Theories of second-language acquisition are various theories and hypotheses in the field of second-language acquisition about how people learn a second language. Research in second-language acquisition is closely related to several disciplines including linguistics, sociolinguistics, psychology, neuroscience, and education, and consequently most theories of second-language acquisition can be identified as having roots in one of them. Each of these theories can be thought of as shedding light on one part of the language learning process; however, no one overarching theory of second-language acquisition has yet been widely accepted by researchers.
From the field of linguistics, the most influential theory by far has been Chomsky's theory of Universal Grammar (UG). The UG model of principles, basic properties which all languages share, and parameters, properties which can vary between languages, has been the basis for much second language research.
From a UG perspective, learning the grammar of a second language is simply a matter of setting the correct parameters. Take the pro-drop parameter, which dictates whether or not sentences must have a subject in order to be grammatically correct. This parameter can have two values: positive, in which case sentences do not necessarily need a subject, and negative, in which case subjects must be present. In German the sentence "Er spricht" (he speaks) is grammatical, but the sentence "Spricht" (speaks) is ungrammatical. In Italian, however, the sentence "Parla" (speaks) is perfectly normal and grammatically correct.[1] A German speaker learning Italian would only need to deduce that subjects are optional from the language he hears, and then set his pro-drop parameter for Italian accordingly. Once he has set all the parameters in the language correctly, then from a UG perspective he can be said to have learned Italian, i.e. he will always produce perfectly correct Italian sentences.
Universal Grammar also provides a succinct explanation for much of the phenomenon of language transfer. Spanish learners of English who make the mistake "Is raining" instead of "It is raining" have not yet set their pro-drop parameters correctly and are still using the same setting as in Spanish.
The main shortcoming of Universal Grammar in describing second-language acquisition is that it does not deal at all with the psychological processes involved with learning a language. UG scholarship is only concerned with whether parameters are set or not, not with how they are set.
Stephen Krashen introduced the acquisition-learning hypothesis, which makes a distinction between conscious language learning and subconscious language acquisition.[2] Krashen argues that only subconscious acquisition can lead to fluency.
A distinction closely related to that made by Krashen (1982) between acquisition and learning is one between implicit and explicit linguistic knowledge.[2] Learners gain implicit knowledge by processing target-language input without consciously giving attention to acquiring the forms and structures of the language. On the other hand, learners get explicit knowledge of a language when they process language input with the conscious intention of discovering the structural rules of the language. A distinction between the implicit learning involved in acquiring a first language (L1) and the mix of implicit and explicit learning that takes place in L2 acquisition has been one analytic route for understanding the virtually universal success of L1 acquisition versus the more limited success of L2 acquisition among adult learners (Hulstijn, 2005).[3] Ellis has found empirical confirmation for the distinct constructs of implicit and explicit language knowledge.[4]
Learners' most direct source of information about the target language is the target language itself. When they come into direct contact with the target language, this is referred to as "input." When learners process that language in a way that can contribute to learning, this is referred to as "intake."
Generally speaking, the amount of input learners take in is one of the most important factors affecting their learning. However, it must be at a level that is comprehensible to them. In his Monitor Theory, Krashen advanced the concept that language input should be at the "i+1" level, just beyond what the learner can fully understand; this input is comprehensible, but contains structures that are not yet fully understood. This has been criticized on the basis that there is no clear definition of i+1, and that factors other than structural difficulty (such as interest or presentation) can affect whether input is actually turned into intake. The concept has been quantified, however, in vocabulary acquisition research; Nation reviews various studies which indicate that about 98% of the words in running text should be previously known in order for extensive reading to be effective.[5]
In his Input Hypothesis, Krashen proposes that language acquisition takes place only when learners receive input just beyond their current level of L2 competence. He termed this level of input “i+1.” However, in contrast to emergentist and connectionist theories, he follows the innate approach by applying Chomsky’s Government and binding theory and concept of Universal grammar (UG) to second-language acquisition. He does so by proposing a Language Acquisition Device that uses L2 input to define the parameters of the L2, within the constraints of UG, and to increase the L2 proficiency of the learner. In addition, Krashen (1982)’s Affective Filter Hypothesis holds that the acquisition of a second language is halted if the leaner has a high degree of anxiety when receiving input. According to this concept, a part of the mind filters out L2 input and prevents uptake by the learner, if the learner feels that the process of SLA is threatening. As mentioned earlier, since input is essential in Krashen’s model, this filtering action prevents acquisition from progressing.
