Head-directionality parameter


In linguistics, head directionality is a proposed parameter that classifies languages according to whether they are head-initial or head-final. The head is the element that determines the category of a phrase: for example, in a verb phrase, the head is a verb.
Some languages are consistently head-initial or head-final at all phrasal levels. English is considered to be strongly head-initial, while Japanese is an example of a language that is consistently head-final. In certain other languages, such as German and Gbe, examples of both types of head direction occur. Various theories have been proposed to explain such variation.
Head directionality is connected with the type of branching that predominates in a language: head-initial structures are right-branching, while head-final structures are left-branching.

Types of phrase

There are various types of phrase in which the ordering of head and complement may be considered when attempting to determine the head directionality of a language, including:
  1. Verb phrase. Here the head is a verb, and the complement are most commonly objects of various types. The ordering here is related to one of the chief questions in the word order typology of languages, namely the normal order of subject, verb and object within a clause.
  2. Noun phrase. Here the head is a noun; various kinds of complementizer phrase and adpositional phrase may be considered to be complements.
  3. Adjective phrase. This contains an adjective as the head, and can take as a complement, for example, an adverbial phrase or adpositional phrase.
  4. Adpositional phrase. Such phrases are called prepositional phrases if they are head-initial, or postpositional phrases if they are head-final. For more on these, see Preposition and postposition. The complement is a determiner phrase.
  5. Determiner phrase. This has a determiner as the head of the phrase. DPs were proposed under generative syntax; not all theories of syntax agree that they exist.
  6. Complementizer phrase. This contains a complementizer, like that in English, as the head. In some cases the head is covert. The complement can be considered to be a tense phrase.
  7. Tense phrase and aspect phrase. These are phrases in which the head is an abstract category representing tense or aspect; the complement is a verb phrase. In more traditional analysis the entire phrase is considered to be simply a verb phrase.
In some cases, particularly with noun and adjective phrases, it is not always clear which dependents are to be classed as complements, and which as adjuncts. Although in principle the head-directionality parameter concerns the order of heads and complements only, considerations of head-initiality and head-finality sometimes take account of the position of the head in the phrase as a whole, including adjuncts. The structure of the various types of phrase is analyzed below in relation to specific languages, with a focus on the ordering of head and complement. In some cases this ordering is found to be the same in practically all types of phrase, whereas in others the pattern is less consistent. Different theoretical explanations of these inconsistencies are discussed later in the article.

Particular languages

English

is a strongly head-initial language. In a typical verb phrase, for example, the verb precedes its complements, as in the following example:
The head of the phrase precedes its complement. Switching the order to " V eat" would be ungrammatical.
Nouns also tend to precede any complements, as in the following example, where the [relative clause that follows the noun may be considered to be a complement:
Nouns do not necessarily begin their phrase; they may be preceded by attributive adjectives, but these are regarded as adjuncts rather than complements. Adjectives themselves may be preceded by adjuncts, namely adverbs, as in extremely happy. However, when an adjective phrase contains a true complement, such as a prepositional phrase, the head adjective precedes it:
English adpositional phrases are also head-initial; that is, English has prepositions rather than postpositions:
On the determiner phrase view, where a determiner is taken to be the head of its phrase, English can be seen to be head-initial in this type of phrase too. In the following example the head is taken to be the determiner any, and the complement is the noun book:
English also has head-initial complementizer phrases, as in this example where the complementizer that precedes its complement, the tense phrase Mary did not swim:
Grammatical words marking tense and aspect generally precede the semantic verb. This indicates that, if finite verb phrases are analyzed as tense phrases or aspect phrases, these are again head-initial in English. In the example above, did is considered a tense marker, and precedes its complement, the verb phrase not swim. In the following, has is a aspect marker; again it appears before the verb which is its complement.
The following example shows a sequence of nested phrases in which each head precedes its complement. In the complementizer phrase in, the complementizer precedes its tense phrase complement. In the tense phrase in, the tense-marking element precedes its verb phrase complement. In the verb phrase in, the verb precedes its two complements, namely the determiner phrase the book and the prepositional phrase on the table. In, where a picture is analyzed as a determiner phrase, the determiner a precedes its noun phrase complement, while in, the preposition on precedes its DP complement your desk.

