Causal clauses as source of sentential complementation: cross-linguistic evidence and methodological issues

In many languages, causal clause markers can also function as – or are formally identical to – complement markers (e.g., Bulgarian če , Twi se , or Latin quod ). This isomorphism is often explained as the result of independent developments from a common source (interrogatives, relativizers, etc.). By contrast, it is also frequently accepted that in some cases the aforementioned identity originates in a type of structural change by which causal clauses are eventually reanalysed as factive complements. However, the nature of the proposed CAUSE > COMPLEMENT development, involving both semantic adjustments and syntactic integration, is not yet fully understood, and it remains unclear how recurrent this phenomenon might be from a cross-linguistic perspective. This article aims to shed light on these questions by analysing diachronic and cross-linguistic data and assessing some of the evidence given for the development of complementation from causal adverbial clauses. The observations reveal significant documentation gaps for postulated models of the emergence of complement clauses and highlight methodological issues that qualify previous explanations about the diachronic relationship between causal and complement clauses.