Conspiring motivations for causative and passive isomorphism:
In many languages a single morphological category exists that expresses both causative and passive functions. Such “causative/passive isomorphism” appears anomalous from the point of view of much work in linguistic typology in that a causative is often considered a “valence increasing” construction, while passive is a “valence decreasing” construction. Nevertheless causative/passive isomorphism is fairly common and can prove stable over multiple generations of language change. In this paper we show how an analytic causative construction can become a morphological permissive causative, and finally take on the functions of canonical passive constructions. This path is motivated by well-documented processes of metaphorical extension, reanalysis and grammaticalization. We illustrate this development with data from Xibe (Sibe), a Tungusic language spoken in Northwestern China.