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Stephen Krashen's Theory of Second Language Acquisition

Stephen Krashen

Language acquisition does not require extensive use of conscious grammatical rules, and does not require tedious drill.

Acquisition requires meaningful interaction in the target language - natural communication - in which speakers are concerned not with the form of their utterances but with the messages they are conveying and understanding.

... 'comprehensible input' is the crucial and necessary ingredient for the acquisition of language.

The best methods are therefore those that supply 'comprehensible input' in low anxiety situations, containing messages that students really want to hear. These methods do not force early production in the second language, but allow students to produce when they are 'ready', recognizing that improvement comes from supplying communicative and comprehensible input, and not from forcing and correcting production.

In the real world, conversations with sympathetic native speakers who are willing to help the acquirer understand are very helpful.


Stephen Krashen (University of Southern California) is an expert in the field of linguistics, specializing in theories of language acquisition and development. Much of his recent research has involved the study of non-English and bilingual language acquisition. During the past 20 years, he has published well over 100 books and articles and has been invited to deliver over 300 lectures at universities throughout the United States and Canada.

This is a brief description of Krashen's widely known and well accepted theory of second language acquisition, which has had a large impact in all areas of second language research and teaching since the 1980s.

Description of Krashen's Theory of Second Language Acquisition

Krashen's theory of second language acquisition consists of five main hypotheses:

The Acquisition-Learning distinction is the most fundamental of all the hypotheses in Krashen's theory and the most widely known among linguists and language practitioners.

According to Krashen there are two independent systems of second language performance: 'the acquired system' and 'the learned system'. The 'acquired system' or 'acquisition' is the product of a subconscious process very similar to the process children undergo when they acquire their first language. It requires meaningful interaction in the target language - natural communication - in which speakers are concentrated not in the form of their utterances, but in the communicative act.

The "learned system" or "learning" is the product of formal instruction and it comprises a conscious process which results in conscious knowledge 'about' the language, for example knowledge of grammar rules. According to Krashen 'learning' is less important than 'acquisition'. (See here our in-depth analysis of the Acquisition/Learning hypothesis and its implications).

The Monitor hypothesis explains the relationship between acquisition and learning and defines the influence of the latter on the former. The monitoring function is the practical result of the learned grammar. According to Krashen, the acquisition system is the utterance initiator, while the learning system performs the role of the 'monitor' or the 'editor'. The 'monitor' acts in a planning, editing and correcting function when three specific conditions are met: that is, the second language learner has sufficient time at his/her disposal, he/she focuses on form or thinks about correctness, and he/she knows the rule.

It appears that the role of conscious learning is somewhat limited in second language performance. According to Krashen, the role of the monitor is - or should be - minor, being used only to correct deviations from "normal" speech and to give speech a more 'polished' appearance.

Krashen also suggests that there is individual variation among language learners with regard to 'monitor' use. He distinguishes those learners that use the 'monitor' all the time (over-users); those learners who have not learned or who prefer not to use their conscious knowledge (under-users); and those learners that use the 'monitor' appropriately (optimal users). An evaluation of the person's psychological profile can help to determine to what group they belong. Usually extroverts are under-users, while introverts and perfectionists are over-users. Lack of self-confidence is frequently related to the over-use of the "monitor".

The Natural Order hypothesis is based on research findings (Dulay & Burt, 1974; Fathman, 1975; Makino, 1980 cited in Krashen, 1987) which suggested that the acquisition of grammatical structures follows a 'natural order' which is predictable. For a given language, some grammatical structures tend to be acquired early while others late. This order seemed to be independent of the learners' age, L1 background, conditions of exposure, and although the agreement between individual acquirers was not always 100% in the studies, there were statistically significant similarities that reinforced the existence of a Natural Order of language acquisition. Krashen however points out that the implication of the natural order hypothesis is not that a language program syllabus should be based on the order found in the studies. In fact, he rejects grammatical sequencing when the goal is language acquisition.

The Input hypothesis is Krashen's attempt to explain how the learner acquires a second language – how second language acquisition takes place. The Input hypothesis is only concerned with 'acquisition', not 'learning'. According to this hypothesis, the learner improves and progresses along the 'natural order' when he/she receives second language 'input' that is one step beyond his/her current stage of linguistic competence. For example, if a learner is at a stage 'i', then acquisition takes place when he/she is exposed to 'Comprehensible Input' that belongs to level 'i + 1'. Since not all of the learners can be at the same level of linguistic competence at the same time, Krashen suggests that natural communicative input is the key to designing a syllabus, ensuring in this way that each learner will receive some 'i + 1' input that is appropriate for his/her current stage of linguistic competence.

Finally, the fifth hypothesis, the Affective Filter hypothesis, embodies Krashen's view that a number of 'affective variables' play a facilitative, but non-causal, role in second language acquisition. These variables include: motivation, self-confidence and anxiety. Krashen claims that learners with high motivation, self-confidence, a good self-image, and a low level of anxiety are better equipped for success in second language acquisition. Low motivation, low self-esteem, and debilitating anxiety can combine to 'raise' the affective filter and form a 'mental block' that prevents comprehensible input from being used for acquisition. In other words, when the filter is 'up' it impedes language acquisition. On the other hand, positive affect is necessary, but not sufficient on its own, for acquisition to take place.

The Role of Grammar in Krashen's View

According to Krashen, the study of the structure of the language can have general educational advantages and values that high schools and colleges may want to include in their language programs. It should be clear, however, that examining irregularity, formulating rules and teaching complex facts about the target language is not language teaching, but rather is "language appreciation" or linguistics.

The only instance in which the teaching of grammar can result in language acquisition (and proficiency) is when the students are interested in the subject and the target language is used as a medium of instruction. Very often, when this occurs, both teachers and students are convinced that the study of formal grammar is essential for second language acquisition, and the teacher is skillful enough to present explanations in the target language so that the students understand. In other words, the teacher talk meets the requirements for comprehensible input and perhaps with the students" participation the classroom becomes an environment suitable for acquisition. Also, the filter is low in regard to the language of explanation, as the students" conscious efforts are usually on the subject matter, on what is being talked about, and not the medium.

This is a subtle point. In effect, both teachers and students are deceiving themselves. They believe that it is the subject matter itself, the study of grammar, that is responsible for the students" progress, but in reality their progress is coming from the medium and not the message. Any subject matter that held their interest would do just as well.