To Top

The Input Hypothesis: Definition and Criticism

The Input Hypothesis: Definition and Criticism

Stephen Krashen is a linguist and educator who proposed the Monitor Model, a theory of second language acquisition, in Principles and practice in second language acquisition as published in 1982. According to the Monitor Model, five hypotheses account for the acquisition of a second language:

However, despite the popularity and influence of the Monitor Model, the five hypotheses are not without criticism. The following sections offer a description of the fourth hypothesis of the theory, the input hypothesis, as well as the major criticism by other linguistics and educators surrounding the hypothesis.

Definition of the Input Hypothesis

The fourth hypothesis, the input hypothesis, which applies only to language acquisition and not to language learning, posits the process that allows second language learners to move through the predictable sequence of the acquisition of grammatical structures predicted by the natural order hypothesis. According to the input hypothesis, second language learners require comprehensible input, represented by i+1, to move from the current level of acquisition, represented by i, to the next level of acquisition. Comprehensible input is input that contains a structure that is “a little beyond” the current understanding—with understanding defined as understanding of meaning rather than understanding of form—of the language learner.

Second language acquisition, therefore, occurs through exposure to comprehensible input, a hypothesis which further negates the need for explicit instruction learning. The input hypothesis also presupposes an innate language acquisition device, the part of the brain responsible for language acquisition, that allows for the exposure to comprehensible input to result in language acquisition, the same language acquisition device posited by the acquisition-learning hypothesis. However, as Krashen cautions, like the time, focus, and knowledge required by the Monitor, comprehensible input is necessary but not sufficient for second language acquisition.

Criticism of the Input Hypothesis

Like for the acquisition-learning hypothesis, the first critique of the input hypothesis surrounds the lack of a clear definition of comprehensible input; Krashen never sufficiently explains the values of i or i+1. As Gass et al. argue, the vagueness of the term means that i+1 could equal “one token, two tokens, 777 tokens”; in other words, sufficient comprehensible input could embody any quantity.

More importantly, the input hypothesis focuses solely on comprehensible input as necessary, although not sufficient, for second language acquisition to the neglect of any possible importance of output. The output hypothesis as proposed by Merrill Swain seeks to rectify the assumed inadequacies of the input hypothesis by positing that language acquisition and learning may also occur through the production of language. According to Swain who attempts to hypothesize a loop between input and output, output allows second language learners to identify gaps in their linguistic knowledge and subsequently attend to relevant input. Therefore, without minimizing the importance of input, the output hypothesis complements and addresses the insufficiencies of the input hypothesis by addressing the importance of the production of language for second language acquisition.

Thus, despite the influence of the Monitor Model in the field of second language learning and acquisition, the input hypothesis, the fourth hypothesis of the theory, has not been without criticism as evidenced by the critiques offered by other linguists and educators in the field.


Gass, Susan M. & Larry Selinker. 2008. Second language acquisition: An introductory course, 3rd edn. New York: Routledge.
Krashen, Stephen D. 1982. Principles and practice in second language acquisition. Oxford: Pergamon.
Swain, Merrill. 1993. The output hypothesis: Just speaking and writing aren’t enough. The Canadian Modern Language Review 50(1). 158-164.
Zafar, Manmay. 2009. Monitoring the ‘monitor’: A critique of Krashen’s five hypotheses. Dhaka University Journal of Linguistics 2(4). 139-146.

More in Information