Behaviorism, proposed by B.F. Skinner, says that we acquire language through external stimuli and experience, while mentalism, a theory associated with Noam Chomsky, says that language is innate, that we are born with the mental capacity to work out the grammar and syntax of a language. http://www.slideshare.net/cupidlucid/behaviourism-and-mentalism-nasir-presentation

As it is evident that first-language acquisition studies infant's acquisition of their native language, social interactionism asserts that much of a child's linguistic growth stems from modeling of and interaction with parents and other adults, explaining behaviorism. And mentalism is in our instincts as physical and psychological phenomena in terms of a creative and interpretative mind. http://en.wikipedia.org/wiki/Social_interactionist_theory