“…They may even be counterbalanced by negative externalities of larger cities and core regions (Broersma and van Dijk, 2008; Harris et al., 2011). To understand the variation in regional productivity fully, scholars have underlined the increase in aggregate productivity and income in the presence of higher skill levels, pointing to the importance of human capital (Abel, Dey and Gabe, 2012; Marrocu and Paci, 2012; Melachroinos and Spence, 2014; Rosenthal and Strange, 2008) and the tendency of more skilled workers to live in densely populated areas (Di Giacinto et al., 2014; Glaeser and Resseger, 2010). Indeed, complementary national and firm-level evidence indicates low levels of skills negatively impact productivity and growth (Crafts and O’Mahoney, 2001; Machin, Vignoles and Galindo-Rueda, 2003; Webber, Boddy and Plumridge, 2007; Wixe, 2015), while regional level evidence has also shown that the positive relationship between productivity and the effect of agglomeration externalities is stronger in more skilled areas (Glaeser and Resseger, 2010; Harris and Moffat, 2015).…”