Input Hypothesis

The input hypothesis is a theory of second language acquisition developed by linguist Stephen Krashen from the late 1970s and set out most fully in his 1985 book The Input Hypothesis: Issues and Implications, proposing that a language learner acquires a new language by being exposed to comprehensible input, language slightly beyond their current level of competence, often denoted i+1, rather than by direct grammar instruction or conscious practice of rules. Krashen argued that acquisition, the largely unconscious process by which a first language is naturally absorbed, is distinct from learning, the conscious study of grammatical rules, and that only comprehensible input drives genuine acquisition, with conscious learning serving mainly to help a speaker monitor and edit output. The hypothesis was highly influential in shaping communicative and comprehension-based approaches to language teaching from the 1980s onward, including approaches that prioritize extensive listening and reading over explicit grammar drills, though critics have long noted the hypothesis is difficult to test empirically because comprehensible input and the level i+1 are not independently measurable.