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Language acquisition is the process by which humans acquire the capacity to perceive, produce, and use words to understand and communicate. This capacity involves the picking up of diverse capacities including syntax and phonetics.  Helen Goodluck sets out language definition in the opening paragraph of her book: "Linguistics is concerned with discovering and defining the form and structure of human languages . . . The study of language acquisition is the study of how and when children get a command of the things linguistics sets out to define."  There have been many theories as regards the acquisition of language by children. Behaviourism, cognitive development, interactionalism and nativism have all formed the basis of theories explaining language acquisition in children. Over the course of this essay I will look at the different schools of thought with regards to language acquisition and discuss their validity.
The behaviourist school of thought maintains that behaviours can be described scientifically without recourse either to internal physiological events or to hypothetical constructs such as the mind  . The behaviourist theory regarding language acquisition was put forward by B.F. Skinner in his book, Verbal Behavior  . Skinner's theory is that children primarily learn language behaviour through imitation, reinforcement and shaping from their environment  i.e. a child imitates the language of it's parents or carers and this is reinforced when the child is praised by the adult or when the child receives what it was asking for - a positive reinforcement of a successful attempt  . Blacker advances several problems with the behaviourist theory  such as:
Language is based on sets of structures and rules which couldn't be worked out by mere imitation of single utterances.
The vast majority of children go through similar if not the same stages of language acquisition (developmental milestones). These stages do not differ based on the different environments that children are brought up in or the attention or treatment that the children receive, except in the most extreme circumstances.
Children are often unable to repeat what an adult says.
The evidence of a critical period for language acquisition. Here Blacker cites the case of Genie. Genie was discovered in 1970 when she was 13. She had been severely neglected, brought up in isolation and deprived of normal human contact. She was disturbed and underdeveloped in many ways. As part of her rehabilitation her carers tried to teach her to speak. Despite some success, mainly in learning vocabulary, she never became a fluent speaker, failing to acquire the grammatical competence of the average five-year-old.
Another issue with the idea of imitation is the development of languages themselves, as Pinker put it "People do not reproduce their parents' language exactly. If they did, we would all still be speaking like Chaucer"  . There are too many issues with Skinner's theory for it to be a viable one, however recently there have been experiments with what is called Relational Frame Theory (RFT). This theory is founded on the expansion of Skinner's behaviourist theory. Essentially RFT says that we learn to derive relationships by viewing lots of examples and then overtime we apply the principle ourselves  . This theory is still being tested.
The cognitive view on language acquisition is that there is relation between the development of the child's cognitive functions and the development of language within the child  and that the acquisition of a language is just one element of a child's intellectual development  . Jean Piaget believed in the cognitive theory. Piaget felt that the child had to be capable of understanding a concept before they could express that concept through language  . An example used to exemplify this theory was that of seriation. At a certain point in a child's intellectual development they can compare objects on the basis of size. Piaget's theory would say that a child that couldn't compare by size couldn't learn or use words such as "bigger" or "smaller"  . According to Blacker this theory works fine for around the first 18 months after which it is hard to connect the cognitive and language development and that some studies show cases where children who had developed abnormally mentally could speak fluently  which goes towards disproving the theory.
The interactionalists theory is formed on the basis that language acquisition occurs as a result of interaction between the child and the child's care-giver  . Interactionalists believe that that child-directed speech (CDS) helps language acquisition. However CDS is not universal. Language acquisition is and so it is reasonable to take the view that CDS is not necessary in order to acquire a language  .
The main nativist theory was put forward by Noam Chomsky. While criticising the behaviourist theory he concluded that children must have an inborn facility for language acquisition  , he referred to this as a Language Acquisition Device (LAD). This LAD enables human children to acquire a language regardless of where in the world they are, their interaction with others, the lack or existence of CDS or the poverty of language (that the adults around the child are not using grammatically correct sentences all the time). Chomsky stated that all human languages have common principles, e.g. things and actions and that it is for the child and the LAD to establish how the specific language they hear expresses those principles. This aspect of the theory has been modified. Dan Issac Slobin proposed that instead of the LAD containing specific knowledge about language, it contained a mechanism which allowed the child to decipher the rules of the language: "It seems to me that the child is born not with a set of linguistic categories but with some sort of process mechanism - a set of procedures and inference rules, if you will - that he uses to process linguistic data. These mechanisms are such that, applying them to the input data, the child ends up with something which is a member of the class of human languages. The linguistic universals, then, are the result of an innate cognitive competence rather than the content of such a competence." 
The theory of an innate language device is the least flawed, and in my opinion the most likely way of describing the language acquisition of children. The acquisition of language is universal as is the LAD, unlike the theories put forth by the behaviourists, interactionalists and those basing ideas on cognitive development.