Krashen, 40 years on

Krashen’s input hypothesis says we acquire a language by understanding messages in it, input pitched slightly beyond our current level. Four decades of research back the core of that claim: learners need large amounts of comprehensible input, and they pick up a lot from it. The strong versions of Krashen’s claims fare worse: that input is all you need, that conscious learning never turns into acquisition, and that teaching grammar does almost nothing.

The input hypothesis is one of five that Stephen Krashen set out in the late 1970s and early 1980s, most fully in Principles and Practice in Second Language Acquisition (1982), which he now makes freely available. If you want the practical, day-to-day version of the idea, start with comprehensible input, explained. This post is the closer look: each hypothesis, the evidence for it, and the critics who pushed back.

The five hypotheses at a glance

1. The acquisition–learning hypothesis

Krashen distinguished acquisition, the subconscious process by which children pick up their first language, from learning, the conscious knowledge of rules you get from a textbook. His most controversial claim was that the two are separate: learned knowledge does not turn into acquired knowledge, no matter how much you practice it. This is often called the “non-interface” position.

What supports it

The broad distinction has aged well. Modern research talks about implicit and explicit knowledge, and most researchers accept that they’re different. You can know the rule for French gender agreement perfectly and still get it wrong in fast speech, and native speakers use rules they could never state. In their 2021 essay “Was Krashen right? Forty years later” in Foreign Language Annals (doi:10.1111/flan.12552), Karen Lichtman and Bill VanPatten argue that the acquisition–learning distinction lives on today as implicit versus explicit learning, often without credit.

What’s contested

The non-interface claim is where the disagreement lies. “Weak interface” positions hold that explicit knowledge can help acquisition indirectly, for example by helping you notice things in input. “Strong interface” positions, associated with skill-acquisition theory, hold that practice can make explicit knowledge automatic. Kevin Gregg’s early critique, “Krashen’s monitor and Occam’s razor” (Applied Linguistics, 1984, doi:10.1093/applin/5.2.79), argued that the two systems were defined in a way that made the claim impossible to test.

2. The monitor hypothesis

According to Krashen, learned knowledge has one job: acting as an editor, or monitor, that checks and corrects what the acquired system produces. And it only works when three conditions are met: you have time, you’re focused on form, and you know the rule.

This explains something real. People make fewer errors on a written test than in conversation, and grammar knowledge does seem most useful when you can slow down. But most researchers found the hypothesis too narrow and too hard to test directly. It’s rarely tested on its own terms today.

3. The natural order hypothesis

Krashen drew on the morpheme order studies of the 1970s, such as Heidi Dulay and Marina Burt’s “Natural sequences in child second language acquisition” (Language Learning, 1974). These found that learners of English acquired grammatical features like progressive -ing, plural -s and third-person -s in a broadly similar order, whatever their first language. Third-person -s (she walks) comes surprisingly late, even though it’s taught early.

What supports it

The idea of ordered development holds up well. Learners pass through similar stages for structures like negation and questions, and teaching a structure before the learner is ready often doesn’t stick. Lichtman and VanPatten count this as another of Krashen’s ideas that survived.

What’s contested

The original studies had methodological problems: they often measured accuracy at one point in time rather than tracking acquisition. And the order itself turns out to be largely explainable. A meta-analysis by Jennifer Goldschneider and Robert DeKeyser (Language Learning, 2001, doi:10.1111/1467-9922.00147) found that a combination of factors, including how salient a form is in speech, how frequent it is and how complex its meaning is, accounted for much of the order. That doesn’t contradict a natural order, but it suggests an explanation grounded in the input rather than a fixed internal program.

4. The input hypothesis

This is the center of the theory. Krashen argued that we acquire language in one way: by understanding messages containing structures slightly beyond our current level, which he wrote as i+1. Speaking, in this view, emerges from acquisition rather than causing it. And if learners get enough comprehensible input, the next structures they need will be in it automatically.

