Source: The Study of Language, Fourth Edition, George Yule, Cambridge Univeristy Press, 2010
The brain doesn't totally split in two (you still have that "corpus callossum" between the two) and c'mon, it's common brain anatomy knowledge. Babies are born with their two brain hemisphere compact together, and the phenomenon appears at the age of 7-8 years, which the scientists also have found that you have to acquire a grammar (that of any language) before
that phenomenon or you'll never be able to acquire one at all in your life. Go read about the case of "Genie", a girl in 1970 that was found impotent at speech and tied to a chair in a small room until the age of 13.
I wasn't speaking of the age of acquaintance of a language, but the critical moment when a child must have learned a language.
What are your
sources, guys? -_^
More sources: http://www.ag.ndsu.edu/pubs/yf/famsci/fs609w.htm
(it even goes as far as saying 10 years old here)
Alrighty then. *cracks knuckles*
The Critical Period Hypothesis is not at all resolved. There are some deep problems with it such that it "cannot plausibly be regarded as a scientiﬁc hypothesis either in the strict Popperian sense of something which can be falsiﬁed or indeed in the rather looser logical positivist sense of something that can be clearly conﬁrmed or supported" (Singleton, "The Critical Period Hypothesis: A coat of many colours"
Even if we admit it as a tenable hypothesis, "the evidence from second-language acquisition research has not provided unequivocal evidence for the critical period hypothesis. The best we can say is that young children generally learn L2 better than older children and adults, at least in the long run. Moreover, the advantage that younger learners display in some studies may be due to biological changes (as assumed in the critical period hypothesis), environmental factors, cognitive changes, or some combination of factors" (Carroll, Psychology of Language
, p 331).
Genie is certainly not a useful case that you can draw reliable conclusions from. Funding for research on Genie was revoked once it occurred to people that the study was inherently anecdotal—there was no possibility for any kind of control, and so there was no way to scientifically draw any strong conclusions. Specifically, we have no way of knowing whether in infancy Genie was healthy/comparable to typical babies, or if she was handicapped from birth; or whether her difficulty with language acquisition resulted merely from a lack of exposure to language, or from trauma arising from other aspects of her abuse. (As a side note, that whole situation raised a bucketful of troubling ethical issues—see Advanced subsidiary psychology: approaches and methods
, p. 74-75.)
In a study of "naturalistic acquisition" of Dutch by families of English speakers who moved to Holland, adolescent and adult learners far outpaced prepubescent learners over the first year (Snow and Hoefnagel-Hohle, "The critical period for language acquisition"
). The advantage of young learners is more of a tortoise-and-hare effect that does not become apparent until much later (Carroll, 331).
If there is a significant biological event that results in decreased ability to acquire language, then we would expect there to be a sharp cut-off in acquisition ability. Johnson and Newport
(1989, PDF) found, in a study of Chinese and Korean immigrants to the US, that such a cutoff did occur at puberty, but subsequent analyses of their data found otherwise. Bialystok & Hakuta (1994)
found that the cutoff was an artifact of the way Newport and Johnson had grouped their data, while Elman et al. (Rethinking innateness: a connectionist perspective on development
, p 187) found that a single curvilinear function could be drawn over the entire dataset. Other studies (such as Hakuta, Bialystok, & Wiley, "Critical Evidence: A Test of the Critical-Period Hypothesis for Second-Language Acquisition"
, 2003) have also found a gradual decline without any sharp cutoffs.
But once again, all of this operates under the assumption that the hypothesis can be usefully confirmed or falsified in the first place. The hypothesis can be reframed and twisted around the data in so many ways as to be scarcely useful at all.
For a synthesis of important issues and research concerning the critical period hypothesis, see Bialystok, Bilingualism in Development
(2001, PDF), particularly pages 71-88. She specifically discusses work by Penfield and Roberts (which concluded that there are two types of language acquisition, "direct" and "indirect," where direct acquisition is highly preferable but only possible before puberty—note the resemblance to your claim about unconscious acquisition vs. willing study) and why it was basically wrong and entirely unfounded.
So for any evidence you find in support of a critical period, there is plenty of evidence against it as well. Feral-child cases like Genie are hardly admissible since they cannot be controlled for the myriad other factors that could affect acquisition capacity.