A great deal of research has taken place on input enhancement, the ways in which input may be altered so as to direct learners' attention to linguistically important areas. Input enhancement might include bold-faced vocabulary words or marginal glosses in a reading text. Research here is closely linked to research on pedagogical effects, and comparably diverse.
Other concepts have also been influential in the speculation about the processes of building internal systems of second language information. Some thinkers hold that language processing handles distinct types of knowledge. For instance, one component of the Monitor Model, propounded by Krashen, posits a distinction between “acquisition” and “learning.”[2] According to Krashen, L2 acquisition is a subconscious process of incidentally “picking up” a language, as children do when becoming proficient in their first languages. Language learning, on the other hand, is studying, consciously and intentionally, the features of a language, as is common in traditional classrooms. Krashen sees these two processes as fundamentally different, with little or no interface between them. In common with connectionism, Krashen sees input as essential to language acquisition.[2]
Further, Bialystok and Smith make another distinction in explaining how learners build and use L2 and interlanguage knowledge structures.[6] They argue that the concept of interlanguage should include a distinction between two specific kinds of language processing ability. On one hand is learners’ knowledge of L2 grammatical structure and ability to analyze the target language objectively using that knowledge, which they term “representation,” and, on the other hand is the ability to use their L2 linguistic knowledge, under time constraints, to accurately comprehend input and produce output in the L2, which they call “control.” They point out that often non-native speakers of a language have higher levels of representation than their native-speaking counterparts have, yet have a lower level of control. Finally, Bialystok has framed the acquisition of language in terms of the interaction between what she calls “analysis” and “control.”[7] Analysis is what learners do when they attempt to understand the rules of the target language. Through this process, they acquire these rules and can use them to gain greater control over their own production.
Monitoring is another important concept in some theoretical models of learner use of L2 knowledge. According to Krashen, the Monitor is a component of an L2 learner’s language processing device that uses knowledge gained from language learning to observe and regulate the learner’s own L2 production, checking for accuracy and adjusting language production when necessary.[2]
Long's interaction hypothesis proposes that language acquisition is strongly facilitated by the use of the target language in interaction. Similarly to Krashen's Input Hypothesis, the Interaction Hypothesis claims that comprehensible input is important for language learning. In addition, it claims that the effectiveness of comprehensible input is greatly increased when learners have to negotiate for meaning.[8]
Interactions often result in learners receiving negative evidence.[8][9] That is, if learners say something that their interlocutors do not understand, after negotiation the interlocutors may model the correct language form. In doing this, learners can receive feedback on their production and on grammar that they have not yet mastered.[8] The process of interaction may also result in learners receiving more input from their interlocutors than they would otherwise.[9] Furthermore, if learners stop to clarify things that they do not understand, they may have more time to process the input they receive. This can lead to better understanding and possibly the acquisition of new language forms.[8] Finally, interactions may serve as a way of focusing learners' attention on a difference between their knowledge of the target language and the reality of what they are hearing; it may also focus their attention on a part of the target language of which they are not yet aware.[10]
In the 1980s, Canadian SLA researcher Merrill Swain advanced the output hypothesis, that meaningful output is as necessary to language learning as meaningful input. However, most studies have shown little if any correlation between learning and quantity of output. Today, most scholars contend that small amounts of meaningful output are important to language learning, but primarily because the experience of producing language leads to more effective processing of input.
Some of the major cognitive theories of how learners organize language knowledge are based on analyses of how speakers of various languages analyze sentences for meaning. MacWhinney, Bates, and Kliegl found that speakers of English, German, and Italian showed varying patterns in identifying the subjects of transitive sentences containing more than one noun.[11] English speakers relied heavily on word order; German speakers used morphological agreement, the animacy status of noun referents, and stress; and speakers of Italian relied on agreement and stress. MacWhinney et al. interpreted these results as supporting the Competition Model, which states that individuals use linguistic cues to get meaning from language, rather than relying on linguistic universals.[11] According to this theory, when acquiring an L2, learners sometimes receive competing cues and must decide which cue(s) is most relevant for determining meaning.
