mental-model

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# Mental Model

A mental model is the user's internal representation of how a system works. When the user's mental model matches the system's actual behavior, interaction is smooth — the user predicts correctly, takes the right actions, and recovers from problems easily. When the mental model diverges from reality, errors and confusion follow. The designer's job is to either build a system that matches the user's existing mental model, or build the system in a way that teaches an accurate model quickly.

## Definition (in our own words)

Every user comes to a design with assumptions about how it will work, drawn from prior experience: how computers behave, how websites navigate, how forms validate, how billing is structured. These assumptions form a mental model — a simplified representation that lets the user predict what will happen if they take a particular action. When the prediction matches reality, the design feels intuitive. When it doesn't, the design feels confusing — or worse, the user proceeds confidently and produces unintended results.

The book distinguishes two complementary mental-model types: **system models** (how the user thinks the system works internally) and **interaction models** (how the user thinks they should interact with the system). Designers tend to have rich system models and weak interaction models; users tend to have weak system models and (with use) increasingly accurate interaction models.

## Origins and research lineage

- **Kenneth Craik**, *The Nature of Explanation* (Cambridge University Press, 1943). The foundational text — Craik proposed that humans build internal models of external reality and run those models to predict consequences. The basis for cognitive psychology's model-based reasoning.
- **Philip Johnson-Laird**, *Mental Models* (Cambridge, 1983). Mental models as the unit of human reasoning.