An area of language acquisition that has attracted considerable scholarly (and lay) interest is the so-called critical period hypothesis. This proposes a critical period in childhood during which people need to acquire a language in order to become fully proficient in it.
Abby Kaplan’s new book Women Talk More than Men: And Other Myths about Language Explained has a helpful chapter on this, investigating whether the ability to acquire a language falls sharply or gradually after a certain age, whether the progressive difficulty in acquiring a second language is universal or admits exceptions, and so on.
In examining whether early childhood exposure to language is vital for its acquisition, Kaplan writes that one source of evidence is ‘the very sad cases of people who weren’t exposed to a language as children, usually due to extreme abuse or neglect’.
A famous example is Genie, who was found in 1970 aged 13 having spent most of her life until then in isolation.
Image of Genie from the Nova documentary ‘Secret of the Wild Child’
Posted by Stan Carey 
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