The three theories of language acquisition hold that language develops in children as a result of: 1) imitation of language being used by adults and older children around the young child; 2) reinforcement of language attempts by the young child; and 3) nativism, or a neurological predisposition allowing for the development of language when a child's neurological development reaches the proper stage.
All of these theories assume that the child is capable of developing and maturing, whether independently or with support and stimulation from the environment around the child. All are addressing the development of ordinary, conversational language through casual means, not a formal study of a language and its application and use.
The imitation theory contends that children acquire language by imitating the speech of others. The problem with this theory is that children quickly begin using words in phrases and sentences that they have not heard from an adult, so they are not imitating in this use of language.
The theory based on reinforcement contends that children receive positive reinforcement for correct or appropriate attempts to use language and negative reinforcement in response to mistakes. This theory does not explain why children enjoy talking to themselves without any feedback or reaction, and it does not address the research that shows many adults tend to reinforce the meaning of a child's verbal language rather than the grammatical content of the language.
The nativism theory holds that
the human brain has a built-in language acquisition device, or LAD, that analyzes the parts of speech in the language that a child hears.
This theory holds that children will begin to acquire language, regardless of imitation or reinforcement, when their brain has developed to the needed stage.