German

, while being predominantly head-initial, is less conclusively so than in the case of English. German also features certain head-final structures. For example, in a nonfinite verb phrase the verb is final. In a finite verb phrase the verb is initial, although it may move to final position in a subordinate clause. In the following example, the non-finite verb phrase es finden is head-final, whereas in the tensed main clause ich werde es finden, the finite auxiliary precedes its complement.
Noun phrases containing complements are head-initial; in this example the complement, the CP der den Befehl überbrachte, follows the head noun Boten.
Adjective phrases may be head-final or head-initial. In the next example the adjective follows its complement.
However, when essentially the same adjective phrase is used predicatively rather than attributively, it can also be head-initial:
Most adpositional phrases are head-initial, as in the following example, where auf comes before its complement den Tisch:
German also has some postpositions, however, and so adpositional phrases can also sometimes be head-final. Another example is provided by the analysis of the following sentence:
Like in English, determiner phrases and complementizer phrases in German are head-initial. The next example is of a determiner phrase, headed by the article der:
In the following example, the complementizer dass precedes the tense phrase which serves as its complement:

Japanese

Japanese is an example of a strongly head-final language. This can be seen in verb phrases and tense phrases: the verb comes after its complement, while the tense marker comes after the whole verb phrase which is its complement.
Nouns also typically come after any complements, as in the following example where the PP New York-de-no may be regarded as a complement:
Adjectives also follow any complements they may have. In this example the complement of quantity, ni-juu-meetoru, precedes the head adjective takai :
Japanese uses postpositions rather than prepositions, so its adpositional phrases are again head-final:
Determiner phrases are head-final as well:
A complementizer comes after its complement, thus Japanese complementizer phrases are head-final:

Chinese

features a mixture of head-final and head-initial structures. Noun phrases are head-final. Modifiers virtually always precede the noun they modify. For examples of this involving relative clauses, see.
In the case of strict head/complement ordering, however, Chinese appears to be head-initial. Verbs normally precede their objects. Both prepositions and postpositions are reported, but the postpositions can be analyzed as a type of noun. For more details and examples of the relevant structures, see Chinese grammar. For a head-direction analysis of Chinese aspect phrases, see the theoretical section below.

Gbe

In Gbe, a mixture of head-initial and head-final structures is found. For example, a verb may appear after or before its complement, which means that both head-initial and head-final verb phrases occur. In the first example the verb for "use" appears after its complement:
In the second example the verb precedes the complement:
It has been debated whether the first example is due to object movement to the left side of the verb or whether the lexical entry of the verb simply allows head-initial and head-final structures.
Tense phrases and aspect phrases are head-initial since aspect markers and tense markers come before the verb phrase.
Gbe noun phrases are typically head-final, as in this example:
In the following example of an adjective phrase, Gbe follows a head-initial pattern, as the head yù precedes the intensifier tàùú.
Gbe adpositional phrases are head-initial, with prepositions preceding their complement:
Determiner phrases, however, are head-final:
Complementizer phrases are head-initial:

