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):
- 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 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).
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.
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.
- The crossover phenomenon was first explored in detail by Postal (1971).
- The distinction between strong and weak crossover is due to Wasow (1972).
- Postal, P. 1971. Crossover phenomena.
- Wasow, T. 1972. Anaphoric relations in English. MIT dissertation.