The existence of spatial strategic interaction between neighbouring local governments is often referred in literature and identified in empirical applications for several countries. This interaction can occur either in expenditures or revenues of local governments. The spatial interaction in local government expenditure finds support on three theoretical explanations: spillover effects, Tiebout competition or mimicking behaviour. Identify the adequate explanation for the local government interaction is not an easy task since the reduced form of the estimated model can generate indistinguishable pattern in spatial interactions. This paper seeks to identify between those theoretical explanations what is the underlying reason for spatial interaction in public expenditures among local governments for the case of a particular sub-area of Portugal. Using differentiated model configurations and the local government expenditures of the municipalities composing the Northern Portuguese mainland, between 1998 and 2008, the paper identifies the structural model that generates the observed spatial auto-correlation in local public expenditures. Among the various theoretical reasons, only the existence of spillovers effects finds support
