In the diachronic development of the modern Romance languages, reflexes of the Latin verb habere have replaced reflexes of esse, to a greater or lesser extent in particular languages. In this article it will be argued that the distribution of esse-reflexes is determined by a hierarchy of unaccusativity based on the semantic distinctions concreteness/abstracteness and movement/staticity, and that habere-reflexes have been spreading systematically from the periphery of this hierarchy towards the core. This process has affected Italian and French to different degrees: Italian has largely retained a syntactically and semantically consistent auxiliary system, whereas the French system, at a more advanced stage of the evolution, shows greater variation and inconsistency.
Evidence is presented from the linguistic intuitions of very advanced Italian non-native speakers of French and French non-native speakers of Italian about equivalent unaccusative verbs, and it is argued that the unaccusative hierarchy conditions both the degree and the directionality of difficulty in second language acquisition. The findings reveal an asymmetric pattern: it is easier for the French learners to fully acquire the facts of essere-selection than for the Italian learners to internalize the facts of être-selection, and the degree of difficulty experienced by the Italian learners with individual unaccusative verbs is correlated to their position along the hierarchy.