Testing causal associations in language change: The replacement of subordinating then with when in Middle English.

Middle English changes the realization of temporal subordinators from a th-form (then) to a wh-form (when). The innovation is quantified with data from four syntactically parsed corpora. The change may have had an antecedent cause in the loss of subject-verb inversion after clause-initial adverb then. This view is supported by the time course of the two developments, the loss of subject-verb inversion slightly preceding the rise of wh-based subordination, as well as by the fact that the presence of alternative subordinating strategies inhibit the presence of wh-subordinators. The paper thus provides quantitative, empirical evidence for language-internally motivated change.