Behaviorism (Skinner) –> language as a product of experience; children entered life with a tabula rasa (blank slate), and learned language only after being exposed to it.
Universal Grammar (Chomsky) –> the idea that every individual, regardless of the language they ultimately spoke, had within their linguistic competence a language acquisition device containing a set of universal principles.
Functional grammar (Halliday) –> believes that language exists to satisfy the communicative needs of its users; that is, that language is a communicative tool. Involves competence and performance

The Development of Grammar
The young child who may be unable to tie his shoes or do even the simplest arithmetic computation masters the complex grammatical structures of her language and acquires a substantial lexicon.
By virtue of Universal Grammar, children are able to acquire language as quickly and effortlessly as they do because they do not have to figure out all the rules of their language, only those that are specific to their own language.

Sign Languages: Evidence for the innateness of language.
The languages used by deaf people in which linguistic units such as morphemes and words as well as grammatical relations are formed by manual and other body movements.
American Sign Language (ASL) – the sign language used by the deaf community in the USA, has its own grammar
Deaf children who are exposed to sign language learn it in stages parallel to those of hearing children learning spoken language.
The fact that deaf children learn sign language shows that the ability to hear or produce sounds is not a prerequisite to language learning. All of the sign languages in the world, which differ as spoken languages do, are visual-gestural systems that are as fully developed an as structurally complex as a spoken language.