Psycholinguists' understanding of how humans acquire language has evolved over time, but many aspects of language learning remain inconclusive. What we do know is that, aside from extreme limitations on cognitive development or language exposure, all humans eventually develop and use language.
Traditional behaviorist studies of language learning view language as one of many behaviors or "habits" into which a child is conditioned through childrearing. The minds of young children are perceived as "blank slates" that are passively trained through exposure and reinforcement to stimuli in the surrounding environment. Any variability in language development is explained as a result of varying degrees of stimulus.
Defenders of more contemporary innatist perspectives of language learning (such as Noam Chomsky) argue that humans are biologically predisposed to acquire language through an internal system in the brain called the Language Acquisition Device (LAD). The LAD enables people to organize language input into categories and to form and test hypotheses about specific language structures, depending on the language of their environment. Universal features included in all languages (such as word order, verb tense, negation, and parts of speech) are set to the specific settings of a given language as the child acquires that language.
The social interactionist theory of language acquisition is more concerned with the functions of language and how they promote language acquisition. Based on Piaget's theories of child development, social interactionists believe that language development is a combination of both nurture and nature. Language is considered a form of acculturation, in which children learn not only vocabulary and syntax, but also a range of paralinguistic elements, such as kinesics (body language), proxemics (personal space), kinesthetics (touching), and prosodics (tone of voice and intonation).