Comprehensible Input

This controversial concept was developed by Stephen Krashen in the 1970s and 1980s in connection with the [Page 164]input hypothesis, which claims that the way humans acquire language is by understanding messages or by receiving comprehensible input. In the late 1970s, Krashen referred to comprehensible input as intake. About a decade earlier, S. Pit Corder had distinguished intake—language that a learner understands, takes in, and uses—from input—any stretch of language available to the learner. Krashen originally claimed that intake alone was both necessary and sufficient for second-language acquisition. His preliminary writings focused on the acquisition of grammatical structures, mainly morphemes. By the mid-1980s, he extended his claims about comprehensible input to include the acquisition of lexical items embedded in messages and the acquisition of literacy.

The input hypothesis asserts that learners become more proficient in a second language when they understand language input that includes grammatical structures slightly beyond their current proficiency levels. Messages directed to the learner that contain language structures too far beyond the learner's current proficiency do not help the learner develop greater or expanded proficiency, because they leave gaps in understanding and therefore in production. Krashen uses the expression i + 1 to capture the idea of language input that is “slightly beyond” the learner's current level of competence. In the expression, the term i equals the learner's current competency level, so that i + 1 is the next level or stage the learner is ready to acquire. Messages to the learner that contain structures that extend well beyond the learner's current proficiency level, say, i + 5 or i + 9, are by definition incomprehensible, and thus, because the learner cannot process the structures in the message, the structures will not be acquired.

Krashen (with Tracy Terrell) points out, however, that this is a theoretical and conceptual portrayal. Comprehensible input does not need to be finely tuned to each learner's i + 1 level to be useful for acquisition. In a classroom where language learners are at different levels of proficiency, a teacher cannot possibly adjust for all the variations in level present in the classroom. The teacher's role is to make sure that learners understand what is being communicated to them orally or in writing. If learners understand the input and there is an ample amount of it, learners are likely to receive i + 1 geared to their acquisition needs. This is what Krashen refers to as “casting a net” of language wide enough to ensure that there are multiple instances of the individual student's i + 1.