coagglomeration patterns to test three of Marshall's theories of industry agglomeration: (1) agglomeration saves transport costs by proximity to input suppliers or final consumers, (2) agglomeration allows for labor market pooling, and (3) agglomeration facilitates intellectual spillovers. While there is some truth behind all three theories, we find that industrial location is far more driven by labor mix than by any of the other explanatory variables.