Input and interactionThe primary factor affecting language acquisition appears to be the input that the learner receives. Stephen Krashen took a very strong position on the importance of input, asserting that comprehensible input is all that is necessary for second language acquisition. Krashen pointed to studies showing that the length of time a person stays in a foreign country is closely linked with his level of language acquisition. Further evidence for input comes from studies on reading: large amounts of free voluntary reading have a significant positive effect on learners' vocabulary, grammar, and writing. Input is also the mechanism by which people learn languages according to the universal grammar model.
The type of input may also be important. One tenet of Krashen's theory is that input should not be grammatically sequenced. He claims that such sequencing, as found in language classrooms where lessons involve practicing a "structure of the day", is not necessary, and may even be harmful.
While input is of vital importance, Krashen's assertion that only input matters in second language acquisition has been contradicted by more recent research. For example, students enrolled in French language immersion programs in Canada still produced non-native-like grammar when they spoke, even though they had years of meaning-focused lessons and their listening skills were statistically native-level. Output appears to play an important role, and among other things, can help provide learners with feedback, make them concentrate on the form of what they are saying, and help them to automatize their language knowledge. These processes have been codified in the theory of comprehensible output.
Researchers have also pointed to interaction in the second language as being important for acquisition. According to Long's interaction hypothesis the conditions for acquisition are especially good when interacting in the second language; specifically, conditions are good when a breakdown in communication occurs and learners must negotiate for meaning. The modifications to speech arising from interactions like this help make input more comprehensible, provide feedback to the learner, and push learners to modify their speech.
Social aspectsAlthough the dominant perspective in second language research is a cognitive one, from the early days of the discipline researchers have also acknowledged that social aspects play an important role.There have been many different approaches to sociolinguistic study of second language acquisition, and indeed, according to Rod Ellis, this plurality has meant that "sociolinguistic SLA is replete with a bewildering set of terms referring to the social aspects of L2 acquisition". Common to each of these approaches, however, is a rejection of language as a purely psychological phenomenon; instead, sociolinguistic research views the social context in which language is learned as essential for a proper understanding of the acquisition process.
Ellis identifies three types of social structure which can affect the acquisition of second languages: socialinguistic setting, specific social factors, and situational factors. Socialinguistic setting refers to the role of the second language in society, such as whether it is spoken by a majority or a minority of the population, whether its use is widespread or restricted to a few functional roles, or whether the society is predominantly bilingual or monolingual. Ellis also includes the distinction of whether the second language is learned in a natural or an educational setting. Specific social factors that can affect second language acquisition include age, gender, social class, and ethnic identity, with ethnic identity being the one that has received most research attention. Situational factors are those which vary between each social interaction. For example, a learner may use more polite language when talking to someone of higher social status, but more informal language when talking with friends.
There have been several models developed to explain social effects on language acquisition. Schumann's acculturation model proposes that learners' rate of development and ultimate level of language achievement is a function of the "social distance" and the "psychological distance" between learners and the second language community. In Schumann's model the social factors are most important, but the degree to which learners are comfortable with learning the second language also plays a role. Another sociolinguistic model is Gardner's socio-educational model, which was designed to explain classroom language acquisition. The inter-group model proposes "ethnolinguistic vitality" as a key construct for second language acquisition. Language socialization is an approach with the premise that "linguistic and cultural knowledge are constructed through each other", and saw increased attention after the year 2000. Finally, Norton's theory of social identity is an attempt to codify the relationship between power, identity, and language acquisition.
Internal factors affecting second language acquisitionThe meaning of things being communicated is more important for second language acquisition than their form. There is a general agreement among researchers that learners must be engaged in decoding and encoding messages in the second language for the conditions to be right for second language learning. Learners must also be engaged in creating pragmatic meaning in order to develop fluency.
Some sort of focus on form does appear to be necessary for second language acquisition, however. Some advanced language structures may not be fully acquired without the opportunity for repeated practice. Schmidt's noticing hypothesis states that conscious attention to specific language forms is necessary for a learner's interlanguage to develop. This attention does not have to be in the form of conscious grammar rules, however; the attention is on how each specific form affects the meaning of what is being said.
Developing subconscious knowledge of the second language is more important than developing conscious knowledge. While conscious language knowledge is important for many aspects of second language acquisition, developing subconscious knowledge is vital for fluency. The knowledge that people use when they are speaking a language is mostly subconscious. It appears that learners can use conscious knowledge in speech if they have time and they are focused on form, but if these conditions are not met then they will fall back on subconscious knowledge. However, if learners have time to plan their speech, grammatical accuracy can improve.
It is not certain exactly how subconscious language knowledge is developed in the mind. According to skill-building theory, subconscious language knowledge is gained by practicing language until it becomes automatic. However, according to emergentist theories subconscious knowledge develops naturally from input and communication. The nature of the interface between conscious and subconscious language knowledge in the brain is also not clear; that is, it is not clear how conscious knowledge can develop into subconscious knowledge. It appears that conscious knowledge and subconscious knowledge are not completely separate, and practice at various aspects of language can lead to language knowledge becoming subconscious. However, studies have found that the two types of knowledge are stored differently in the brain, and this has led to the idea that conscious knowledge merely primes language acquisition processes rather than being directly involved. Both of these issues are still under debate.
The way learners process sentences in their second language is also important for language acquisition. According to MacWhinney's competition model, learners can only concentrate on so many things at a time, and so they must filter out some aspects of language when they listen to a second language. Learning a language is seen as finding the right weighting for each of the different factors that learners can process.
Similarly, according to processability theory, the sequence of acquisition can be explained by learners getting better at processing sentences in the second language. As learners increase their mental capacity to process sentences, mental resources are freed up. Learners can use these newly freed-up resources to concentrate on more advanced features of the input they receive. One such feature is the movement of words. For example, in English, questions are formed by moving the auxiliary verb or the question word to the start of the sentence (John is nice becomes Is John nice?) This kind of movement is too brain-intensive for beginners to process; learners must automatize their processing of static language structures before they can process movement