Monitor theory

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Not to be confused with the 'monitor hypothesis', one component of monitor theory.[1]

Monitor theory comprises five hypotheses about second language acquisition (SLA)[2] developed by Stephen Krashen: the acquisition-learning hypothesis; the monitor hypothesis; the natural order hypothesis; the input hypothesis; and the affective filter hypothesis. Each hypothesis relates to conditions that are necessary for subconscious emergence of language to take place, and also assume that conscious learning can improve communication but does not lead to true acquisition.

Krashen's "five hypotheses"

Acquisition-learning hypothesis

Monitor theory, as defined by Krashen, distinguishes two processes that enable learners to develop their language ability: subconscious acquisition and conscious learning. Acquisition takes place subconsciously and instinctively, with the user developing true competence in the structures of the new language as they are exposed to and interact with it; it is only in this way that 'input' can become actual 'intake'. Learning, meanwhile, can only be used for 'monitoring' (see below).[3]

Monitor hypothesis

Language learners may very well experience formal teaching and learning of the target language, such as study of grammar, rote memorisation, or exam tests where thinking about the structure of the language is required. Such conscious learning, according to Krashen, is only available as a 'monitor', i.e. learners can consciously 'edit' their 'output' (utterances or written work) to make themselves more fluent or comprehensible, based on what they have formally learned about the second language. This, however, has no effect on subconscious, true acquisition, and in its absence, output will be less 'accurate' or native speaker-like.[4]

Natural order hypothesis

This hypothesis points towards a fairly fixed sequence of acquisition that adult language users go through when monitoring does not interfere much, and which is closer to first language acquisition by children. For example, grammatical morphemes appear to emerge in a particular order, with e.g. in English the -ing ending being produced earlier than the third person singular -s. Research reports some acquisition order consistency in support of this hypothesis.[5]

Input hypothesis

Krashen states that, fundamentally, there is only one way to acquire language: through 'comprehensible input'. This means that the linguistic material that the learner experiences has to be both processable and slightly beyond their current level of acquisition: Krashen refers to this level as i + 1 ,where i is the current level of proficiency. The comprehension and processing of meaningful 'input' is therefore the priority for the early learner, while production-based activities, such as an early focus on speaking, may inhibit this. Similarly, a focus on the form of language, such as detailed discussion and practice of grammar rules, as opposed to a focus on meaning, may block comprehensible input from becoming intake.[6]

Affective filter hypothesis

The learner's attitude to the target language, their learning situation and the amount of stress they experience all have an impact on the extent of acquisition. A learner who is comfortable with their learning has a low 'affective filter', meaning that they are receptive to new input and more likely to convert it to intake. Pressure to perform, on the other hand, sets the filter high, blocking new acquisition. Krashen sees this phenomenon as one explanation for why second-language attainment varies a great deal from learner to learner, even in the same environment.[7]


Though monitor theory would broadly appear to be supported by many linguists and teachers - e.g. that lots of input is necessary, that there is a difference between acquisition and learning, etc.[8], it has also been strongly criticised due to the prevailing mood in applied linguistics that learned knowledge does form part of true acquisition, and that more than just comprehensible input is necessary or helpful.[9]


  1. Authors often refer to the five hypotheses as monitor theory, e.g. VanPatten and Williams (2015: 25-33); Markee (1997: 25), though Stephen Krashen rarely uses the term in his books. He does refer to them as one model, however, e.g. Krashen and Terrell (1983: 26), and in academic discussion he has explicitly used the term monitor theory to mean the five hypotheses, e.g. Krashen (1981b: 219-220).
  2. e.g. Krashen (1981a); see also VanPatten and Williams (2015: 25-33); Markee (1997: 25).
  3. Krashen (1981a: 1-2).
  4. Krashen (1981a: 1-2); VanPatten and Williams (2015: 26).
  5. e.g. Larsen-Freeman (1975).
  6. VanPatten and Williams (2015: 26-27).
  7. VanPatten and Williams (2015: 27).
  8. See e.g. Scrivener (2005:19), a handbook for teachers which points out that the alternative of a strong focus on explicit instruction has proved of little help to beginning learners.
  9. See Gregg (1984) for a strong critique.