to reach broad generalizations across languages. Syntacticians have attempted to explain the causes of word-order variation within individual languages and cross-linguistically. Much of such work has been done within the framework of generative grammar, which holds that syntax depends on a genetic endowment common to the human species. In that framework and in others, linguistic typology and universals have been primary explicanda. Alternative explanations, such as those by functional linguists, have been sought in language processing. It is suggested that the brain finds it easier to parse syntactic patterns that are either right- or left-branching but not mixed. The most-widely held approach is the performance–grammar correspondence hypothesis by John A. Hawkins, who suggests that language is a non-innate adaptation to innate cognitive mechanisms. Cross-linguistic tendencies are considered as being based on language users' preference for grammars that are organized efficiently and on their avoidance of word orderings that cause processing difficulty. Some languages, however, exhibit regular inefficient patterning such as the VO languages Chinese, with the adpositional phrase before the verb, and Finnish, which has postpositions, but there are few other profoundly exceptional languages. More recently, it is suggested that the left- versus right-branching patterns are cross-linguistically related only to the place of role-marking connectives (adpositions and subordinators), which links the phenomena with the semantic mapping of sentences. Theoretical syntactic models Dependency grammar Dependency grammar is an approach to sentence structure in which syntactic units are arranged according to the dependency relation, as opposed to the constituency relation of phrase structure grammars. Dependencies are directed links between words. The (finite) verb is seen as the root of all clause structure and all the other words in the clause are either directly or indirectly dependent on the root. Some prominent dependency-based theories of syntax are the following: Recursive categorical syntax, or algebraic syntax Functional generative description Meaning–text theory Operator grammar Word grammar Lucien Tesnière (1893–1954) is widely seen as the father of modern dependency-based theories of syntax and grammar. He argued vehemently against the binary division of the clause into subject and predicate that is associated with the grammars of his day (S → NP VP) and remains at the core of most phrase structure grammars. In the place of that division, he positioned the verb as the root of all clause structure. Categorial grammar Categorial grammar is an approach in which constituents combine as function and argument, according to combinatory