Interlanguage theory posits that a dormant psychological framework in the human brain is activated with study of a second language.
Selinker noted in 1972 that in a given situation, the utterances of a learner differ from those of a native speaker to convey an identical meaning.
This comparison suggests a separate linguistic system, which can be observed in the utterances of a learner attempting to convey meaning in L2.
The principal theory of second-language (L2) development had been contrastive analysis, which assumed that learner errors were caused by the difference between L1 (their first language) and L2.
[2] In 1957, Robert Lado said that contrastive analysis should be viewed as hypothetical unless it was based on systematic analyses of learner speech data.
Selinker wrote that in a given situation, the utterances of a learner differ from what a native speaker would produce to convey the same meaning.
[5] Variability is observed when comparing a learner's conversational utterances with form-focused tasks, such as memory-based oral drills in a classroom.
Those with a Chomskyan perspective on second-language acquisition typically regard variability as performance errors unworthy of systematic inquiry.
[6] Preference may also be based on linguistic variables, such as the phonological environment or neighboring features denoting formality or informality.
Free variation in the use of a language feature is usually taken as a sign that it has not been fully acquired; the learner is still figuring out which rules govern the use of alternate forms.
Linguistic factors are usually local; for example, a learner in an earlier stage of acquisition will often systematically vary the correct tense.
According to communication accommodation theory, a learner may adapt their speech to converge with or diverge from their interlocutor's usage; for example, they may deliberately use a non-target form like "me no" to an English teacher to assert their identity with a non-mainstream ethnic group.
[10] Affective factors also play a role in systematic variation; learners in a stressful situation, such as a formal exam, may produce fewer target-like forms than they would in a comfortable setting.
According to interlanguage theory, this apparent progression and regression of language learning indicates the learner's increased understanding of L2 grammar.
Fossilization is freezing the transition between L1 and L2; it is the final stage of interlanguage development, and can occur even in motivated learners who are continuously exposed to L2 and have adequate learning support.