<P> Another key idea within the theory of social interactionism is that of the zone of proximal development . Briefly, this is a theoretical construct denoting the set of tasks a child is capable of performing with guidance, but not alone . As applied to language, it describes the set of linguistic tasks (proper syntax, suitable vocabulary usage, etc .) a child cannot carry out on their own at a given time, but can learn to carry out if assisted by an able adult . </P> <P> The relational frame theory (RFT) (Hayes, Barnes - Holmes, Roche, 2001), provides a wholly selectionist / learning account of the origin and development of language competence and complexity . Based upon the principles of Skinnerian behaviorism, RFT posits that children acquire language purely through interacting with the environment . RFT theorists introduced the concept of functional contextualism in language learning, which emphasizes the importance of predicting and influencing psychological events, such as thoughts, feelings, and behaviors, by focusing on manipulable variables in their context . RFT distinguishes itself from Skinner's work by identifying and defining a particular type of operant conditioning known as derived relational responding, a learning process that, to date, appears to occur only in humans possessing a capacity for language . Empirical studies supporting the predictions of RFT suggest that children learn language via a system of inherent reinforcements, challenging the view that language acquisition is based upon innate, language - specific cognitive capacities . </P> <P> Emergentist theories, such as MacWhinney's competition model, posit that language acquisition is a cognitive process that emerges from the interaction of biological pressures and the environment . According to these theories, neither nature nor nurture alone is sufficient to trigger language learning; both of these influences must work together in order to allow children to acquire a language . The proponents of these theories argue that general cognitive processes subserve language acquisition and that the end result of these processes is language - specific phenomena, such as word learning and grammar acquisition . The findings of many empirical studies support the predictions of these theories, suggesting that language acquisition is a more complex process than many believe . </P> <P> As syntax began to be studied more closely in the early 20th century, in relation to language learning, it became apparent to linguists, psychologists, and philosophers that knowing a language was not merely a matter of associating words with concepts, but that a critical aspect of language involves knowledge of how to put words together--sentences are usually needed in order to communicate successfully, not just isolated words . A child will use short expressions such as Bye - bye Mummy or All - gone milk, which actually are combinations of individual nouns and an operator, before it begins to use gradually more complex sentences . In the 1990s within the Principles and Parameters framework, this hypothesis was extended into a maturation - based Structure building model of child language regarding the acquisition of functional categories . In this model, children are seen as gradually building up more and more complex structures, with Lexical categories (like noun and verb) being acquired before Functional - syntactic categories (like determiner and complementiser). When acquiring a language, it is also often found, in languages such as English, that the most frequently used verbs are irregular verbs . Young children first begin to learn the past tense of verbs individually; however, when they acquire a "rule", such as adding - ed to form the past tense, they begin to exhibit occasional overgeneralization errors (e.g. "runned", "hitted") alongside correct past - tense forms . One influential proposal to the origin of these errors is as follows: the adult state of grammar stores each irregular verb form in memory as well as a "block" on the use of the regular rule for forming that type of verb . In the developing child's mind, retrieval of that "block" may fail, causing the child to erroneously apply the regular rule instead of retrieving the irregular . </P>

The emergentist model of language acquisition claims that language emerges primarily from