Input Hypothesis

SLA

The Input Hypothesis, proposed by Stephen Krashen, claims that language is acquired when learners understand messages slightly beyond their current level. This "comprehensible input" is the only causative factor in acquisition—not grammar study, not practice, not output.

The i+1 Formula

i = current competence level +1 = input just beyond that level

Learners progress by understanding input containing structures slightly ahead of their current stage. Context, background knowledge, and extralinguistic cues help make this input comprehensible.

Core Claims

Comprehensible input is necessary: No acquisition without understanding
Comprehensible input is sufficient: Understanding alone causes acquisition
Output is not necessary: Speaking is a result, not a cause, of acquisition
Grammar instruction has limited value: Conscious learning can't become acquisition

Part of Krashen's Five Hypotheses

The Input Hypothesis is one of five interconnected hypotheses:

Hypothesis	Claim
Acquisition-Learning	Acquisition (subconscious) and learning (conscious) are separate
Natural Order	Grammatical structures are acquired in a predictable sequence
Monitor	Conscious learning only serves as an editor, not a source
Input	Acquisition comes from comprehensible input (i+1)
Affective Filter	Anxiety, low motivation block input from reaching the LAD

How i+1 Works

Krashen argues we acquire the next structure in the natural order by understanding messages containing it:

Learner is at stage i
Receives input containing i+1
Uses context and knowledge to understand
The Language Acquisition Device processes the input
Learner acquires i+1 subconsciously

The exact "distance" between i and i+1 cannot be too large—input must be only slightly beyond current competence.

Classroom Implications

If the Input Hypothesis is correct:

Do This	Avoid This
Provide abundant comprehensible input	Excessive grammar explanation
Use context, visuals, gestures	Forcing premature production
Allow a silent period	Drilling and mechanical practice
Lower the affective filter	High-anxiety correction
Read extensively	Focus on discrete grammar points

Evidence Cited

Krashen points to:

Children acquire L1 without instruction
Immersion programs outperform grammar instruction
Extensive reading produces acquisition
Silent periods before production are natural

Criticisms

The Input Hypothesis has faced substantial criticism:

Theoretical Problems

Unfalsifiable: i+1 is defined vaguely—can't be tested
Acquisition vs. learning: The distinction is hard to operationalize
No mechanism: Doesn't explain how input becomes knowledge

Empirical Challenges

Output matters: Merrill Swain's Output Hypothesis shows production aids acquisition
Noticing is needed: Richard Schmidt argues conscious attention is necessary
Instruction helps: Research shows grammar teaching can accelerate acquisition

Practical Limits

Providing perfect i+1 for every learner is unrealistic
Some structures may need explicit attention
Learners often want and benefit from grammar explanation

The i+1 vs. ZPD Confusion

Some have equated i+1 with Vygotsky's Zone of Proximal Development. This is incorrect—they are incommensurable constructs:

i+1	ZPD
Property of language	Property of the learner
Individual exposure	Social interaction
Input alone sufficient	Mediation essential

Legacy

Despite criticisms, the Input Hypothesis transformed language teaching by:

Emphasizing meaningful communication over drill
Validating extensive reading
Shifting focus from teaching to providing input
Inspiring communicative and content-based approaches

Stephen Krashen - Developer of the Input Hypothesis
Language Acquisition Device - The innate faculty that processes input
Nativist Theory - Theoretical foundation
Output Hypothesis - Swain's complementary/contrasting view
Interaction Hypothesis - Long's extension of input theory
Natural Approach - Teaching method based on Input Hypothesis
ZPD and i+1 - Incommensurable Constructs - Why i+1 ≠ ZPD