Here’s an interesting discussion about this on the r/askscience subreddit on Reddit.
I personally like the theory supported by John McWorther: as languages are left to their own devises (mainly lack of interaction with speakers of different languages or dialects), they tend to outgrow quite a bit in complexity. That is mainly because children have a seemingly nearly infinitive capacity to learn difficult structures in a language, and this proceeds over generations, with each subsequent generation just picking up all the structures that the previous generation had invented naturally and adding to it – be it excessive grammatical forms, superfluous context-adding phrases and so on.
However, if languages are exposed to second language speakers, those speakers tend to simplify the language so as to facilitate interaction, and native speakers do too in order to be better understood. Thus, in those scenarios, languages drop a lot of their complexity that is not necessary for communication and become much simpler. An extreme example of this is found in pidgins – the first stage of such development, which later develop into creoles when children start speaking them, but the creoles are much simpler than the original languages – compare English to Tok Pisin or Dutch to Afrikaans.
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