Crossover effects

In linguistics, crossover effects are restrictions on possible binding or coreference found between certain phrases and pronouns.^[1] 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).^[2] The phenomenon occurs in English and related languages, and it may be present in all natural languages that allow fronting.

Examples

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):

a. Who₁ said he₁ was hungry? - Crossover absent, intentional coreferential reading available

b. *Who₁ did he₁ say __₁ was hungry. - Crossover present (strong), intentional coreferential reading unavailable

a. Who told Jill₁ that Fred would call her₁? - Crossover absent, intentional coreferential reading available

b. *Who₁ did Jill₁ say that Fred should call __₁? - Crossover present (strong), intentional coreferential reading unavailable

a. Who₁ will call his₁ mother? - Crossover absent, coreferential reading available

b. ?Who₁ will his₁ mother call __₁? - Crossover present (weak), coreferential reading unlikely

a. Which student₁ called her₁ instructor? - Crossover absent, coreferential reading available

b. ?Which student₁ did her₁ instructor call __₁ - Crossover present (weak), coreferential reading unlikely

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.

Strong and weak crossover

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

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.

a. *Which politician₁ did Susan₁ say that they should vote for __₁? - Strong crossover, intentional coreferential reading impossible

b. *Who₁ do you think he₁ trusts __₁? - Strong crossover, intentional coreferential reading impossible

c. *Which man₁ did he₁ promise __₁ would get a promotion. - Strong crossover, intentional coreferential reading impossible

While these sentences are actually grammatical, the reading indicated by the subscripts is robustly unavailable. For instance, the first sentence (sentence a) cannot be interpreted to mean that Susan told them to vote for herself (i.e. Susan).

Weak crossover

Crossover is "weak" when the coreferential reading is marginal, that is, when the coreferential reading is not clearly unacceptable, but rather just quite unlikely. Typical cases of weak crossover occur when the expression that has been "crossed over" is a possessor inside a noun phrase, e.g.

a. ?Which players₁ does their₁ coach distrust __₁? - Weak crossover, indicated reading possible, but unlikely

b. ?Which beer₁ does its₁ brewer never advertise __₁? - Weak crossover, indicated possible, but unlikely

c. ?Who₁ do her₁ parents worship __₁? - Weak crossover, indicated reading possible, but unlikely

The pronoun that has been crossed over in each of these examples is embedded inside a noun phrase. For some reason that has to do with binding possibilities, such cases of crossover are not impossible, but rather just unlikely. In other words, the coreferential reading can actually occur. Semantics and pragmatics seems to play a major role determining the likelihood of this reading.

Comments

Crossover is a particular manifestation of binding, which is one of the most explored and discussed areas of theoretical syntax. The factors that determine when the coreferential reading is possible have been extensively debated. Simple linear order plays a role, but the other key factor might be c-command as associated (primarily) with Government and Binding, or it might be o-command as associated with Head-Driven Phrase Structure Grammar. Weak crossover is also interesting with respect to how it interacts with parasitic gaps. A parasitic gap can obviate what would otherwise be an occurrence of weak crossover.

Notes

↑ The crossover phenomenon was first explored in detail by Postal (1971).
↑ The distinction between strong and weak crossover is due to Wasow (1972).

References

Postal, P. 1971. Crossover phenomena.
Wasow, T. 1972. Anaphoric relations in English. MIT dissertation.

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