Part III – The Input Hypothesis

The Input hypothesis is Krashen’s bread and butter.  If you’ll recall, we talked about the difference between acquisition and learning.

But how do you “acquire” a language rather than “learn” it?

If Krashen’s theories are correct, that question is the vital question. Answer it and you’ll have the key to knowing and speaking another language.

Krashen’s answer: comprehensible input.

(For an explanation of this series on the second language acquisition theories of Stephen Krashen, see this post. Also, I went straight to the horse’s mouth and got the information from “Principles and Practice in Second Language Acquisition,” which is freely available on Krashen’s website.)

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The Input hypothesis states that a student acquires a language by being exposed to comprehensible input in that language.

What’s comprehensible input? Basically, it’s material–stuff that you listen to or read–that you are able to understand. This idea is often expressed as i + 1, where i is our current level of competence. If you are exposed to language structures that are slightly above your current level of competence, you’ll acquire it.

Furthermore, the student’s focus shouldn’t be on the language structures themselves–the grammar or vocab–but on the communication. If the input is a story, the focus should be on understanding what’s going on and being entertained. If it’s part of a game, such as Simon Says, the focus should be on trying to win.

Here’s the most controversial part of this: Krashen believes that after you’ve been exposed to enough input and have in turn acquired enough language, speaking will come naturally. Trying to speak before you’re ready, he believes, is unnatural, unnecessary, and perhaps even counterproductive.


The implications are huge. If you think about it, what Krashen is saying is that real acquisition–i.e., really knowing a language–comes as a result of one thing and one thing only: exposure to language that you can understand.

That’s it. Grammar instruction? Might help learning, but not acquisition. Drills? Same. Speaking practice? Nope.

These are cornerstones of traditional language classrooms, and Krashen is saying their effects are minimal at best and harmful at worst.

The job of the teacher (or yourself, if you’re a self-learner like myself) is to provide comprehensible input and the job of the student is to pay attention. Then when the student is ready, he or she will talk.

My Take

It might be tempting to nitpick the i+1 formula, but to Krashen’s credit, he doesn’t want people getting too rigid with all of this. He offers a lot of suggestions in his writings:

  • Teachers shouldn’t try to provide only i+1 material. If there is input text that the student actually understands, then enough i+1 content will be mixed in to promote acquisition.
  • Teachers should focus on providing “roughly-tuned input” for the class, meaning that they give a best guess on what the students need. Children learned their first language from their caretakers, who didn’t obsess over the structure of the language.
  • Teachers shouldn’t teach content according to a grammatical hierarchy (as one might identify using the natural order hypothesis). There are a lot of reasons for this, such as the fact that it’s unnatural, it takes the focus off of communication and on the text’s structures, and that you risk exposing students to a particular grammatical construction only once.

That’s great advice, but is the Input hypothesis valid in the first place?

From my personal observations, you can’t go wrong exposing yourself to more of the foreign language. There’s absolutely something magical that happens when you listen to and read a lot of texts. Your brain puts things together, and soon you get that gut sense of how a language works.

I do take issue with a few things, though.

First, as I discussed before, I don’t think that acquisition and learning are completely distinct. As I discussed in an earlier post in this series, I know firsthand that learning can lead to acquisition. I definitely wouldn’t get rid of grammar or vocab instruction altogether.

I wouldn’t get rid of lecture altogether.
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Second, speaking–as a skill–is distinct from listening and reading. It has its own challenges that make it uniquely difficult.

No, you can’t acquire grammar or vocabulary by speaking. And you’ll need that acquired grammar and vocabulary to speak. But speaking requires you to make a link between your mouth and the acquired knowledge in your brain, and that requires practice actually speaking.

Wrapping Up

If you put your textbooks in storage and spent the next three months listening to and reading foreign language material that you understand, would your language improve?

I think it would, yes. In fact, I think that you would get much, much better results from that than you would if you only used the textbook. And I could understand why someone would make comprehensible input the bedrock of their language-training schedule, because that’s similar to what I do in my own studying.

But why limit yourself? Do it all. Get the comprehensible input in. Get some formal “learning” in. Get some speaking practice in.

Then you’ll really start seeing some results.

  • Pingback: Series Introduction: Stephen Krashen’s Theory of Second Language Acquisition ← Language Surfer

  • Seb Falk

    Thanks for this – an interesting and clear summary of a subject I had a vague notion of, but whose implications I hadn’t properly thought through. I’ve drawn on (and linked to) it in the blog post I’ve just written about my experiences learning Arabic in Amman – you can read it at if you’re interested.

    Comments welcome – and advice about learning Arabic, of course!

    • Ron G.

      Hey Seb, thanks for the note and the link to your excellent blog post. (Looking forward to checking out more of your whole site soon.) In particular, this really stood out: “But it’s surely true to say that for a foreign-language learner, a solid
      knowledge of grammar can only help with language acquisition.”

      I think you’re right. It really gets confusing on where the “learning” ends and the “acquisition” begins. Lately, I would say in my own studies that explicit vocab and grammar “learning” are great for making texts comprehensible, and then consuming those texts as naturally as possible is great for acquiring language. …I go back and forth as I reexamine this, so we’ll see how I feel about that in another couple months or so…