Phenomena involving the displacement of syntactic units are widespread in human languages. The term displacement refers here to a dependency relation whereby a given syntactic constituent is interpreted simultaneously in two different positions. Only one position is pronounced, in general the hierarchically higher one in the syntactic structure. Consider a wh-question like (1) in English:
(1) Whom did you give the book to <whom>
The phrase containing the interrogative wh-word is located at the beginning of the clause, and this guarantees that the clause is interpreted as a question about this phrase; at the same time, whom is interpreted as part of the argument structure of the verb give (the copy, in <> brackets). In current terms, inspired by minimalist developments in generative syntax, the phrase whom is first merged as (one of) the complement(s) of give (External Merge) and then re-merged (Internal Merge, i.e., movement) in the appropriate position in the left periphery of the clause. This peripheral area of the clause hosts operator-type constituents, among which interrogative ones (yielding the relevant interpretation: for which x, you gave a book to x, for sentence 1). Scope-discourse phenomena—such as, e.g., the raising of a question as in (1), the focalization of one constituent as in TO JOHN I gave the book (not to Mary)—have the effect that an argument of the verb is fronted in the left periphery of the clause rather than filling its clause internal complement position, whence the term displacement. Displacement can be to a position relatively close to the one of first merge (the copy), or else it can be to a position farther away. In the latter case, the relevant dependency becomes more long-distance than in (1), as in (2)a and even more so (2)b:
(2)
a Whom did Mary expect [that you would give the book to<whom >]
b Whom do you think [that Mary expected [that you would give the book to <whom >]]
50 years or so of investigation on locality in formal generative syntax have shown that, despite its potentially very distant realization, syntactic displacement is in fact a local process. The audible position in which a moved constituent is pronounced and the position of its copy inside the clause can be far from each other. However, the long-distance dependency is split into steps through iterated applications of short movements, so that any dependency holding between two occurrences of the same constituent is in fact very local. Furthermore, there are syntactic domains that resist movement out of them, traditionally referred to as islands. Locality is a core concept of syntactic computations. Syntactic locality requires that syntactic computations apply within small domains (cyclic
domains), possibly in the mentioned iterated way (successive cyclicity), currently rethought of in terms of Phase theory. Furthermore, in the Relativized Minimality tradition, syntactic locality requires that, given X . . . Z . . . Y, the dependency between the relevant constituent in its target position X and its first merge position Y should not be interrupted by any constituent Z which is similar to X in relevant formal features and thus intervenes, blocking the relation between X and Y. Intervention locality has also been shown to allow for an explicit characterization of aspects of children’s linguistic development in their capacity to compute complex object dependencies (also relevant in different impaired populations).
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