[Apologies if I'm adding something that's already been brought up -- I'm
trying to catch up on my list-reading quickly]

"Critical Period" effects primarily apply to acquisition of
pronunciation; grammar shows much less of an age-related effect. Adult
second-languge learners show gaps in mastery of "grammar" typically in
areas where one needs a very large amount of exposure to grasp the
nuances (e.g. definite article usage) or a very large amount of exposure
to encounter a particular item enough times in the first place (e.g.
uncommon idioms, colloquialisms). Given enough exposure, adults can
demonstrably pass as native speakers *in writing* (Joseph Conrad's
writing is distinctive -- but not because it's Polish-y). It's the
speech that gives them away. 


Pronunciation is the part of language most closely tied to motor
routines -- "muscle habits" and the like -- and motor routines solidify
fairly early on. Non-linguistic motor routines show this kind of effect
as well; if you want to be a virtuoso pianist, you'd better start young.
And, while there is thus (arguably) a "critical period" for piano
virtuosity, claims for such as a specific genetic endowment would seem a
bit strained. 

Bill Spruiell

Dept. of English
Central Michigan University

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