Syntactic change in diachrony versus contact-induced change: Two sides of the same coin?

Recent studies (e.g. Kupisch, Tanja & Maria Polinsky. 2022. Language history on fast forward: Innovations in heritage languages and diachronic change. Bilingualism: Language and Cognition 25. 1–12) have rekindled an old debate concerning whether language change in contact (CIC) and language change in diachrony (CID) proceed along the same developmental path, or whether they diverge from one another in fundamental and predictable ways. This paper contributes to this ongoing debate; we propose a new heuristic to determine similarities and differences in syntactic change in CIC and CID. We postulate that the primary distinction boils down to the type of features related to the domain of syntax under investigation, i.e., situations involving (formal) ϕ-features lead to similar trajectories of change in both CIC and CID, while those driven by discourse-features show divergence. We test our hypothesis on a host of different empirical data, e.g., indexicals, (subject) clitics, and differential object marking (DOM) as evidence for our claim.