These findings also relate to Connectionism. Connectionism attempts to model the cognitive language processing of the human brain, using computer architectures that make associations between elements of language, based on frequency of co-occurrence in the language input.[12] Frequency has been found to be a factor in various linguistic domains of language learning.[13] Connectionism posits that learners form mental connections between items that co-occur, using exemplars found in language input. From this input, learners extract the rules of the language through cognitive processes common to other areas of cognitive skill acquisition. Since connectionism denies both innate rules and the existence of any innate language-learning module, L2 input is of greater importance than it is in processing models based on innate approaches, since, in connectionism, input is the source of both the units and the rules of language.
Attention is another characteristic that some believe to have a role in determining the success or failure of language processing. Schmidt states that although explicit metalinguistic knowledge of a language is not always essential for acquisition, the learner must be aware of L2 input in order to gain from it.[14] In his “noticing hypothesis,” Schmidt posits that learners must notice the ways in which their interlanguage structures differ from target norms. This noticing of the gap allows the learner’s internal language processing to restructure the learner’s internal representation of the rules of the L2 in order to bring the learner’s production closer to the target. In this respect, Schmidt’s understanding is consistent with the ongoing process of rule formation found in emergentism and connectionism.
Some theorists and researchers have contributed to the cognitive approach to second-language acquisition by increasing understanding of the ways L2 learners restructure their interlanguage knowledge systems to be in greater conformity to L2 structures. Processability theory states that learners restructure their L2 knowledge systems in an order of which they are capable at their stage of development.[15] For instance, In order to acquire the correct morphological and syntactic forms for English questions, learners must transform declarative English sentences. They do so by a series of stages, consistent across learners. Clahsen proposed that certain processing principles determine this order of restructuring.[16] Specifically, he stated that learners first, maintain declarative word order while changing other aspects of the utterances, second, move words to the beginning and end of sentences, and third, move elements within main clauses before subordinate clauses.
Thinkers have produced several theories concerning how learners use their internal L2 knowledge structures to comprehend L2 input and produce L2 output. One idea is that learners acquire proficiency in an L2 in the same way that people acquire other complex cognitive skills. Automaticity is the performance of a skill without conscious control. It results from the gradated process of proceduralization. In the field of cognitive psychology, Anderson expounds a model of skill acquisition, according to which persons use procedures to apply their declarative knowledge about a subject in order to solve problems.[17] On repeated practice, these procedures develop into production rules that the individual can use to solve the problem, without accessing long-term declarative memory. Performance speed and accuracy improve as the learner implements these production rules. DeKeyser tested the application of this model to L2 language automaticity.[18] He found that subjects developed increasing proficiency in performing tasks related to the morphosyntax of an artificial language, Autopractan, and performed on a learning curve typical of the acquisition of non-language cognitive skills. This evidence conforms to Anderson’s general model of cognitive skill acquisition, supports the idea that declarative knowledge can be transformed into procedural knowledge, and tends to undermine the idea of Krashen[2] that knowledge gained through language “learning” cannot be used to initiate speech production.
Michael T. Ullman has used a declarative/procedural model to understand how language information is stored. This model is consistent with a distinction made in general cognitive science between the storage and retrieval of facts, on the one hand, and understanding of how to carry out operations, on the other. It states that declarative knowledge consists of arbitrary linguistic information, such as irregular verb forms, that are stored in the brain’s declarative memory. In contrast, knowledge about the rules of a language, such as grammatical word order is procedural knowledge and is stored in procedural memory. Ullman reviews several psycholinguistic and neurolinguistic studies that support the declarative/procedural model.[19]
Perhaps certain psychological characteristics constrain language processing. One area of research is the role of memory. Williams conducted a study in which he found some positive correlation between verbatim memory functioning and grammar learning success for his subjects.[20] This suggests that individuals with less short-term memory capacity might have a limitation in performing cognitive processes for organization and use of linguistic knowledge.
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