Which are the fundamental causes of the Italian "dualism"? This paper explores the effect of "Social Infrastructure", defined by Hall and Jones (1999) as the set of economic institutions of a Country or Region, which are crucial for understanding the persistent lag of the "Mezzogiorno". We provide an indicator able to measure the quality of this concept and we demonstrate that it accounts for gaps in GDP per capita among Italian Regions better than other factors stressed by the literature, i.e. human capital, physical capital and public capital. Evidences suggest that the "Mezzogiorno" is trapped in a low-quality State: the convergence in Italy's regional GDP per capita will be possible only if the institutional question behind the Italian "dualism" will be tackled.