According to the Unaccusative Hypothesis (Perlmutter 1978, Burzio 1986), there are two types of intransitive verbs, unaccusative and unergative, with distinct syntactic properties. The essential insight (variously expressed by different syntactic theories) is that the subject of unaccusative verbs is syntactically comparable to the object of a transitive verb, while the subject of an unergative verb is a true subject. Evidence for the distinction is both syntactic and semantic: for example, in several European languages unaccusative verbs generally select BE as a perfective auxiliary while unergative verbs select HAVE; semantically, the subject of unaccusative verbs tends to be a patient while that of unergative verbs is an agent. However, it has proved difficult to fit many verbs unambiguously into one class or the other. On the one hand, there are verbs that do not satisfy unaccusativity diagnostics in consistent ways, both within and across languages; on the other hand, there are verbs that can display either unaccusative or unergative syntax depending on the characteristics ofthe predicate.
One of the main challenges posed by the Unaccusative Hypothesis is therefore to account for the variable behaviour of verbs. Theoretical linguistic research in the last 10 years - so-called ‘projectionist’ and ‘constructionist’ approaches - has tended to focus on the complex mappings between a lexical-semantic level of representation and the level of syntactic structure. However, all these approaches have limitations and inparticular cannot account for the systematic variation that is attested in many languages. I have proposed (Sorace 2000, 2004) that intransitive verbs are organized in a Split Intransitivity Hierarchy (SIH), defined primarily by aspectual notions (telicity/atelicity), and secondarily by the degree of agentivity of the verb (Figure 1).
| CHANGE OF LOCATION > | categorical unaccusative syntax | |
| CHANGE OF STATE> | 
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| CONTINUATION OF STATE > | ||
| EXISTENCE OF STATE > | ||
| UNCONTROLLED PROCESS > | ||
| MOTIONAL PROCESS > | ||
| NON-MOTIONAL PROCESS | categorical unergative syntax |
The SIH therefore identifies the notion of "telic change" at the core of unaccusativity and that of "atelic non motional activity" at the core of unergativity. The closer to the core a verb is, the more determinate its syntactic status as either unaccusative or unergative. Sensitivity to contextual or compositional factors correlates with the distance of a verb from the core. Verbs that are stative and non-agentive are the most indeterminate. Evidence for the SIH comes from various domains, including language typology, diachronic change, second language acquisition, first language attrition, language processing and language disorders. I will present some of this evidence, including data from native and non-native Japanese, and outline how this generalization might be explained within a formal model of the syntax-lexicon interface.