Do internal EU borders matter equally? Inequality and spatial configurations in Spanish–Portuguese and Spanish–French corridors

Internal European Union borders are often assumed to have lost economic relevance as market integration, freedom of movement and cross-border cooperation have advanced. This article compares the Spanish-Portuguese and Spanish-French border corridors over 2000–2023. Using NUTS-3 data, the analysis combines a population-weighted decomposition of the Theil index with Global Moran's I. The former measures the share of GDP per capita inequality associated with national divisions, while the latter captures spatial configurations. The results show that internal EU borders still shape territorial inequality, but with markedly different intensity. In the Spanish–Portuguese corridor, the between-country component accounts for 52.9% of total inequality on average. In the Spanish–French corridor, by contrast, the national divide explains only 10.6%, with inequality driven mainly by within-country heterogeneity. Robustness checks confirm that this contrast is not explained by the definition of spatial contiguity, although the Spanish-French results are sensitive to Haute-Garonne and the Toulouse metropolitan area. Spatial autocorrelation differs descriptively between the corridors but is not statistically significant. The findings show that internal EU borders operate as heterogeneous territorial institutions shaped by peripheral, metropolitan and functional contexts.