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 langauge acquisition particularly in the United States, but have received criticism from some academics. Two of the main criticisms are that the hypotheses are untestable, and that they assume a degree of separation between acquisition and learning that has not been proven to exist.
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 reviews various studies which indicate that about 98% of the words in running text should be previously known in order for extensive learning to be effective.