Taking the early U.S. automobile industry as an example, we evaluate four competing hypotheses on regional industry agglomeration: intra-industry local externalities, inter-industry local externalities, employee spinouts, and location fixed-effects. Our findings suggest that inter-industry spillovers, particularly the development of the carriage and wagon industry, play an important role. Spinouts play a secondary role and only contribute to agglomeration at later stages of industry evolution. The presence of other firms in the same industry has a negligible (or maybe even negative) effect on agglomeration. Finally, location fixed-effects account for some agglomeration, though to a lesser extent than inter-industry spillovers and spinouts.
NBER Working Paper version (subscription required): http://www.nber.org/papers/w18973