Monitor Model

The Monitor Model is Stephen Krashen's unified theory of second language acquisition, developed across the late 1970s and 1980s. It comprises five interrelated hypotheses that together make a strong claim: acquisition is driven by Comprehensible [[Input|comprehensible input]], and conscious learning plays only a marginal editing role. Despite extensive criticism, the model remains one of the most widely known and debated frameworks in SLA, and its core ideas have profoundly shaped communicative and input-based approaches to language teaching.

The Five Hypotheses

1. The Acquisition-Learning Hypothesis

There are two independent ways of developing knowledge of a second language:

Acquisition -- A subconscious process identical in character to first language acquisition. It occurs through meaningful interaction with the target language where the focus is on communication, not form.
Learning -- A conscious process that results in explicit knowledge about the language (rules, patterns, metalinguistic awareness).

Krashen's central claim is that these systems are entirely separate: acquired knowledge drives fluent production, while learned knowledge can only function as a monitor (see below). This strict non-interface position is the model's most controversial aspect.

2. The Monitor Hypothesis

Consciously learned knowledge serves only as a monitor -- an editor that checks and corrects output after it has been initiated by the acquired system. The monitor can only operate when three conditions are met simultaneously:

The learner has sufficient time to reflect
The learner is focused on form (not meaning)
The learner knows the rule

In spontaneous conversation, these conditions are rarely all present, which is why Krashen argues that learning contributes little to real-time communication. He identified three learner types: Monitor over-users (hesitant, self-conscious), Monitor under-users (fluent but error-prone), and optimal Monitor users (who deploy conscious knowledge appropriately).

3. The Natural Order Hypothesis

Grammatical structures are acquired in a predictable order, and this order is largely independent of the order in which structures are taught. Drawing on morpheme studies by Dulay and Burt (1974) and others, Krashen argued that the acquisition sequence is driven by internal processing mechanisms, not instructional sequencing. This does not mean grammar teaching is useless, but that it cannot override natural developmental sequences.

4. The Input Hypothesis

Acquisition occurs when learners receive Comprehensible Input at a level slightly beyond their current competence, expressed as i+1 (where i = current level). The learner uses context, world knowledge, and extralinguistic information to make sense of input containing unfamiliar structures, and acquisition happens as a byproduct of understanding messages.

Key claims:

Input is both necessary and sufficient for acquisition
Speaking is a result of acquisition, not a cause of it
Output practice does not directly contribute to acquisition
If enough comprehensible input is provided, i+1 is automatically present

This is the hypothesis that has had the greatest practical impact, influencing extensive reading/listening programmes, sheltered instruction, and content-based teaching.

5. The Affective Filter Hypothesis

Emotional variables -- motivation, self-confidence, and anxiety -- act as a filter that can block input from reaching the language acquisition device. When the affective filter is high (the learner is anxious, unmotivated, or self-conscious), even comprehensible input will not be acquired. A low affective filter creates conditions favourable for acquisition.

Criticisms

The Monitor Model has been challenged on multiple fronts:

Unfalsifiability -- The acquisition-learning distinction is not empirically testable. If a learner produces correct language, Krashen can attribute it to acquisition; if they self-correct, he attributes it to learning. There is no observable evidence that would disprove the separation.
The non-interface position -- Research on Focus on Form (Long, 1991) and the Noticing Hypothesis (Schmidt, 1990) provides evidence that conscious attention to form does contribute to acquisition, not just monitoring. The Interaction Hypothesis and Output Hypothesis show that production and negotiation of meaning also drive acquisition.
Vagueness of i+1 -- Krashen never operationalised what constitutes "slightly beyond" current competence. Without a way to measure i or i+1, the concept is difficult to apply systematically.
Natural order evidence -- The morpheme studies underpinning the Natural Order Hypothesis have been criticised for methodological limitations and for conflating accuracy orders with acquisition orders.
Sufficiency of input -- Immersion programme research (Swain, 1985) showed that learners who received massive comprehensible input over years still had persistent accuracy problems, suggesting input alone is not sufficient.

Legacy and Influence

Despite its flaws, the Monitor Model shifted the field's centre of gravity from teaching to acquisition, from form to meaning, and from output practice to input provision. Krashen's advocacy for free voluntary reading, comprehensible input, and low-anxiety classrooms has had lasting practical impact. The model is best understood not as a complete theory but as a set of powerful hypotheses that provoked more rigorous research -- including the Interaction Hypothesis, Output Hypothesis, and Noticing Hypothesis that were developed partly in response to its limitations.

The Monitor Model connects to the Input Hypothesis and Comprehensible Input as its most influential component, to the Noticing Hypothesis and Focus on Form as direct challenges to its non-interface position, to the Interaction Hypothesis and Output Hypothesis as alternative accounts of what drives acquisition, and to Accuracy and Fluency as the tension the Monitor Hypothesis attempts to explain.