Specializations > Historical Linguistics
There is no doubt that Sprachbunds exist. How though do they come about when the members are not closely related? What makes the speakers of one language adopt a feature of another? It is difficult to imagine people thinking: "Postpositive articles! We'll have some of that!"
Close contact. Sprachbunds aren't a sharply defined phenomenon, but just the extreme end of prolonged close contact between languages. Bilingualism results in convergence, along with borrowing and other factors.
--- Quote ---It is difficult to imagine people thinking: "Postpositive articles! We'll have some of that!"
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But that's not the case for internal change either. Language change is just the result of language use. When a language is used a certain way, that shapes its future. When languages are used together, that shapes their futures.
Languages meet and mix in all sorts of ways for various reasons. I can see that when people need to communicate they will make concessions, but the grammar of one language "interfering" with another seems extreme. Adoption by a group of languages of grammatical features does not in fact increase mutual intelligibility between them, so it is puzzling why it happens.
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