What supports it

The evidence that input drives acquisition is broad and consistent:

• Extensive reading. In a well-known “book flood” study in Fiji, Warwick Elley and Francis Mangubhai (Reading Research Quarterly, 1983, doi:10.2307/747337) gave primary schools a supply of high-interest illustrated storybooks in English. Children who read them made larger gains than those taught with the standard program. Takayuki Nakanishi’s 2015 meta-analysis in TESOL Quarterly (doi:10.1002/tesq.157) pooled 34 studies of extensive reading and found a medium effect in group comparisons (d = 0.46) and a larger one in pre–post comparisons (d = 0.71).
• Incidental vocabulary learning. Study after study finds that learners pick up words from reading and viewing, as long as they understand what’s going on. The gains from a single encounter are small, which is why volume matters.
• Coverage research. Work on lexical coverage, covered in our post on the 98% rule, shows how strongly comprehension depends on knowing nearly all the words, which fits Krashen’s emphasis on comprehensible.

No mainstream theory of second language acquisition today denies that input is essential. That’s a real win for Krashen.

Criticism: “i+1” can’t be measured

Barry McLaughlin, in Theories of Second-Language Learning (1987), made the most cited criticism: the theory’s key terms were never defined precisely enough to test. What exactly is a learner’s “i”? How would you know if a text is “+1” rather than “+2”? If any learning is explained after the fact by “there must have been i+1 in the input,” the claim can’t be proven wrong. Gregg made a similar point.

This is a fair criticism of i+1 as a scientific claim. It matters less for you as a learner, because “mostly understandable, slightly stretching” is a perfectly good practical guide even if it isn’t a measurable quantity.

Criticism: input isn’t enough (Swain)

Merrill Swain’s challenge came from Canadian French immersion programs, where English-speaking children studied school subjects in French for years. If input were sufficient, these students should have been in great shape. They understood French extremely well, but their speaking and writing remained noticeably less accurate than those of native speakers, even after years of input.

In her 1985 chapter “Communicative competence: Some roles of comprehensible input and comprehensible output in its development” (in Gass and Madden’s Input in Second Language Acquisition), Swain proposed the output hypothesis. When you have to produce language, you discover gaps you can skip over while listening, because comprehension lets you get the meaning from context without processing every grammatical detail. Speaking and writing push you to process the grammar.

Criticism: conversation matters (Long)

Michael Long’s interaction hypothesis didn’t reject input. It refined it. Long’s research in the 1980s looked at conversations between native and non-native speakers, and found that a great deal of input becomes comprehensible through negotiation of meaning: asking for clarification, checking understanding, rephrasing. In his updated version (in Ritchie and Bhatia’s Handbook of Second Language Acquisition, 1996), interaction helps because it connects input, the learner’s attention and output, and gives learners feedback right when they need it.

The interaction hypothesis is arguably closest to Krashen of all the critiques: comprehensible input is still the engine, but conversation is an especially good way to get it and fine-tune it.

Criticism: you learn what you notice (Schmidt)

Richard Schmidt’s “The role of consciousness in second language learning” (Applied Linguistics, 1990, doi:10.1093/applin/11.2.129) drew partly on his own experience learning Portuguese in Brazil. He noticed that forms he’d been hearing for months only started appearing in his speech after he had consciously noticed them in input. His noticing hypothesis claimed that input only becomes “intake” when you notice it. He later softened this to a weaker claim: people learn about what they attend to, and don’t learn much about what they don’t.

This directly contradicts the idea of purely subconscious acquisition. The strong version has been hard to test, because it’s hard to measure whether someone noticed something. But the weaker version, that attention matters, is now widely accepted.

Criticism: teaching grammar does help

Krashen argued that grammar instruction has little effect on acquisition. A large meta-analysis by John Norris and Lourdes Ortega (Language Learning, 2000, doi:10.1111/0023-8333.00136) of 49 studies found the opposite: targeted instruction produced large gains, explicit instruction outperformed implicit instruction, and the effects lasted.

There’s a caveat Krashen’s defenders raise, and Norris and Ortega noted it themselves: many studies used tests that favor explicit knowledge, such as grammar tests, so the effect on spontaneous use may be smaller. Still, the claim that instruction does nothing is hard to sustain.

