Typological consistency

The theory of typological consistency states that languages can be classified as belonging to a certain type to which they ideally conform in all domains of their morpho-syntax.

Typological consistency does not imply that all languages are consistent in type at all stages in their history. This should be clear from "the fact that few languages are entirely consistent in one type or the other means that these constraints cannot be in any way absolute" (Fox 1995:262). Yet, all languages being inconsistent in type are taken to be in a transition (or drift) from one type to another.

In the standard work on typological consistency two basic types of language are distinguished, those where (direct) objects precede the verb (OV), and those where they follow the verb (VO).

History and proponents of the concept
Greenberg's Universals of word order typology (1963) have revealed a striking positive correlation between the order of head and modifier in the verb phrase (VP) and the ordering of head noun and the modifiers in the noun phrase (NP). Winfred P. Lehmann generalized over Greenberg’s data and came up with a theory that claimed the existence of to basic types of languages depending on whether the verb precedes the (direct) object (VO) or the object precedes the verb (OV).

"In VO languages, nominal modifiers such as relative, adjectival, and genitival expressions follow nouns; in OV languages they precede nouns." (Lehmann 1973:48). Apart from the tendency to conform to one head-type within different parts of syntax, Lehmann (1973) noted a tendency of OV languages to be agglutinating and VO languages to be inflecting, a property he also believed to be related to the type of the language.

Next to Lehmann, Theo Vennemann is one of the main proponents of the theory of typological consistency. His two types of languages are labeled as VX and XV rather than VO/OV, yet those labels are interchangeable. He formulates Lehmann's idea as that of "a mechanical correlation of such kind that when a language changes its basic verb position, the correlated phenomena will also change in time" and criticizes that no further explanations are given on the nature of this correlation or its motivation (Vennemann 1974:345).

Alternate explanations and criticism
As noted before one important factor within the theory of typological consistency is the impact of the ordering of constituents within the VP. In other words, the order within the VP, namely the order of verb (head) and object (modifier), is treated as the core criterion the rest of the grammar will eventually conform to. A change of head-structure in the VP will eventually lead to a change in the noun phrase. Givón (1975:89) notes that this has generally been assumed to be "the work of analogy", yet he proposes a different explanation for the observed patter. He claims that OV nominal compounds (derived from object initial VPs) are analyzed as NN compounds (Genitive initial NPs), and thus he labels the Genitive as "the spearhead of the invasion of OV syntax into the noun phrase." (Givón 1975:90).

Comrie (1981:204ff) criticizes the concept of typological consistency in two general points. First, since the concept of drift is introduced, and most actual languages are indeed believed to be in a transitory state from one type to the other, the theory is not able to make "any predictions about the distribution of language types". And secondly, that it gives no account on "why the inconsistency should have arisen" at all.