Grammaticalization — the historical process by which lexical items and constructions come to serve grammatical functions, and once grammaticalized continue to develop new grammatical functions — is standardly assumed to be unidirectional, moving from lexical to grammatical and from less to more grammatical. For adverbial connectives the documented cross-linguistic baseline runs from temporal to conditional ('when' > 'if'), and conditionals are generally taken to be the more grammatical category, so development from conditional to temporal should be especially unlikely. Kuo (2026, Folia Linguistica; first published online 2025) assembles diachronic evidence from Chadic languages, Japanese, and Mandarin showing temporal and conditional connectives developing into each other in both directions, and argues that such changes are regular processes mediated by morphosyntactic vagueness — contexts in which the two categories share distributional and functional properties so that their distinction is underdetermined and not at issue. The open problem is whether such vagueness-mediated bidirectional change is a regular mechanism of grammatical change generally, and whether grammaticalization should accordingly be defined in terms of regularity rather than unidirectionality. The literature has not adjudicated this: the unidirectionality debate remains a decades-old standoff, the specific proposal is recent, and its reception so far is confined to the author's own research program and sympathetic adjacent work on cyclic change.

Background

In historical linguistics, grammaticalization is the process whereby lexical items and constructions, in highly constrained pragmatic and morphosyntactic contexts, come to serve grammatical functions — for example, the English verb 'go' developing into the future marker 'be going to' — and, once grammaticalized, continue to develop new grammatical functions. A foundational generalization of the framework is the unidirectionality hypothesis: such changes are claimed to proceed systematically in one direction, from lexical to grammatical and from less to more grammatical. Strong early formulations asserted that the reverse direction is impossible: Bybee & Pagliuca's 1985 chapter 'Cross-linguistic comparison and the development of grammatical meaning' is quoted in later typological work as stating that 'the opposite direction of development is not possible'. The hypothesis has been debated since the 1990s: Haspelmath (1999) argued that irreversibility follows from general diachronic mechanisms and treats attested reversals as marginal; Campbell (2001) used counterexamples to question what the hypothesis actually claims; Norde (2009) devoted a monograph to degrammaticalization — changes from grammatical to less grammatical status — and concluded that it is genuine but rare, which is why most theorists treat unidirectionality as a strong tendency rather than a law. A further concept is needed for the present problem: morphosyntactic vagueness, the state of contexts in which two grammatical categories share distributional and functional properties so that the category membership of an item is underdetermined (Denison 2017); such vague contexts are natural sites of category change because the category distinction is simply not at issue.

Subordinating connectives — the markers that introduce adverbial clauses, such as temporal clauses ('when P') and conditional clauses ('if P') — are a classical testing ground for directionality. The protasis is the if-clause of a conditional; temporal and conditional clauses overlap semantically because both situate one event relative to another, and the standard grammaticalization path runs temporal > conditional: Traugott (1985) showed that conditional markers frequently develop from temporal ones, and Kortmann's (1997) typology of adverbial subordinators across European languages established the tendency for temporal meanings to grammaticalize into causal, conditional, and concessive relations. In hierarchical models of clause structure, conditionals are also taken to be more grammatical than temporals (conditional markers sit at the clause periphery, an area reserved for grammaticalized, scope-taking elements). Under the unidirectionality hypothesis, then, a development from conditional connective to temporal connective — a more grammatical item becoming a less grammatical one — should be doubly unexpected. Yet temporal and conditional connectives are formally intertwined in many languages: in Chadic languages (a branch of the Afroasiatic family spoken around Lake Chad), conditionals are formally indistinguishable from certain temporal constructions (documented descriptively for Makary Kotoko by Allison 2017), and languages such as German use one subordinator, 'wenn', for both 'when' and 'if'. Synchronic overlap of this kind does not by itself settle the direction of the historical changes, which is precisely what makes the temporal–conditional pairing a sharp test case.

The source paper (Kuo 2026, Folia Linguistica 60(1): 531–560; first published online 2025) assembles diachronic evidence from Chadic languages, Japanese, and Mandarin that temporal and conditional connectives developed into each other — including conditionals developing into temporals, the 'wrong' direction — and argues that these are not random counterexamples. On its proposal, the changes originate in contexts where formal and functional equivalence held between temporals and conditionals and their morphosyntactic distinction was vague, so that the changes are regular processes mediated by grammatical equivalence; grammatical change may thus be regularly bidirectional, and grammaticalization, if defined in terms of regularity rather than unidirectionality, may be bidirectional too. This extends the same author's documented bidirectionality between modal and conditional constructions in Mandarin (Kuo 2022, Diachronica; Kuo 2024, Journal of Linguistics), where the definitional argument is stated in full: grammaticalization should be defined as the development of grammatical categories, not in terms of 'non-equivalence' such as unidirectionality or increasing grammaticality. The empirical stakes of the temporal–conditional extension are higher, because conditionals are widely agreed to be more grammatical than temporals, so a conditional > temporal development is a reversal under anyone's grammaticality metric.

Problem Statement

Determine whether bidirectional change between temporal and conditional connectives — and between grammatical categories more generally — is a regular mechanism of grammatical change, and correspondingly whether grammaticalization should be defined in terms of regularity rather than unidirectionality. The problem has three linked questions. (1) The documentary question: whether dated attestations from languages and families beyond the Chadic, Japanese, and Mandarin cases show that connectives developed temporal > conditional and conditional > temporal as genuine grammaticalization paths — descent through contexts where formal and functional equivalence held and the morphosyntactic distinction was vague — rather than as chance formal mergers, homophony without change, or borrowing. (2) The regularity question: determine whether such bidirectional changes recur systematically wherever the proposed enabling conditions hold (distributional and functional similarity between the categories, i.e. morphosyntactic vagueness), so that they are regular, constrained, principled processes of grammatical change rather than sporadic counterexamples to unidirectionality; this includes assessing the frequency asymmetry between the two directions on a balanced cross-linguistic sample. (3) The definitional question: adjudicate whether a concept of grammaticalization defined as the development of grammatical categories — without built-in unidirectionality or increasing grammaticality — is superior to the unidirectionality-based definition, taking an explicit stance on how the documented reversals are then to be classified (e.g. against alternatives such as degrammaticalization, insubordination — the historical recruitment of subordinate-clause structures as independent clauses — or merger). A negative resolution equally counts: demonstrating that the documented conditional > temporal cases reduce to chance mergers, polysemy without diachronic change, or contact effects, or that bidirectional cases are too sporadic to constitute a regular mechanism, would settle the problem in favor of the unidirectionality-based view.

References

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