An ELT Glossary : The Monitor Hypothesis

Definition : One of the five hypotheses which make up Krashen's Monitor Model, which he proposed in his work in the seventies and eighties.

For Krashen, the Monitor was part of the distinction between acquisition and learning. Although utterances would be generated from acquired language, this could then be "monitored" for error - ie checked for accuracy and appropriateness - and corrected using learnt knowledge. This, however, requires time, and there may be a payoff between accuracy and fluency - monitoring will improve accuracy but impede fluency.

Criticisms of this depend not so much on what it says - but the reasons for it. It is obvious that, under conditions of high communicative challenge such as having to listen to and understand what other interlocutors say, then think of the reply, formulate it in English and actually say it, mistakes are liable to be made. But this is true of native speakers too and is explained by Chomsky's concepts of competence and performance. The fact that someone "knows" the language doesn't prevent slips and disfluencies from occurring. 

As Scott Thornbury has written :   Stephen Krashen once said (only half-jokingly, I suspect) that, more important than having new ideas is giving old ideas new names. 

Further reading :