Stephen Krashen's Theory of Second Language Acquisition
"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." Stephen Krashen
Introduction
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 hypothesis：● the Acquisition－Learning hypothesis
● the Monitor hypothesis
● the Natural Order hypothesis
● the Input hypothesis
● and the Affective Filter hypothesis
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.
The Acquisition－Learning hypothesis
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'.
the Monitor hypothesis
The Monitor hypothesis encapsulates the relationship between acquisition and learning and defines the role of 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
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. 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%, 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
The Input hypothesis is Krashen's attempt to explain how the learner acquires a second language. In other words, this hypothesis is Krashen's explanation of how second language acquisition takes place. So, 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.The Affective Filter hypothesis
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.Krashenの五つの仮説（TESOLより）
(a) 習得と学習の仮説（Acquisition-Learning Hypothesis）
習得とは「言語を身につける」ことであり，一方，学習は「言語について知る」ことである。誤りを訂正するのは，無意識のうちの習得には何ら影響（効果）がないが，意識的な学習には有効である。(b) 自然習得順序（Natural Order Hypothesis）
文法構造の習得にも順序があり，例えば，進行形のingと複数のsは最初に習得される形態素で，三人称・単数・現在のｓと所有のｓはだいぶ後になってから習得される。(c) モニター仮説（Monitor Hypothesis）
学習はモニターの役割しか果たさず，モニターする力はあるが限られた効果しかない。モニター使用にも個人差があるが，使いすぎる人は，文法教育だけを受けた犠牲者で，正確に話すことを強く意識しすぎ，流暢に話すことができない。入試のために文法ばかりを学んでいる日本人のことを話しているようだ。逆に，モニターをほとんど使わない人は，習得法だけに頼り，誤りを訂正されても何ら影響を受けない。書いたり，演説の草案のときにはモニターを使用するが，日常会話のときにはモニターを使わないという人が，一番理想的ではないだろうか。
(d) インプット仮説（Input Hypothesis）
クラッシェンは言語習得には，理解可能なインプット(comprehensible input)が必要であると述べている。そのインプットが，学習者の言語能力よりもやさしい場合は無駄だが，すこし難しいものは大変効果的だ。学習者が言葉として表現できない時期を経て，ある日アウトプットされ，それが習得という形で表れてくる。使える言語を教えるには，この理解可能なインプットをどんどん与えるのが効果的な方法だ。しかし，ここで環境が問題になってくる。どこを向いても日本語という世界で，第二言語のインプットは難しいことである。教室内でのインプットだけという場合もある。しかし，学習者にとって教室こそ理解可能なインプットを与えられる最適な場なのである。
(e) 情意フィルター仮説（Affective Filter Hypothesis）
デュレイとバートが１９７７年に情意フィルターの概念を提示し，第二言語習得にいかに関連しているか述べている。動機付けも自信もあり，不安のない人ほどフィルターが低く，第二言語習得に適している。すばらしい語学の先生とは，学生に不安を与えずに理解可能なインプットをどんどん送り込む人である。英語を母語としていない日本では，英語に接する機会がほとんどない。日本の英語教育は，入学試験という大きな障壁があるため，言語学習が中心となり，言語習得に重点が置かれていない。
しかし，教室こそインプットが与えられる最良の場なのだから，文法事項の暗記や文型練習，模倣だけを叩き込む学習ではなく，あくまでもコミュニケーションを行う場として，言語習得の方向に改善されていかなくてはならない。