Language acquisition
Language acquisition is the process by which the language capability develops in a human.  First language acquisition concerns the development of language in children, while second language acquisition focuses on language development in adults as well. Historically, theorists are often divided between emphasising either nature or nurture (see Nature versus nurture) as the most important explanatory factor for acquisition.
One hotly debated issue is whether the biological contribution includes capacities specific to language acquisition, often referred to as universal grammar. For fifty years, linguists Noam Chomsky and the late Eric Lenneberg argued for the hypothesis that children have innate, language-specific abilities that facilitate and constrain language learning.
Other researchers, including Elizabeth Bates, Catherine Snow, and Michael Tomasello, have hypothesized that language learning results only from general cognitive abilities and the interaction between learners and their surrounding communities. Recent work by William O'Grady proposes that complex syntactic phenomena result from an efficiency-driven, linear computational system. O'Grady describes his work as "nativism without Universal Grammar." One of the most important advances in the study of language acquisition was the creation of the CHILDES database by Brian MacWhinney and Catherine Snow.

# Nativist theories
Nativist theories hold that children are born with an innate propensity for language acquisition, and that this ability makes the task of learning a first language easier than it would otherwise be. These "hidden assumptions"  allow children to quickly figure out what is and isn't possible in the grammar of their native language, and allow them to master that grammar by the age of three.  Nativists view language as a fundamental part of the human genome, as the trait that makes humans human, and its acquisition as a natural part of maturation, no different from dolphins learning to swim or songbirds learning to sing.
Chomsky originally theorized that children were born with a hard-wired language acquisition device (LAD) in their brains . He later expanded this idea into that of Universal Grammar, a set of innate principles and adjustable parameters that are common to all human languages. According to Chomsky, the presence of Universal Grammar in the brains of children allow them to deduce the structure of their native languages from "mere exposure".
Much of the nativist position is based on the early age at which children show competency in their native grammars, as well as the ways in which they do (and do not) make errors. Infants are born able to distinguish between phonemes in minimal pairs, distinguishing between bah and pah, for example. Young children (under the age of three) do not speak in fully formed sentences, instead saying things like 'want cookie' or 'my coat.' They do not, however, say things like 'want my' or 'I cookie,' statements that would break the syntactic structure of the Phrase, a component of universal grammar. Children also seem remarkably immune from error correction by adults, which Nativists say would not be the case if children were learning from their parents.
The possible existence of a Critical Period for language acquisition is another Nativist argument. Critical periods are time frames during which environmental exposure is needed to stimulate an innate trait. Young chaffinches, for example, must hear the song of an adult chaffinch before reaching maturity, or else would never be able to sing. Nativists argue that if a Critical Period for language acquisition exists (see below), then language acquisition must be spurred on by the unfolding of the genome during maturation.
Linguist Eric Lenneberg stated in 1964 that the crucial period of language acquisition ends around the age of 12 years. He claimed that if no language is learned before then, it could never be learned in a normal and fully functional sense. This was called the "Critical period hypothesis."
Detractors Template:Who of the "Critical Period Hypothesis" say 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. The "Critical Period" theory of brain plasticity and learning capacity has been called into question. Other factors may account for differences in adult and child language learning. Childrens apparently effortless and rapid language acquisition may be explained by the fact that the environment is set up to engage them in frequent and optimal learning opportunities. By contrast, adults seem to have an initial advantage in their learning of vocabulary and syntax, but may never achieve native-like pronunciation.
A more up-to-date view of the Critical Period Hypothesis is represented by the University of Maryland, College Park instructor Robert DeKeyser. DeKeyser argues that although it is true that there is a critical period, this does not mean that adults cannot learn a second language perfectly, at least on the syntactic level. DeKeyser talks about the role of language aptitude as opposed to the critical period.
More support for the innateness of language comes 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 were unable to learn syntax.
Derek Bickerton's (1981) landmark work with Hawaiian pidgin speakers studied immigrant populations where first-generation parents spoke highly-ungrammatical "pidgin English". Their children, Bickerton found, grew up speaking a grammatically rich language -- neither English nor the  syntax-less 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.
Debate within the nativist position now revolves around how language evolved. Derek Bickerton suggests a single mutation, a "big bang", linked together previously evolved traits into full language. Others like Steven Pinker argue for a slower evolution over longer periods of time.

# Criticism and Non-Nativist Theories
Non-nativist theories include Relational Frame Theory, the competition model, functionalist linguistics, usage-based language acquisition,  social interactionism and others. 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.
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, but it is vulnerable to the clear evidence of the availability of negative input given by Conversational analysis. In addition, meta analysis has shown that there is a large amount of corrections made . Meork (1994) conducted a meta-analysis of 40 studies and found substantial evidence that corrections do indeed play a role. From this work, corrections are not only abundant but contingent on the mistakes of the child. (see behavior analysis of child development ).
Many criticisms of the basic assumptions of generative theory have been put forth, with little response from its champions. The concept of LAD is unsupported by evolutionary anthropology which shows a gradual adaptation of the human body to the use of language, rather than a sudden appearance of a complete set of binary parameters (which are common to digital computers but not to neurological systems such as a human brain) delineating the whole spectrum of possible grammars ever to have existed and ever to exist.
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. Examples of alternative theories that do not utilize movement and empty categories are Head-driven phrase structure grammar, Lexical functional grammar, and several varieties of Construction Grammar. 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. Relational Frame Theory (Hayes, Barnes-Holmes, Roche, 2001), provides a wholly selectionist/learning account of the origin and development of language competence and complexity.  Bolstered by a significant and growing base of experimental and applied research, RFT posits a "functional contextualist" approach to the understanding, prediction and control of language phenomenon.