Input Hypothesis

Krashen's Input Hypothesis (1977, formalized in The Input Hypothesis: Issues and Implications, 1985) is the engine of the Monitor Model. It claims that acquisition occurs when learners receive comprehensible input containing structures slightly beyond their current competence, expressed as i+1. Speaking emerges as a result of acquisition; producing output plays no causal role.

The i+1 Formula

i represents the learner's current state of interlanguage competence. The "+1" denotes structures one increment beyond i in the acquisition sequence. Learners parse i+1 by combining linguistic knowledge with context, world knowledge, and extra-linguistic cues (visuals, gesture, situation). Acquisition happens automatically when meaning is grasped despite the unfamiliar form.

Two conditions are necessary:

• The message must be understood.
• The input must contain forms beyond current competence so there is something new to acquire.

Krashen further argues that i+1 is delivered automatically whenever learners receive enough comprehensible input in meaningful contexts. Teachers do not need to grade input precisely: sufficient quantity guarantees that i+1 will be encountered.

Strong Form: Input Is Necessary AND Sufficient

The hypothesis makes a maximalist claim:

Claim	Implication
Input is necessary	No acquisition without exposure (uncontroversial)
Input is sufficient	Output, instruction, and correction are unnecessary (highly contested)
Acquisition is subconscious	Conscious learning cannot become acquisition (the non-interface position)
Speaking is a result	Production emerges when readiness is reached; forcing output is futile

The strong form is what most researchers reject. The weak form, that comprehensible input is necessary, is broadly accepted across SLA frameworks.

Operating Conditions

Even Krashen's own model imposes constraints on the hypothesis:

• The Affective Filter must be low. Anxiety, low motivation, or poor self-image block input from being processed.
• The input must be at i+1, not i+5. Input far above current competence is incomprehensible noise.
• The input must be compelling (Krashen's later refinement): interesting enough that the learner forgets they are processing a foreign language.

Criticisms

• Unfalsifiability. If a learner fails to acquire after extensive input, the theory blames an opaque Affective Filter or claims the input was not truly comprehensible. This makes the hypothesis difficult to disprove.
• Vagueness of i+1. Krashen never operationalized how to identify i or measure "+1." Without a mapped acquisition sequence, calibration is impossible.
• The output evidence. Swain's (1985) Output Hypothesis emerged from Canadian French immersion data showing that learners with massive comprehensible input still produced persistently inaccurate language. Input was not sufficient.
• The interaction evidence. Long's Interaction Hypothesis argues that negotiation during interaction, not bare input, drives acquisition by tailoring input to the learner's level at the moment of need.
• The noticing problem. Schmidt's Noticing Hypothesis directly contradicts the subconscious claim: input must be consciously noticed to become intake.
• Passive learner model. Treats acquisition as absorption rather than active, embodied, social construction.

Legacy

Despite the criticisms, the Input Hypothesis reshaped language teaching. It powered the shift from grammar-translation and audiolingualism toward meaning-focused approaches, extensive reading and listening programs, sheltered instruction, and methods like TPRS. Most contemporary SLA frameworks treat input as one essential ingredient alongside output, interaction, noticing, and feedback, a position the strong form would reject but the weak form anticipates.

References

• Krashen, S. (1985). The Input Hypothesis: Issues and Implications. Longman.
• Krashen, S. (1982). Principles and Practice in Second Language Acquisition. Pergamon.
• Swain, M. (1985). Communicative competence: Some roles of comprehensible input and comprehensible output in its development. In Gass & Madden (Eds.), Input in Second Language Acquisition. Newbury House.