The manner in which a child acquires language is a matter long debated by linguists and child psychologists alike. The father of most nativist theories of language acquisition is Noam Chomsky, who brought greater attention to the innate capacity of children for learning language, which had widely been considered a purely cultural phenomenon based on imitation. Nativist linguistic theories hold that children learn through their natural ability to organize the laws of language, but cannot fully utilize this talent without the presence of other humans. This does not mean, however, that the child requires formal tutelage of any sort. Chomsky claims that children are born with a hard-wired language acquisition device (LAD) in their brains. They are born with the major principles of language in place, but with many parameters to set (such as whether sentences in the language(s) they are to acquire must have explicit subjects). According to nativist theory, when the young child is exposed to a language, their LAD makes it possible for them to set the parameters and deduce the grammatical principles, because the principles are innate.
This is still a very controversial view, and many linguists and psychologists do not believe language is as innate as Chomsky argues. There are important arguments both for and against Chomsky's view of development. One idea central to the Chomskian view is the idea of Universal Grammar, which posits that all languages have the same basic underlying structure, and that specific languages have rules that transform these underlying structures into the specific patterns found in given languages. Another argument is that without a propensity for language, human infants would be unable to learn such complete speech patterns in a natural human environment where complete sentences are the exception. This is sometimes mis-characterised as the poverty of stimulus argument. Psychologists such as Catherine Snow at Harvard, who study parent-child interaction, point out that children do not have to deduce the principles of language from impoverished and ungrammatical scraps of talk. Many studies of child directed speech or CDS have shown that speech to young children is slow, clear, grammatical, and very repetitious, rather like traditional language lessons. Social interactionists like Snow theorize that adults play an important part in children's language acquisition.
However, some researchers claim that the empirical data on which theories of social interactionism are based have often been over-representative of middle class American and European parent-child interactions. Various anthropological studies of other human cultures, as well as anecdotal evidence from western families, suggests rather that many, if not the majority, of the world's children are not spoken to in a manner akin to traditional language lessons, but nevertheless grow up to be fully fluent language users. Many researchers now take this into account in their analyses. Furthermore, as any parent knows, children often pay scarce attention to what they are told to say, instead sticking to their own ungrammatical preferences.
Nevertheless, Snow's criticisms might be powerful against Chomsky's argument if the argument from the poverty of stimulus were indeed an argument about degenerate stimulus, but it is not. The argument from the poverty of stimulus is that there are principles of grammar that cannot be learned on the basis of positive input alone, however complete and grammatical that evidence is. This argument is not vulnerable to objection based on evidence from interaction studies such as Snow's.
However, a powerful argument against Chomskian views of language acquisition lies in Chomskian theory itself. The theory has several hypothetical constructs, such as movement, empty categories, complex underlying structures, and strict binary branching, that cannot possibly be acquired from any amount of input. Since the theory is, in essence, unlearnably complex, then it must be innate. A different theory of language, however, may yield different conclusions. While all theories of language acquisition posit some degree of innateness, a less convoluted theory might involve less innate structure and more learning. Under such a theory of grammar, the input, combined with both general and language-specific learning capacities, might be sufficient for acquisition.
Linguist Eric Lenneberg states that the crucial period of language acquisition ends around the age of 12 years. He claims that if no language is learned before then, it can never be learned in a normal and fully functional sense. This is known as the "Critical Period Hypothesis".
An interesting example of this is the case of Genie, otherwise known as "The Wild Child". A thirteen-year-old victim of lifelong child abuse, Genie was discovered in her home on November 4th, 1970, strapped to a potty chair and wearing diapers. She appeared to be entirely without language. Her father had judged her retarded at birth and had chosen to isolate her, and so she had remained up until her discovery.
It was an ideal (albeit horrifying) opportunity to test the theory that a nurturing environment could somehow make up for a total lack of language past the age of 12. Unfortunately, she was unable to acquire language completely. Due to this and other complications, she eventually ended up in an adult foster care home.
Detractors of the "Critical Age Hypothesis" point out that in this example and others like it (see Feral children), the child is hardly growing up in a nurturing environment, and that the lack of language acquisition in later life may be due to the results of a generally abusive environment rather than being specifically due to a lack of exposure to language.
However, there exists emerging evidence of both innateness of language and the "Critical Age Hypothesis" from the deaf population of Nicaragua. Until approximately 1986, Nicaragua had neither education nor a formalized sign language for the deaf. As Nicaraguans attempted to rectify the situation, they discovered that children past a certain age had difficulty learning any language. Additionally, the adults observed that the younger children were using gestures unknown to them to communicate with each other. They invited Judy Kegl , an American linguist from MIT, to help unravel this mystery. Kegl discovered that these children had developed their own, distinct, Nicaraguan Sign Language with its own rules of "sign-phonology" and syntax. She also discovered some 300 adults who, despite being raised in otherwise healthy environments, had never acquired language, and turned out to be incapable of learning language in any meaningful sense. While it was possible to teach vocabulary, these individuals seem to be unable to learn syntax.
The developmental period of most efficient language learning coincides with the time of rapid post-natal brain growth and plasticity in both humans and chimps. Prolonged post-natal brain growth in humans allows for an extended period of the type of brain plasticity characteristic of juvenile primates and an extended time window for language learning. The neotenic pattern of human brain development is associated with persistence of considerable language learning capacity into human adulthood.
Derek Bickerton's (1981) landmark work with Hawaiian pidgin speakers studied immigrant populations where first-generation parents spoke highly-ungrammatical "pidgin English". Their children, it was found, grew up speaking a grammatically rich language -- neither English nor the broken pidgin of their parents. Furthermore, the language exhibited many of the underlying grammatical features of many other natural languages. The language became "creolized," and is known as Hawaii Creole English. This was taken as powerful evidence for children's innate grammar module. See Bickerton, D. (1981). Roots of language. Ann Arbor, MI: Karoma. See: Wug Test, Jean Berko Gleason, fis phenomenon, babbling, Steven Pinker.
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