Comprehensible Output

The comprehensible output hypothesis, developed by Merrill Swain, is a theory of second language acquisition holding that learning takes place when a learner encounters a gap between what they want to say in the second language and what they are actually able to say, and, by noticing that gap, becomes aware of it and may modify their own output to learn something new about the language. Swain does not claim comprehensible output is solely or even mostly responsible for language acquisition, but argues that under some conditions it facilitates second language learning in ways that differ from and enhance input, because of the mental processes connected with actually producing language. She defines three functions served by output: a noticing function, in which learners encounter gaps between what they want to say and what they are able to say; a hypothesis-testing function, in which uttering something tests a tacit hypothesis, such as about grammar, and draws feedback that can be reprocessed; and a metalinguistic function, in which reflecting on the language being learned lets learners control and internalize linguistic knowledge.