Population agglomeration of urban agglomerations, spatial externalities, and industry-university-research collaborative innovation

Agglomeration externalities are a core theoretical explanation for why spatial concesntration enhances innovation. However, existing research has largely emphasised population density within individual cities and has paid less attention to whether cross-city population agglomeration within urban agglomerations shapes industry-university-research collaborative innovation through spatial externalities. Drawing on patent data from A-share listed firms on the Shanghai and Shenzhen stock exchanges between 2004 and 2022, we test this relationship using a two-way fixed effects model. The results show that a doubling of the agglomeration scale generated by other cities within the same urban agglomeration is associated with a 24.1% increase in local industry-university-research collaborative innovation. This relationship is mediated by market integration and knowledge spillovers and is more pronounced in cities with stronger intellectual property protection, smaller cultural differences, and inter-provincial urban agglomeration structures. In addition, Spatial Durbin Model estimates indicate that population agglomeration of urban agglomeration produces positive spillover effects on neighbouring cities. Overall, this study advances research on agglomeration externalities by showing that innovation benefits arise not only from within-city density but also from cross-city population concentration at the urban agglomeration level, thereby enriching our understanding of the spatial foundations of collaborative innovation.