Input hypothesis

The input hypothesis, also known as the monitor model, is a framework for understanding second-language acquisition. Developed by the linguist Stephen Krashen during the 1970s and 1980s, it consists of five distinct hypotheses. These ideas suggest that language is not just something we study, but something we acquire through understanding. The model emphasizes that exposure to meaningful messages is the primary driver of linguistic growth.

At the heart of this model is the concept of comprehensible input (CI). Krashen argues that learners progress when they understand messages that are slightly beyond their current level. He uses the formula "i+1" to describe this process. In this equation, "i" represents the learner's current interlanguage, which includes their existing linguistic competence and extra-linguistic knowledge. The "+1" represents the next increment of new language structure. This new information must be understandable through context, such as knowledge of the world or the specific situation.

To understand how this works, we must look at the acquisition–learning hypothesis. Krashen claims there is a strict separation between these two processes. Acquisition is a natural, intuitive, and subconscious process. It is similar to how children learn their native language by focusing on meaning rather than form. Learning, however, is a conscious process. This often involves formal instruction in a classroom where students study abstract rules and grammar. According to Krashen, only acquisition leads to spontaneous language production.

This leads to the monitor hypothesis, which explains how conscious learning interacts with speech. The monitor is an internal mechanism that uses learned rules to check language output. When a learner speaks, they use this system to scan for errors and perform self-correction. For the monitor to work, three specific conditions must be met. First, the learner must actually know the specific rule. Second, the learner must be focused on correctness rather than meaning. Third, the learner must have enough time to apply the rule.

While the monitor can help with accuracy, it has significant limitations. Using the monitor requires a person to slow down, which can make the flow of conversation painfully slow. If a speaker over-monitors, they may focus so much on form that they exchange very little information. Furthermore, Krashen suggests that the monitor is best used during activities like writing. This is because writing allows more time for precision than spontaneous speech. The monitor is a tool for checking, but it is not the source of language itself.

The model also includes the natural order hypothesis. This states that all learners acquire language structures in a predictable, linear sequence. This order is independent of how easy a feature is to teach. For example, in English, the third-person "-s" (as in "he runs") is easy to teach but is often acquired later in the natural sequence. This hypothesis was supported by morpheme studies conducted by researchers Dulay and Burt. They found that certain linguistic forms are predictably learned before others.

Finally, the affective filter hypothesis addresses the emotional side of learning. The affective filter is a psychological barrier that can impede language acquisition. Negative emotions, such as anxiety, self-doubt, or boredom, cause the filter to go "up." When the filter is high, it acts as a blockage that prevents input from being processed. To lower the filter, teachers can create low-anxiety environments. They might also allow for a "silent period," where students are not expected to speak until they have received enough input.

Krashen's work has had a massive influence on language education, especially in the United States. However, the model has also faced criticism from academics. Some critics argue that the hypotheses are untestable through scientific means. Others suggest that the theory assumes a separation between acquisition and learning that may not actually exist. Despite these debates, the idea of comprehensible input remains a central concept in how we think about mastering new languages.