Learner-external factors

The study of learner-external factors in SLA is primarily concerned with the question: How do learners get information about the target language? Study has focused on the effects of different kinds of input, and on the impact of the social context.

Social effects
The process of language learning can be very stressful, and the impact of positive or negative attitudes from the surrounding society can be critical. One aspect that has received particular attention is the relationship of gender roles to language achievement. Studies across numerous cultures have shown that women, on the whole, enjoy an advantage over men. Some have proposed that this is linked to gender roles. Doman (2006) notes in a journal devoted to issues of Cultural affects on SLA, "Questions abound about what defines SLA, how far its borders extend, and what the attributions and contributions of its research are. Thus, there is a great amount of heterogeneity in the entire conceptualization of SLA. Some researchers tend to ignore certain aspects of the field, while others scrutinize those same aspects piece by piece."
Community attitudes toward the language being learned can also have a profound impact on SLA. Where the community has a broadly negative view of the target language and its speakers, or a negative view of its relation to them, learning is typically much more difficult. This finding has been confirmed by research in numerous contexts. A widely-cited example is the difficulty faced by Navajo children in learning English as a second language.
Other common social factors include the attitude of parents toward language study, and the nature of group dynamics in the language classroom. Additionally, early attitudes may strengthen motivation and facility with language in general, particularly with early exposure to the language.

Input and intake
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 (2001) 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.
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.

Interaction
Long's interaction hypothesis proposes that language acquisition is strongly facilitated by the use of the target language in interaction. In particular, the negotiation of meaning has been shown to contribute greatly to the acquisition of vocabulary (Long, 1990). In a review of the substantial literature on this topic, Nation (2000) relates the value of negotiation to the generative use of words: the use of words in new contexts which stimulate a deeper understanding of their meaning.
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.

Pedagogical effects
The study of the effects of teaching on second language acquisition seeks to systematically measure or evaluate the effectiveness of language teaching practices. Such studies have been undertaken for every level of language, from phonetics to pragmatics, and for almost every current teaching methodology. It is therefore impossible to summarize their findings here. However, some more general issues have been addressed.
Research has indicated that many traditional language-teaching techniques are extremely inefficient. (P Lightbrown, 1990) However, today a broad consensus of SLA scholars acknowledge that formal instruction can help in language learning.
Another important issue is the effectiveness of explicit teaching: can language teaching have a constructive effect beyond providing learners with enhanced input? Because explicit instruction must usually take place in the learner's first language, some have argued that it simply starves learners of input and opportunities for practice.[citation needed] Research on this at different levels of language has produced quite different results. Traditional areas of explicit teaching, such as phonology, grammar and vocabulary, have had decidedly mixed results. The positive effect of explicit instruction at this level seems to be limited to helping students notice important aspects of input.[citation needed] Interestingly, the higher-level aspects of language such as sociopragmatic and discourse competence have shown the most consistently strong effects from explicit instruction. Research has also shown a distinct effect of age on the effectiveness of explicit instruction: the younger learners are, the less benefit they show.[citation needed]
However, research has again and again shown that early exposure to a second language increases a child's capacity to learn language, even their first language.[citation needed]