Comprehensible Input


In Part 1 of this article we addressed the Acquisition-Learning Hypothesis, the Natural Order Hypothesis, and the Monitor Hypothesis.  However, it is the final two hypotheses – the Input Hypothesis and the Affective Filter Hypothesis – which have the most practical application for both language acquisition and language teaching.  It is to these hypotheses that we turn our attention now.

The Input Hypothesis

The Input Hypothesis addresses the question, “How do we acquire language?”  Notice that it relates to language acquisition, not learning.  In a nutshell, the input hypothesis states that that “we acquire … only when we understand language that contains structure that is ‘a little beyond’ where we are now.”  It’s often stated mathematically that if i represents our current stage of language competence, then to move to stage i+1 requires understanding input that contains i+1.

Consider just how radical this suggestion is.  As Krashen puts it:

The input hypothesis runs counter to our usual pedagogical approach in second and foreign language teaching.  … our assumption has been that we first learn structures, then practice using them in communication, and this is how fluency develops.  The input hypothesis says the opposite.  It says we acquire by ‘going for meaning’ first, and as a result, we acquire structure!

This hypothesis further states suggests that “speaking fluency cannot be taught directly,” but rather emerges when the acquirer reaches a state of readiness.  “The best way, and perhaps the only way, to teach speaking, according to this view, is simply to provide comprehensible input.”

Comprehensible InputComprehensible input is simply speech we hear or text we read in our new language that we can understand by drawing upon our current skills plus deductions from context.  Consider, for example, the way that parents and caretakers speak with young children: speech is simplified to help them understand.  Similarly, when speaking with a foreigner who struggles with English, we may simplify our speech seeking to express ourselves roughly at the level of the listener.  The input hypothesis predicts that this type of “natural, communicative, roughly-tuned, comprehensible input” will actually be more helpful than exercises designed to teach a particular point of grammar.

Krashen points out several problems with typical, grammatically organized language syllabi.  This is not to say that students make no progress with typical deductive methods focused on grammar and translation.  However, the input hypothesis predicts “that an approach that provides substantial quantities of comprehensible input will do much better.”

The Affective Filter Hypothesis

Krashen begins by pointing to research confirming the effects in second language acquisition of affective variables such as motivation, self-confidence, and low anxiety levels.  In other words, our attitudes and emotional states affect our ability to acquire language.  In particular, our affect, it is suggested, works like a filter.

Consider how a filter works, an air filter for example.  An air filter necessarily slows down (never speeds up) the flow of air, and some particles are simply stopped and never get through at all.

Affective Filter Diagram

According to the affective filter hypothesis, those with low motivation, low self-confidence, and high anxiety level have a high affective filter.  “Even if they understand the message, the input will not reach the part of the brain responsible for language acquisition.”  By contrast, those with more optimal attitudes have a lower filter, are more open to input, and that input has a deeper impact.

Conclusions for Language Acquisition and Effective Teaching

Comprehensible input paired with a low affective filter (high motivation and low anxiety) are the keys to second language acquisition, and language acquisition is more important than language learning.

If you’re seeking to acquire fluency in a new language and are strongly motivated, then seek out all the comprehensible input you can.  That is, focus on listening and reading – especially when you can understand some of what you hear/read, but still find it challenging.  It’s okay to delay speaking; it comes after understanding.  Of course, eventually, the more speaking you do, the more fluent you will become.

As Glossika founder Michael Campbell points out, “Glossika courses start with the assumption that you have basic understanding already, and you’re looking to build upon it. Glossika is also an excellent way to recover/revive or re-learn a language you studied in school many years ago.”  So, “If you have a strong reason to learn a language and are strongly motivated, then go for it!”  Check out our review of Glossika, and take advantage of their free trial.


  • Stephen D. Krashen, Principles and Practice in Second Language Acquisition, 1982 (first internet edition 2009), chapter 2 “Second Language Acquisition Theory.” All quotes reference this source.
  • Michael Campbell, “Second Language Acquisition vs Learning,” Glossika Blog September 3, 2015.


  1. Good study of language learning theory and skills. I can appreciate the motivation factors discussed. It looks like you offer a clear explanation of the learning methods that would give anyone with a desire to learn another language a good way to assess the method that would work best for them. A well thought out and informative website.

  2. This is a very informational article. My native language is not English, hence, speaking with westerners who are obviously very fluent in English, can sometimes be nerve-wracking.

    I did not know that anxiety will affect the filter in our brain. But it kinda makes sense to me, because some people who are not very sociable, get really anxious and thus, their speech suffers. Or am I wrong to assume this?

    Thanks for this great article!

    1. That’s exactly the case. Our anxiety (and other aspects of our affective state) affect our ability to acquire language fluency and our ability to express what we have already acquired.

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