Theoretical views

The idea that syntactic structures reduce to binary relations was introduced by Lucien Tesnière within the framework of dependency theory, which was developed during the 1960s. Tesnière distinguished two structures that differ in the placement of the structurally governing element : centripetal structures, in which heads precede their dependents, and centrifugal structures, in which heads follow their dependents. Dependents here may include complements, adjuncts, and specifiers.
Joseph Greenberg, who worked in the field of language typology, put forward an implicational theory of word order, whereby:
The first set of properties make heads come at the start of their phrases, while the second set make heads come at the end. However, it has been claimed that many languages do not fulfill the above conditions, and that Greenberg's theory fails to predict the exceptions.
Noam Chomsky's Principles and Parameters theory in the 1980s introduced the idea that a small number of innate principles are common to every human language, and that these general principles are subject to parametric variation. In this theory, the dependency relation between heads, complements, specifiers, and adjuncts is regulated by X-bar theory, proposed by Jackendoff in the 1970s. The complement is sister to the head, and they can be ordered in one of two ways. A head-complement order is called a head-initial structure, while a complement-head order is called a head-final structure. These are special cases of Tesnière's centripetal and centrifugal structures, since here only complements are considered, whereas Tesnière considered all types of dependents.
In the principles and parameters theory, a head-directionality parameter is proposed as a way of classifying languages. A language which has head-initial structures is considered to be a head-initial language, and one which has head-final structures is considered to be a head-final language. It is found, however, that very few, if any, languages are entirely one direction or the other. Linguists have come up with a number of theories to explain the inconsistencies, sometimes positing a more consistent underlying order, with the phenomenon of phrasal movement being used to explain the surface deviations.
According to the Antisymmetry theory proposed by Richard Kayne, there is no head-directionality parameter as such: it is claimed that at an underlying level, all languages are head-initial. In fact, it is argued that all languages have the underlying order Specifier-Head-Complement. Deviations from this order are accounted for by different syntactic movements applied by languages. Kayne argues that a theory that allows both directionalities would imply an absence of asymmetries between languages, whereas in fact languages fail to be symmetrical in many respects. Kayne argues using the concept of a probe-goal search, whereby a head acts as a probe and looks for a goal, namely its complement. Kayne proposes that the direction of the probe-goal search must share the direction of language parsing and production. Parsing and production proceed in a left-to-right direction: the beginning of sentence is heard or spoken first, and the end of the sentence is heard or spoken last. This implies an ordering whereby probe comes before goal, i.e. head precedes complement.
Some linguists have rejected the conclusions of the Antisymmetry approach. Some have pointed out that in predominantly head-final languages such as Japanese and Basque, the change from an underlying head-initial form to a largely head-final surface form would involve complex and massive leftward movement, which is not in accordance with the ideal of grammatical simplicity. Some take a "surface true" viewpoint: that analysis of head direction must take place at the level of surface derivations, or even the Phonetic Form, i.e. the order in which sentences are pronounced in natural speech. This rejects the idea of an underlying ordering which is then subject to movement, as posited in Antisymmetry and in certain other approaches. It has been argued that a head parameter must only reside at PF, as it is unmaintainable in its original form as a structural parameter.
Some linguists have provided evidence which may be taken to support Kayne's scheme, such as Lin, who considered Standard Chinese sentences with the sentence-final particle le. Certain restrictions on movement from within verb phrases preceding such a particle are found to be consistent with the idea that the verb phrase has moved from its underlying position after its head. However, Takita observes that similar restrictions do not apply in Japanese, in spite of its surface head-final character, concluding that if Lin's assumptions are correct, then Japanese must be considered to be a true head-final language, contrary to the main tenet of Antisymmetry. More details about these arguments can be found in the Antisymmetry article.

Statistical classifications

Some scholars, such as Tesnière, argue that there are no absolute head-initial or head-final languages. According to this approach, it is true that some languages have more head-initial or head-final elements than other languages do, but almost any language contains both head-initial and head-final elements. Therefore, rather than being classifiable into fixed categories, languages can be arranged on a continuum with head-initial and head-final as the extremes, based on the frequency distribution of their dependency directions. This view was supported in a study by Haitao Liu, who investigated 20 languages using a dependency treebank-based method. For instance, Japanese is close to the head-final end of the continuum, while English and German, which have mixed head-initial and head-final dependencies, are plotted in relatively intermediate positions on the continuum.
Polinsky identified the following five head-directionality sub-types:
She identified a strong correlation between the head-directionality type of a language and the ratio of verbs to nouns in the lexical inventory. Languages with a scarcity of simple verbs tend to be rigidly head-final, as in the case of Japanese, whereas verb-rich languages tend to be head-initial languages.