Nativist theories - These theories suggest that all humans have an inbuilt or innate capacity for language acquisition.

Social Interaction theories - These theories suggest that to acquire and develop language skills a child must have social and verbal interaction.

Cognitive theorists - The cognitive theorist believes that language is acquired as part of a wider development of understanding and cognitive ability.

BF Skinner (1957) having performed experiments using rats and pigeons discovered that they could perform a variety of tasks, as long as they were broken into stages, and received rewards for 'correct' actions.  From this Skinner formed his 'Operant Conditioning Theory'.  This was then applied to human learning, claiming that children learn language through the simple process of imitation and reinforcement.  Skinner also claimed that no complicated internal mechanisms were needed for language acquisition and that children learning language was virtually the same as lab rats learning to complete tasks.

Eric Lenneburg (1962) argued against Skinner's theory, in that children who are unable to speak due to illness are able to gain a normal comprehension of language without the ability to imitate adults, or by having their utterances reinforced.  Hart and Risley (1995) argued that differences in social classes in the USA and the verbal interaction received by children in these classes can have an influence on their social and economic development in their later life. 

Noam Chomsky - is a linguist and nativist theorist.  He argued against Skinner's interactional theory and suggested that children acquire language because all humans have an innate ability to speak.  Essentially he stated that the human brain is 'hard-wired' for language.  The LAD (Language Acquisition Device) enables a child to hear and extrapolate the rules of their own language and understand the words and structures.  Chomsky suggested that all languages have a Universal Grammar, stating that under the surface of all languages a similar grammatical structure is present.  Looking at features of child language acquisition, such as virtuous errors and overgeneralization, Chomsky noted that all children created these features without having heard adults use the same language and he concluded that there must be more than interaction and imitation at work to explain this phenomenon. 

Theorist Steven Pinker, looked at Chomsky's work and further developed the LAD idea and created his own PPT (Principles and Parameter Theory).  This theory essentially states that by hearing the principles and parameters of the individual’s native language, the rules become fixed.  In a sense Pinker is saying that the PPT is the software used by the LAD hardware to create language.  Once the parameters are defined through hearing speech, the principles become defined and are retained.  In the light of cognitive theories about child language acquisition, Chomsky has reflected on his own work and moved towards the ideas put forward by cognitive theorists.

Lenneburg, who worked as Chomsky's colleague added to the LAD theory stating that there must also be a 'critical period' when the LAD needs to be 'activated' with a sufficient amount of input, or the child's language development may be impaired.  This relates to 'feral - children', such as 'Genie'.  A child who was 'found' in 1970 and who had never had any positive interaction to develop language acquisition.  Forced to keep quiet, she never developed the ability to vocalise langauge properly, suggesting as per Lenneburg's idea, that she had passed the 'critical period' of activation and input; however, it should be noted that she did acquire the ability to communicate through sign language.  ASL (American Sign Language) does have grammatical rules, not the same as English, but still as Genie was able to acquire this ability and communicate with those around her, the interactional theory by Skinner is given more weight, as she would have learnt ASL through imitation and positive reinforcement by her care givers.

Jean Piaget - a cognitive theorist suggested that language development is due to a wider development in cognitive ability and that a child cannot articulate ides or concepts that they are uanble to understand.   Piaget argued that if a child needs to understand a concept such as the past, to be able to use language terms to refer to it.  Also the concept of seriation ( the ability to place items in a series, such as ascending or decending orders) needs to be understood before a child could learn to use superlatives and comparatives.  Object permanence features in this idea too.  The idea that the object still exists even if it is not visible needs to be understood too, so that the child comprehends that everything has it's own separate identity and will still exist even when the child cannot see it.

Lee Vygotsky -  a Russian pyschologist had similar views to Piaget.  He stated that langauge has two roles, one for communication and the other as a tool for further developing understanding, as he believed that language and thought become related in a relatively short period of time.  He also believed that collaborative play has an influence and essential part in a child's early development.  In instances where the emphasis is put more on play than teaching a child is required to stretch their cognitive abilities in and understanding new concepts or ideas without even realising  they are being taught.  Vygotsky said "What a child can do in co-operation today, he can do alone tomorrow".

(theorist research from another blog)