WordieWhat Is the Input Hypothesis?
Input Hypothesis: Stephen Krashen's theory that people acquire a language by understanding messages slightly beyond their current level (i+1), and that speaking ability emerges from this comprehension rather than being taught directly.
Also known as: i+1, Krashen's input hypothesis
The input hypothesis is a theory of second language acquisition proposed by Stephen Krashen. It states that we acquire language in one way: by understanding messages that contain structures a little beyond our current level, written as i+1. On this view, speaking is a result of acquisition, not its cause, and fluency "emerges" once enough comprehensible input has been processed.
Where the input hypothesis comes from
Krashen developed the idea in the late 1970s and presented it in Principles and Practice in Second Language Acquisition (1982) and The Input Hypothesis: Issues and Implications (1985). It is one of five linked hypotheses often called the Monitor Model:
| Hypothesis | Core claim |
|---|---|
| Acquisition-learning | Subconscious acquisition and conscious learning are separate systems. |
| Natural order | Grammatical features are acquired in a broadly predictable order. |
| Monitor | Consciously learned rules only act as an editor on output, when there is time. |
| Input | Acquisition happens through understanding input at i+1. |
| Affective filter | Anxiety and low motivation block input from being acquired. |
Input hypothesis vs comprehensible input
Comprehensible input is the material: a text or conversation the learner understands. The input hypothesis is the theory about why that material matters, including the strong claim that it is sufficient. Krashen also argued that if input is understood and plentiful, i+1 is supplied automatically, so teachers do not need to target the next structure deliberately.
An example of i+1
A learner can use the simple past ("I walked") but has not acquired the present perfect. They hear:
"Have you ever eaten sushi? I've eaten it twice, once in Tokyo and once in London."
The situation, the familiar verb eat and the words ever and twice make the meaning clear. Under the input hypothesis, repeatedly understanding sentences like this, rather than memorizing the rule have + past participle, is what eventually lets the learner produce the form.
Why it matters for vocabulary learning
The input hypothesis gave strong theoretical support to reading and listening as the main way to grow vocabulary, and it influenced extensive reading programs and immersion-style teaching. Vocabulary research largely agrees that input is crucial: many words are learned incidentally through repeated encounters in context (Nagy, Herman and Anderson 1985). It also shows the limit. Incidental gains per encounter are small, and studies with graded readers found that many new words are forgotten unless met again (Waring and Takaki 2003). For adults with goals and deadlines, input plus some deliberate study is usually faster than input alone.
Criticisms and limitations
- Hard to test. Neither i nor +1 is defined precisely enough to measure, so critics such as McLaughlin (1987) and Gregg (1984) argued the hypothesis cannot be falsified.
- Output matters. Swain (1985) observed that Canadian French immersion students understood a great deal but still made persistent errors, and proposed that producing language pushes learners to notice gaps.
- Interaction matters. Long's interaction hypothesis holds that negotiating meaning in conversation (asking for clarification, rephrasing) makes input more useful.
- Noticing matters. Schmidt's noticing hypothesis (1990) argues that learners acquire forms they consciously notice, which challenges the sharp split between acquisition and learning.
- Explicit instruction helps. Later meta-analyses of form-focused instruction (for example Norris and Ortega 2000) found that explicit teaching produces real, measurable gains.
How to apply it sensibly
- Make input the base of your routine. Read and listen daily to material you mostly understand.
- Aim for i+1 in practice. A rough guide for reading is knowing about 95–98% of the words. Check a sample with the lexical coverage calculator.
- Add noticing. When a new word or pattern recurs, save it and look at how it is used.
- Add output and review. Speak, write and test yourself with active recall so that understood words become usable ones.
For a broader overview, read comprehensible input explained.
Sources: Krashen, S. (1982) Principles and Practice in Second Language Acquisition; Krashen, S. (1985) The Input Hypothesis: Issues and Implications; Gregg, K. (1984) Krashen's monitor and Occam's razor, Applied Linguistics; McLaughlin, B. (1987) Theories of Second-Language Learning; Swain, M. (1985) Communicative competence: some roles of comprehensible input and comprehensible output; Schmidt, R. (1990) The role of consciousness in second language learning, Applied Linguistics; Norris, J. & Ortega, L. (2000) Effectiveness of L2 instruction, Language Learning; Waring, R. & Takaki, M. (2003) Reading in a Foreign Language.
Frequently asked questions
What does i+1 mean?
In Krashen's notation, i is the learner's current level of competence and +1 is the next step. Input at i+1 contains language slightly beyond what the learner already knows but is still understandable from context.
Who proposed the input hypothesis?
Stephen Krashen, an American linguist. He set it out as part of his Monitor Model in Principles and Practice in Second Language Acquisition (1982) and developed it in The Input Hypothesis: Issues and Implications (1985).
What are the main criticisms of the input hypothesis?
Critics argue that i+1 cannot be defined or measured, so the theory is hard to test; that output and interaction also drive learning (Swain, Long); and that learners need to notice language forms, not only understand meaning (Schmidt).
Is the input hypothesis still accepted?
Its core insight, that large amounts of understandable input are essential, is widely accepted and shapes modern teaching. Its strong claim that input is the only cause of acquisition is not accepted by most second language researchers.
Related terms
Comprehensible Input
language that a learner can understand, mostly from context and prior knowledge, even though it contains some words or structures they do not yet know.
Affective Filter
a hypothesized mental barrier, raised by anxiety, low motivation or low self-confidence, that prevents a learner from fully acquiring language from input they otherwise understand.
Extensive Reading
an approach to language learning in which you read large amounts of easy, self-chosen material quickly and for enjoyment, focusing on overall meaning rather than on studying every word.
Zone of Proximal Development
the gap between what a learner can do independently and what they can do with guidance from a teacher or more capable peer; Vygotsky saw learning in this zone as the driver of development.
Incidental Vocabulary Learning
the learning of new words as a by-product of doing something else, such as reading, listening or watching for meaning, without deliberately setting out to memorize them.
Articles about Input Hypothesis
Sentence Mining: The Complete Guide to Learning Vocabulary from Real Sentences
Sentence mining explained: how to find i+1 sentences, the best sources, card formats that work, and a full workflow from browsing to Anki via CSV export.
Comprehensible Input Explained: Krashen's Theory, Its Critics, and What Actually Helps Learners
Comprehensible input explained: Krashen's input hypothesis, the main criticisms, what research supports, and how to use extensive reading to grow vocabulary.
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