Second language acquisition theories are distinctly different and distinguished from natural first language acquisition by children. There is no single accepted explanation of how people, particularly adults with mature cognitive skills, learn a second language. Several related disciplines — education, linguistics, psychology and neurology — have contributed theoretical ideas and research studies. The many theories and hypotheses also make a clear distinction between a multilingual person and the process of a person acquiring fluency in additional languages.
In general, most second language acquisition theories parse the task. There is a native first language, sometimes abbreviated L1; the second language to be learned is sometimes referred as the “target language” and abbreviated L2. As new vocabulary and rules of grammar and syntax are learned, they are initially retained in a “lexical memory” which is essentially a store of definitions and declarations. An “input” event in the second language is processed, perhaps compared to the known information in memory, to derive at new conclusions and interpretations. The resulting “output” provides either positive or negative feedback to refine both memory and process.
The greatest influence on second language acquisition theories is the idea proposed by Noam Chomsky, the social philosopher regarded as the “father of modern linguistics,” that all languages have a “universal grammar.” Learning a second language is as easy as habituating all the minor rules of grammar peculiar to the language. The second most significant influence on second language acquisition theories is the concept of an “interlanguage.” A second language learner develops an indefinable, but systematic, third language that mediates between the first and second. Awkward mistakes such as, “I goed shopping yesterday,” are believed to be derived from this interlanguage.
Understanding the acquisition of a second language is an abstract theoretical exercise, as even modern brain imaging technologies cannot definitively map the cognitive process. Most studies, and their theoretical conclusions, concern the linguistic characteristics of a second language input, and a learner’s resulting output. The learner is categorized for level of proficiency. Of particular interest is linguistic mistakes in their output. The mistakes are scientifically replicable and reveal something of the underlying processes which cause them.
A set of hypotheses collectively called the Monitor Theory posits in essence that a learner possesses evaluative filters for both input and output, as well as one for the accuracy or effectiveness between them. For example, the input must be at least at a learner’s comprehension level; otherwise the input does not pass through the filter. Additionally, particularly in the early stages of learning, people exercise a great deal of control and deliberation over their output.
Theories emphasizing input and output have direct application in teaching a second language. The Monitor Theory, for example, includes an affective component to the filters that inhibits both input and output when subjected to stress or anxiety. A second language school or course will strive to establish ease in a learner, to allow for mistakes in understanding, and replying to, an unfamiliar utterance.
There are several other significant second language acquisition theories and hypotheses. The role of memory — both short term and long term, both lexical such as vocabulary and procedural such as grammar — is clearly very important. Some theorists note that learning occurs at certain cognitive thresholds. The theory of processability, for example, suggests that only when an interlanguage has established a restructuring order of the native language can a second language be learned. Other theories attempt the ultimate ambition of blurring the distinction between acquisition and learning, so that learning a second language might someday become as effortless as acquiring a first language.