5. The affective filter hypothesis

Krashen proposed that anxiety, low motivation and low self-confidence raise a mental “filter” that keeps input from reaching the parts of the brain that acquire language.

The evidence that emotions matter is solid. A meta-analysis by Yasser Teimouri, Julia Goetze and Luke Plonsky (Studies in Second Language Acquisition, 2019, doi:10.1017/S0272263118000311) covered almost 20,000 learners and found a moderate negative correlation between language anxiety and achievement (r = −.36).

But correlation runs both ways. Anxious learners may do worse, and learners who are doing worse may become anxious. And the “filter” itself is a metaphor rather than a mechanism anyone has identified. The practical advice survives, even if the model doesn’t: relaxed, interested learners tend to keep going, and keeping going is most of the battle.

Krashen’s input hypothesis today: was he right?

Partly, and in the ways that matter most to self-directed learners.

What holds up:

• Comprehensible input is necessary, and it’s the main source of acquisition.
• Large amounts of reading and listening produce real gains in vocabulary, grammar and fluency.
• Implicit knowledge is different from explicit knowledge, and implicit knowledge is what fluent use depends on.
• Learners develop in a broadly predictable order that teaching can’t easily override.
• Anxiety and boredom get in the way.

What doesn’t:

• That input is sufficient on its own for everyone, for every skill.
• That conscious learning and attention play no role.
• That grammar instruction is essentially useless.
• That “i+1” is a precise, testable quantity.

Most researchers today would describe input as the foundation, with attention, output, interaction and some explicit teaching each adding something on top. Lichtman and VanPatten’s conclusion is a good summary: many of Krashen’s ideas are alive, often under new names, even where his specific formulations were rejected.

What this means for how you learn

If you’re an input-first learner, the research suggests a few adjustments rather than a change of direction:

1. Make input the bulk of your time. Reading and listening at the right level is where most acquisition happens.
2. Pay attention, lightly. Schmidt’s point is easy to apply: notice the words and patterns you meet. Looking up a word, saving a phrase or highlighting a form you keep seeing all count.
3. Use a little explicit knowledge. A quick look at a grammar point can help you notice it in input. It doesn’t replace input.
4. Add output and conversation eventually. Swain and Long both suggest that speaking and writing push you further than input alone, especially for accuracy.
5. Keep it enjoyable. Whatever you call the affective filter, you’ll do more of something you like.

Lumo, the reading and listening app we’re building, is designed around the first two points. It labels every text by how much of it you already understand, so you can find input at the right level, and it keeps the words you’re learning highlighted so you notice them each time they come back. It isn’t available yet. Its job is to make the input part easy; the conversation part is up to you and whoever you talk to.

FAQ

What are Krashen’s five hypotheses?

The acquisition–learning hypothesis, the monitor hypothesis, the natural order hypothesis, the input hypothesis and the affective filter hypothesis. Together they’re often called Monitor Theory, and they were set out most fully in Krashen’s 1982 book Principles and Practice in Second Language Acquisition.

What is the main criticism of Krashen’s input hypothesis?

The most common criticism, from Barry McLaughlin and Kevin Gregg, is that the key terms, especially “i+1,” are too vague to test. Others, like Merrill Swain, argued that input alone isn’t sufficient, because learners with years of input in immersion programs still had gaps in accuracy.

What is the difference between Krashen’s input hypothesis and Swain’s output hypothesis?

Krashen argued that acquisition comes from understanding input, and that speaking is a result of acquisition, not a cause. Swain argued that producing language also drives learning, because it forces you to notice gaps in your knowledge and process grammar you can skip while listening.

Is comprehensible input enough to become fluent?

For comprehension, large amounts of input take you a very long way. For accurate speaking and writing, most research suggests you also need practice producing language and some feedback. Input is the foundation, not the whole building.

Is Krashen’s theory still accepted today?

Not as a complete theory, but many of its ideas are. The importance of comprehensible input, the difference between implicit and explicit knowledge, and ordered development are all part of mainstream research. The claims that input is sufficient and that instruction doesn’t help are widely rejected.