Behaivioursism is also known as the immitation theory.
BF Skinner in 1957 (a phsycologist) observed rats and pigeons to develop the idea that Language is a learned behaviour so children can be trained to use it correctly.
Children are taught this through reinforcment. This can either be posotive or negative or verbal and physical.
Meaning that posotive reinformant includes things such as 'well done' 'good girl' or a pat on the back, when a child has said something correctly.
Negative reinforcment includes 'no youre wrong' when the chd ilhas said something incorrectly.
Evidence for this theory -
Gesstalt words are words that sound like 1 word but are infact two. Examples of this are 'shoeson' 'toppon'. Children say utterences like this as they have heard it from people around them meaning they are imitating.
Question construction is a process where children learn to ask questions. The first stage of this is rising intonation which children have picked up from adults meaning they are imitating the pitch and intonation of others around them supporting this theory.
Regression which is based on Roger Brown's U shaped graph of regression, shows how children are copying adult grammer and are therefore copying and imitating adults.
Katherine Nelsons 4 first words include naming social action and modifying
Naming explosion show how a child is demonstrating a demand for adults input
Pragmatics and politeness features are taught which shows how children are learning through adults and immitating,.
Evidence Against -
Children over generalise grammatical rules e.g I swimmed. They are adding free morphemes because they understand and are creatively applying what they know to patterns. This shows how they don't simply immitate.
Derevational morphology means that children are creating new words showing they understand the sounds that create utterneces in order to create their own.
The wugs experiment shows how children are applying what they know when looking at words they have never heard before showing they understand and don't just immitate.
Adults are more interested in the truth of the statement rather than the grammar and syntax that the child is using.
Also a big factor here for evidence against is that the experiment was done on pigeons and rats and not children and children are a lot more sophistacted.