The input hypothesis is a hypothesis in second language acquisition developed by Stephen Krashen, which states that a language learner gains the most benefit from receiving linguistic input that is just beyond his or her current interlanguage, or level of grammatical understanding. This type of input is known as comprehensible input or "i + 1," where "i" refers to the learner's interlanguage. According to Krashen, comprehensible input is most likely to be gained from interacting with another speaker of the language.
In some respects, the input hypothesis is fairly intuitive. Someone who understands only a few basic phrases of Chinese will not derive much benefit from listening to a scientific discourse in Chinese since it will be incomprehensible. Likewise, someone who is nearly fluent in Italian will not gain much grammatical knowledge from a child's picture book, because it will not introduce any new grammatical features.
Krashen, however, draws on more complex theories of second language acquisition to make his claim. The interlanguage hypothesis states that learners acquire the grammatical features of a language in a predictable order, and that at any given time the learner has an internally consistent grammatical framework known as the interlanguage. As the learner progresses, the interlanguage becomes increasingly similar to the target language's actual grammar. The input hypothesis states that input one stage closer to the target language — or i + 1 input — helps the learner acquire the next set of grammatical features. It is not enough, however, for a learner to receive comprehensible input passively; he or she must then analyze the new data in order to move the interlanguage forward.
The input hypothesis states that the best way for learners to gather comprehensible input is through a sort of trial-and-error process of communication. The learner seeks out conversation partners, who modify their speech until it becomes comprehensible to the learner. This process can be aided by nonverbal communication, such as by gestures and by feedback from the learner. When the process is successful, the learner's interlanguage grows to accommodate new grammatical features that he or she has observed.
Krashen claims that output, or speech production, has little to no relevance in second language acquisition. Many other researchers have criticized this position, however, saying that more advanced language learning and syntactic processing have to come through the trial-and-error process not only of comprehending, but also of speaking. Language output allows the speaker to test out his or her grammatical hypotheses and modify them if communication is not successful.