Crossover effects

In linguistics, crossover effects are restrictions on possible binding or coreference found between certain phrases and pronouns. Coreference (or coindexation) that is normal and natural when a pronoun follows its antecedent becomes impossible, or at best just marginally possible, when "crossover" is deemed to have occurred, e.g. ?Who₁ do his₁ friends admire __₁? The term itself refers to the traditional transformational analysis of sentences containing leftward movement (e.g. wh-movement, topicalization), whereby it appears as though the fronted constituent crosses over the expression (usually a pronoun) with which it is coindexed on its way to the front of the clause. Crossover effects are divided into strong crossover (SCO) and weak crossover (WCO). The phenomenon occurs in English and related languages, and it may be present in all natural languages that allow fronting.

The following sentences illustrate crossover phenomena. The a-sentences are questions in which crossover has not occurred and are given here for the sake of comparison, and the b-sentences illustrate crossover. The subscripts mark coindexation (≈coreference); they indicate that the words bearing the subscripts are supposed to refer to the same one person. The gaps in the b-sentences mark the canonical position of the wh-expression (before movement):

The acceptability contrast here is curious upon first analysis. In both the a- and b-sentences, the order of the wh-expression and the coindexed pronoun is the same, the wh-expression preceding the pronoun. But only the reading indicated with the a-sentences is (fully) possible. Note that the first two b-sentences actually allow an accidental coreferential reading. This accidental reading is, however, unlike the intentional coreferential reading that the a-sentences allow. The relevant difference is that in the b-sentences, the wh-expression appears to have been moved across the pronoun on its way to the front of the sentence, whereas there is no such crossover in the a-sentences.

As just indicated by the examples, there are two types of crossover, strong and weak. The following two sections consider each of these types.

Strong crossover occurs when the pronoun is in an argument position, i.e. it is an argument of the relevant verb. This means that it is not contained inside a noun phrase. Instances of strong crossover are clearly impossible, that is, the coreferential reading is strongly unavailable, e.g.

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