Second-language acquisition

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Second-language acquisition (SLA), sometimes called second-language learning—otherwise referred to as L2 (language 2) acquisition, is the process by which people learn a second language. Second-language acquisition is also the scientific discipline devoted to studying that process. The field of second-language acquisition is regarded by some but not everybody as a sub-discipline of applied linguistics but also receives research attention from a variety of other disciplines, such as psychology and education.

A central theme in SLA research is that of interlanguage: the idea that the language that learners use is not simply the result of differences between the languages that they already know and the language that they are learning, but a complete language system in its own right, with its own systematic rules. This interlanguage gradually develops as learners are exposed to the targeted language. The order in which learners acquire features of their new language stays remarkably constant, even for learners with different native languages and regardless of whether they have had language instruction. However, languages that learners already know can have a significant influence on the process of learning a new one. This influence is known as language transfer.

Two important distinctions made in the field of second language learning are those between language input (i.e. listening, reading or seeing) and language output (i.e. speaking, writing or signing) and between explicit knowledge of a language (conscious awareness of rules) and their implicit knowledge (automatic use of the rules in practice). Many debates centre on the relative importance of these factors in language acquisition. On the issue of the relative significance of input and output, competing views are the comprehensible input hypothesis, the comprehensible output hypothesis and the interaction hypothesis. On the issue of the relative significance of explicit and implicit knowledge and the connection between the two, views range from the notion that explicit learning is crucial for implicit learning and to some degree can serve as a basis for it, as seen in skill-based theories of second-language acquisition and the noticing hypothesis, to the idea that connections between the two are minimal and explicit learning has very limited relevance for true acquisition, as seen in the acquisition-learning hypothesis.

Research on how exactly learners acquire a new language spans several different areas. Focus is directed toward providing proof of whether basic linguistic skills are innate (nature), acquired (nurture), or a combination of the two attributes. Cognitive approaches to SLA research deal with the processes in the brain that underpin language acquisition, for example how paying attention to language affects the ability to learn it, or how language acquisition is related to short-term memory and long-term memory. Sociocultural approaches reject the notion that SLA is a purely psychological phenomenon and attempt to explain it in a social context. Some key social factors that influence SLA are the level of immersion, connection to the L2 community, and gender. Linguistic approaches consider language separately from other kinds of knowledge and attempt to use findings from the wider study of linguistics to explain SLA. There is also a considerable body of research about how SLA can be affected by individual factors such as age and learning strategies. A commonly discussed topic regarding age in SLA is the critical period hypothesis, which suggests that individuals lose the ability to fully learn a language after a particular age in childhood. Another topic of interest in SLA is the differences between adult and child learners. Learning strategies are commonly categorized as learning or communicative strategies and are developed to improve their respective acquisition skills. Affective factors are emotional factors that influence an individual's ability to learn a new language. Common affective factors that influence acquisition are anxiety, personality, social attitudes, and motivation. In the domain of personality, introversion and extroversion in particular can affect learning.

Individuals may also lose a language through a process called second-language attrition. This is often caused by a lack of use or exposure to a language over time. The severity of attrition depends on a variety of factors including level of proficiency, age, social factors, and motivation at the time of acquisition. Finally, classroom research deals with the effect that language instruction has on acquisition.