The input hypothesis is one of five hypotheses in linguist Stephen Krashen’s Monitor Model of second-language acquisition, developed during the 1970s and 1980s.
It proposes that people acquire language when they understand spoken or written input containing language slightly beyond their current competence. Krashen represented this as i + 1: i is the learner’s current competence, while + 1 represents the next language features available for acquisition.
The input hypothesis gives comprehensible input a central role in subconscious language acquisition. The stronger claim that comprehensible input is the only mechanism that produces acquisition belongs to Krashen’s broader theory and remains debated in second-language acquisition research.
Krashen’s five hypotheses
The input hypothesis
Learners develop their language competence when they understand input containing language slightly beyond their current level. The input must be comprehensible; simply encountering unfamiliar language is not sufficient.
The acquisition–learning hypothesis
Krashen distinguishes acquisition, a subconscious process resembling first-language development, from learning, the conscious study of rules. In the model, learned knowledge does not become acquired competence.
The monitor hypothesis
Consciously learned knowledge can act as a monitor that checks or edits language output when a learner has enough time, knows the relevant rule, and focuses on correctness. It does not generate spontaneous language in the model.
The natural order hypothesis
Grammatical features are acquired in a broadly predictable order that cannot be directly controlled by the sequence of explicit instruction. This does not mean every learner progresses identically or that all language features have one fixed universal order.
The affective filter hypothesis
Emotional factors can affect how readily learners use the input available to them. Anxiety, embarrassment, low confidence, or low motivation are said to raise the affective filter, making acquisition less likely even when the input is understandable.