"Heavy NP shift" is a particular manifestation of shifting where a "heavy" noun phrase (NP) appears in a position to the right of its canonical position under certain circumstances. Although the term is coined according to the tradition of transformational grammar, which describes the process to be done by movement of the NP, the term is also used by linguists who do not believe in transformational grammar. Heaviness is determined by the grammatical complexity, such as a noun modified by a relative clause, but length is also correlated to complexity.
The following pair shows (a) a sentence without heavy NP shift and (b) the same sentence after the operation has applied:
The operation cannot usually apply to smaller NPs, as shown by the unacceptability of (2b):
In the preceding examples, heavy NP shift has inverted the order of the direct object and indirect object. The operation can also invert the order of the direct/indirect object and an adjunct:
John R. Ross observed that heavy NP shift obeys a "right roof constraint"; that is, an NP cannot be shifted outside of its containing clause, as shown by the unacceptability of (4b):[1]
There have been a number of different analyses of heavy NP shift within generative syntax. The traditional assumption was that there is a rightward movement operation (i.e. a transformation) which optionally moves any heavy NP to the right edge of the Verb phrase in which it occurs. In an alternative formulation, heavy NP shift may apply freely to any NP regardless of weight, but light NPs which undergo shifting are filtered out in the phonological component. More recently, Richard Larson has suggested that sentences such as (1b) are derived as follows using only leftward movement: [2]
This analysis has the benefit of explaining Ross's right roof constraint without stipulation. It has become especially popular following the ban on rightward movement imposed by Richard Kayne's theory of Antisymmetry.
Larson's analysis of heavy NP shift has been criticized by Ray Jackendoff, amongst others [3][4] (see also Simpler Syntax). Jackendoff argues that heavy NP shift does not result from movement, but rather from a degree of optionality in the ordering of a verb's complements. The preferred order in English is for the indirect object to follow the direct object, and for adjuncts to follow objects of all kinds, but if the direct object is "heavy", the opposite order may be preferred (since this leads to a more right-branching tree structure which is easier to process).
A mysterious property of heavy NP shift is that, in the case of ditransitive verbs, a shifted direct object prevents extraction of the indirect object via wh-movement:
This would arguably be unexpected if heavy NP shift merely resulted from optionality in complement ordering, or from a rightward movement operation. Larson's analysis may be able to explain this restriction.