Behaviourist psychologists developed their theories while carrying out a series of experiments on animals. They observed that rats or birds, for example, could be taught to perform various tasks by encouraging habit-forming.
Researchers rewarded desirable behaviour. This was known as positive reinforcement. Undesirable behaviour was punished or simply not rewarded — negative reinforcement. The behaviourist B. F. Skinner then proposed this theory as an explanation for language acquisition in humans. In Verbal Behaviour (1957), he stated: “The basic processes and relations which give verbal behaviour its special characteristics are now fairly well understood. Much of the experimental work responsible for this advance has been carried out on other species, but the results have proved to be surprisingly free of species restrictions. Recent work has shown that the methods can be extended to human behaviour without serious modifications.” (cited in Lowe and Graham, 1998, p.68)

Skinner suggested that a child imitates the language of its parents or carers.
Successful attempts are rewarded because an adult who recognises a word spoken by a child will praise the child and/or give it what it is asking for. The linguistic input was key — a model for imitation to be either negatively or positively reinforced. Successful utterances are therefore reinforced while unsuccessful ones are forgotten. No essential difference between the way a rat learns to negotiate a maze and a child learns to speak.