Katarineko · Guides

The input hypothesis and i+1, explained practically

The input hypothesis is Stephen Krashen's claim that we acquire language by understanding messages, and i+1 is his name for input just above your current level. It is contested as science and useful as practice. This guide covers both, then shows how to use the idea when picking what to read tonight; Katarineko's weekly stories are the idea put into practice.

What Krashen claimed

In the early 1980s the linguist Stephen Krashen proposed that we acquire language in one way only: by understanding messages. Grammar study and drills produce conscious knowledge that can monitor your output, but the language you own arrives through input you understood.

i+1 is his description of the input that teaches best: your current competence (i) plus one step. New language sticks when it rides in on structure you already understand: one unknown held in place by nine knowns. Krashen also argued that anxiety and boredom block acquisition; he called this the “affective filter”.

The criticisms

The input hypothesis has been argued about for forty years, and you should know the shape of the argument. The central objection: it is hard to falsify. Nobody can measure your “i”, so “+1” cannot be pinned down in an experiment, and a theory that can explain any outcome sits uneasily as science. Others, notably Merrill Swain, argued output matters too: being forced to produce language exposes gaps that input alone lets you gloss over. Mainstream second-language research today treats input as necessary but debates how sufficient it is.

What is not seriously contested: input is central, and it must be largely comprehended to do anything. You also need a lot of it. The related coverage research is concrete where i+1 is vague. Hu and Nation's studies put comfortable unassisted reading near 98% known words, with comprehension degrading quickly below that.

How to use it

Treat i+1 as a selection rule for material rather than a formula. In practice it means: read and listen to things where you meet a few new words per sentence-cluster, not per sentence. If you are pausing to look something up every five seconds, it is i+10; drop down. If you never meet anything new, it is i+0; pleasant, but move up.

Almost no native material sits at i+1 for a beginner, and material written at i+1 (textbooks, graded readers) is rarely about anything you love. That tension, and the way out of it, is the subject of our comprehensible input guide. The short version: derive the material from the destination. A film like Kiki's Delivery Service reaches 70% dialogue coverage with its top 244 words. Stories written from that vocabulary, pitched just above your measured level, sit at i+1 automatically, and every “+1” is a word the film is about to say to you. That is the mechanism Katarineko runs weekly.

Common questions

Is i+1 proven?

As a testable mechanism, no; see above. As a heuristic for choosing material, it holds up: mostly comprehensible, slightly novel, high volume, interesting. Every successful method converges on those four properties, whatever it calls them.

Does grammar study contradict the input hypothesis?

No. Even in Krashen's own account, conscious knowledge helps you monitor your output and helps make input comprehensible in the first place. A grammar primer is scaffolding for input.

What is i+1 in vocabulary terms?

A rule of thumb from the coverage research: aim for material where you know roughly 95 to 98% of the running words. Below 90%, context stops working and reading becomes decoding.

Referenced: Krashen, Principles and Practice in Second Language Acquisition (1982); Swain's comprehensible output work; Hu & Nation (2000) on vocabulary coverage and comprehension.