Reduced relative clause
A reduced relative clause is a relative clause that is not marked by an overt relative pronoun or complementizer (such as who, which or that). An example is the clause I saw in the English sentence This is the man I saw. (Alternative unreduced forms of this relative clause would be that I saw, who I saw or whom I saw.) Another type is the "reduced object relative passive clause", a type of non-finite clause headed by a past participle, such as found here in the sentence The animals found here can be dangerous.
Finite types 
Relative clauses are a special class of dependent clause (also called "subordinate clause") that usually serve to modify a noun. In English, similarly to most other Indo-European languages, relative clauses are often introduced by one of the relative pronouns who, whom, whose, what, or which, or by the word that, which can be analyzed either as a relative pronoun or as a relativizer (complementizer)—see English relative clauses.
Reduced relative clauses, on the other hand, have no relative pronoun or complementizer introducing them. The example below contrasts an English non-reduced relative clause and reduced relative clause.
|Relative clause:||The man||who/whom/that I saw||was big.|
|Relative clause||Predicate of
|Reduced relative clause:||The man||I saw||was big.|
|Reduced relative clause||Predicate of
Because of the omission of function words, the use of reduced relative clauses, particularly when nested, can give rise to sentences which, while theoretically correct grammatically, are not readily parsed by listeners. A well-known example put forward by linguists is "Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo", which contains the reduced relative clause Buffalo buffalo buffalo (meaning "which buffalo from Buffalo (do) buffalo").
Non-finite types 
In English, the similarity between the active past tense form of verbs (i.e., "John kicked the ball") and the passive past tense (i.e., "the ball was kicked") can give rise to confusion concerning a special form of reduced relative clause, called the reduced object relative passive clause (so called because the noun being modified is the direct object of the relative clause, and the relative clause is in passive voice), the most famous example of which is
In sentences such as this, when the reader or hearer encounters the verb, he or she can interpret it in two different ways: as a main verb, or the first verb of a reduced relative clause. Linguist David W. Carrol gives the example of "the florist sent...", which could either go on to form a sentence such as "the florist sent the flowers to the elderly widow" (in which "sent" is the main verb), or one such as "the florist [who was] sent the flowers was very pleased" (in which "sent" is the beginning of a reduced relative clause). Sentences like this often produce a garden path effect—an effect whereby a reader begins a sentence with one interpretation, and later is forced to backtrack and re-analyze the sentence's structure. The diagram below illustrates the garden path effect in the sentence "the florist sent the flowers was pleased," where (1) represents the initial structure assigned to the sentence, (2) represents the garden path effect elicited when the reader encounters "was" and has nowhere to put it, and (3) represents the re-analysis of the sentence as containing a reduced relative clause.
While reduced relative clauses are not the only structures that create garden path sentences in English (other forms of garden path sentences include those caused by lexical ambiguity, or words that can have more than one meaning), they are the "classic" example of garden path sentences, and have been the subject of the most research.
Use in psycholinguistic research 
Across languages, reduced relative clauses often give rise to temporary ambiguity (garden path effects), since the first word of a reduced clause may initially be interpreted as part of the main clause. Therefore, reduced relative clauses have been the subject of "an enormous number of experiments" in psycholinguistics, especially for investigating whether semantic information or information from the context can affect how a reader or listener initially parses a sentence. For example, one study compared sentences in which the garden path effect was more likely because the reduced relative verb was one that was likely to be used as a main verb for its subject (as in "the defendant examined...[by the lawyer]", where the subject "defendant" is animate and could be the do-er of the action) and sentences in which the garden path effect was less likely (as in "the evidence examined...[by the lawyer]", where the subject "evidence" is not animate and thus could not be doing the examining). Reduced relative clauses have also been used in studies of second-language acquisition, to compare how native speakers handled reduced relatives and how non-native speakers handle them.
In languages with head-final relative clauses, such as Chinese, Japanese, and Turkish, non-reduced relative clauses may also cause temporary ambiguity because the complementizer does not precede the relative clause (and thus a person reading or hearing the relative clause has no "warning" that they are in a relative clause).
- "Subordinate Clauses". The Internet Grammar of English. University College of London. 1998. Retrieved 15 March 2009.
- Li & Thompson 1981:579–580.
- Carrol 2008:294.
- Carrol 2008:136.
- Townsend & Bever 2001:247
- 彭聃龄 (Peng Danling); 刘松林 (Liu Songlin) (1993). "汉语句子理解中语义分析与句法分析的关系 (Syntactic and semantic analysis in Chinese sentence comprehension)". 心理学报 (Acta Psychologica Sinica) 2: 132–139.
- McKoon, Gail; Roger Ratcliff (2003). "Meaning Through Syntax: Language Comprehension and the Reduced Relative Clause Construction". Psychological Review 110 (3): 490–525. doi:10.1037/0033-295X.110.3.490. PMC 1403829. PMID 12885112.
- Carrol 2008:5.
- Townsend & Bever 2001:248.
- Carrol 2008:137.
- Juffs, Alan (1998). "Main Verb Versus Reduced Relative Clause Ambiguity Resolution in L2 Sentence Processing". Language Learning 48 (1).
- See, e.g., Peng (1993), and Hsu, Natalie (2006). Issues in head-final relative clauses in Chinese: Derivation, processing, and acquisition. Ph.D. dissertation, University of Delaware.
- Carrol, David W (2008). Psychology of Language (5 ed.). Belmont: Thomson & Wadsworth.
- Li, Charles N; Sandra A Thompson (1981). Mandarin Chinese: A Functional Reference Grammar. Los Angeles: University of California Press.
- Townsend, David J; Thomas G Bever (2001). Sentence Comprehension: The Integration of Habits and Rules. Cambridge: MIT Press.