The unidirectionality hypothesis is the idea that grammaticalization, the development of lexical elements into grammatical ones, or less grammatical into more grammatical, is the preferred direction of linguistic change, that a grammatical item is much less likely to move backwards rather than forwards on Hopper & Traugott's cline of grammaticalization.
In the words of Bernd Heine, "grammaticalization is a unidirectional process, that is, it leads from less grammatical to more grammatical forms and constructions". This is one of the strongest claims about grammaticalization, and is often cited as one of its basic principles. In addition, unidirectionality refers to a general developmental orientation which all (or the large majority) of the cases of grammaticalization have in common, and which can be paraphrased in abstract, general terms, independent of any specific case.
The idea of unidirectionality is an important one when trying to predict language change through grammaticalization (and for making the claim that grammaticalization can be predicted). Lessau notes that "unidirectionality in itself is a predictive assertion in that it selects the general type of possible development (it predicts the direction of any given incipient case)," and unidirectionality also rules out an entire range of development types that do not follow this principle, hereby limiting the amount of possible paths of development.
Read more about this topic: Grammaticalization
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... Although unidirectionality is a key element of grammaticalization, it is not absolute ... coupled with their rarity, is given as evidence for the general operating principle of unidirectionality ...
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