Input hypothesis

The input hypothesis, also known as the monitor model, is a group of five hypotheses of second-language acquisition developed by the linguist Stephen Krashen in the 1970s and 1980s. Krashen originally formulated the input hypothesis as just one of the five hypotheses, but over time the term has come to refer to the five hypotheses as a group. The hypotheses are the input hypothesis, the acquisition–learning hypothesis, the monitor hypothesis, the natural order hypothesis and the affective filter hypothesis. The input hypothesis was first published in 1977.

The hypotheses put primary importance on the comprehensible input (CI) that language learners are exposed to. Understanding spoken and written language input is seen as the only mechanism that results in the increase of underlying linguistic competence, and language output is not seen as having any effect on learners' ability. Furthermore, Krashen claimed that linguistic competence is only advanced when language is subconsciously acquired, and that conscious learning cannot be used as a source of spontaneous language production. Finally, learning is seen to be heavily dependent on the mood of the learner, with learning being impaired if the learner is under stress or does not want to learn the language.

Krashen's hypotheses have been influential in language education, particularly in the United States, but have received criticism from some academics. Two of the main criticisms state that the hypotheses are untestable, and that they assume a degree of separation between acquisition and learning that has not been proven to exist.

The five hypotheses that Krashen proposed are as follows:

If i represents previously acquired linguistic competence and extra-linguistic knowledge, the hypothesis claims that we move from i to i+1 by understanding input that contains i+1. Extra-linguistic knowledge includes our knowledge of the world and of the situation, that is, the context. The +1 represents new knowledge or language structures that we should be ready to acquire.

The comprehensible input hypothesis can be restated in terms of the natural order hypothesis. For example, if we acquire the rules of language in a linear order (1, 2, 3...), then i represents the last rule or language form learned, and i+1 is the next structure that should be learned. It must be stressed, however, that just any input is not sufficient; the input received must be comprehensible. According to Krashen, there are three corollaries to his theory.

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