Tracing the roots of syntax with Bayesian phylogenetics

The ordering of subject, verb, and object is one of the fundamental components of the syntax of natural languages. The distribution of basic word orders across the world's languages is highly nonuniform, with the majority of languages being either subject-object-verb (SOV) or subject-verb-object (SVO). Explaining this fact using psychological accounts of language acquisition or processing requires understanding how the present distribution has resulted from ancestral distributions and the rates of change between orders. We show that Bayesian phylogenetics can provide quantitative answers to three important questions: how word orders are likely to change over time, which word orders were dominant historically, and whether strong inferences about the origins of syntax can be drawn from modern languages. We find that SOV to SVO change is more common than the reverse and VSO to SVO change is more common than VSO to SOV, and that if the seven language families we consider share a common ancestor then that common ancestor likely had SOV word order, but also that there are limits on how confidently we can make inferences about ancestral word order based on modern-day observations. These results shed new light on old questions from historical linguistics and provide clear targets for psychological explanations of word-order distributions.