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In the early 19th century, a period that saw a renewed interest in the diachronic development of languages, both as individual entities and in relation to each other, von Humboldt ^[1] (p. 57) recognized that ‘Language itself is not a product (ergon) but an activity (energeia)’. This acknowledgment of the dynamic nature of language, which had its earliest echoes in antiquity in the works of Plato, Aristotle and Varro, was complemented by the identification of correspondences between ancestor languages and their offsprings by several of von Humboldt’s contemporaries, who were focusing on sound change. Jacob Grimm and August Schleicher were among the first to formulate the idea that sound change follows regular patterns, hence explicitly rejecting randomness. Their observations were later reformulated in stronger terms by the so-called Neogrammarians, a group of German scholars who focused on sound change: Leskien ^[2] was the first to claim that sound laws have no exceptions, a statement later mirrored in what has become known as the Neogrammarian’s manifesto, ‘Every sound change, insofar as it occurs mechanically, takes place according to laws that admit no exception’ (^[3] (p. xv)). The scene was set for the recognition of the general systematic nature of language change.

By the 20th century, the focus of investigation for language change had shifted to syntactic structures: identifying recurring patterns in the diachronic change of lexical items, Meillet ^[4] recognized the existence of a process by which an autonomous word evolves into a grammatical element. Implicit in this is the generalization that language change is directional, one of the core tenets of grammaticalization theory, which took form in the second half of the 20th century. The predominantly observed path sees a lexical item (a free word) gradually weakening on the phonetic (erosion), semantic (bleaching) and syntactic (loss of independence) levels, and becoming an inflectional affix (a morpheme). This generalization is captured by Givón’s ^[5] (p. 413) famous observation that ‘today’s morphology is yesterday’s syntax’.

Much work in grammaticalization theory in the second half of the 20th century focused on identifying recurring directional paths of change, so-called ‘grammaticalization clines’, sequences of sequential and ordered steps that captured the gradual nature of change and were found to hold across languages. Underpinning the validity of such clines was the recognition that ‘forms tend to undergo the same kinds of changes’ ^[6] (p. 6).

The reasons adduced for such systematic and predictable changes vary depending on the theoretical perspective. Two of the main approaches to language change are the usage-based, or cognitive, and the formal mentalist, or generative. Their diverging views stem from different understandings of what language is, where change occurs and what drives it. In the rest of this article, we explore these two main positions and, focusing on the usage-based approach, we explore four main motivations for language change (and grammaticalization): pragmatic inference, frequency, (inter)subjectification and discourse management in the form of turn-taking.