The Input Hypothesis
The input hypothesis is Stephen Krashen's theory that a second language is acquired in only one way — by understanding messages slightly above the learner's current level (i + 1) — and that comprehension, not output or grammar study, is what causes acquisition.
The input hypothesis is one of five hypotheses in Krashen's broader theory of second-language acquisition. It draws a sharp line between acquisition — the subconscious process by which we pick up language from understanding it — and learning — the conscious study of rules. Krashen argues that only acquisition produces fluent, spontaneous use.
The mechanism is comprehension of i + 1 input: language a bit beyond your current ability, made understandable by context, gestures, pictures, and prior knowledge. When you focus on meaning and succeed in understanding, the new structures are acquired as a by-product — you do not have to consciously notice them.
A practical consequence is that speaking is a result of acquisition, not its cause. Krashen claims fluency "emerges" once enough comprehensible input has been absorbed, which is why input-based approaches prioritise listening and reading before pushing learners to produce.
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FAQ
What is the input hypothesis?
It is Stephen Krashen's claim that we acquire a second language by understanding input that is a little beyond our current level (i + 1), and that comprehension — not speaking or grammar drills — is what causes acquisition.
What is the difference between acquisition and learning?
Acquisition is the subconscious process of absorbing language by understanding it; learning is the conscious study of rules. Krashen argues only acquisition leads to fluent, automatic use.
Does the input hypothesis say speaking is unimportant?
It says speaking is a result of acquisition rather than its cause. Output and interaction help, but fluency emerges from large amounts of comprehensible input first.