Textbook of Usability

A mental model is a user's internal representation of how a system works — a simplified understanding that guides expectations, predictions, and actions. Users build mental models through experience, instruction, and analogy with familiar systems.

Mental models are often:

• Incomplete (users don't know everything the system does)
• Inaccurate (beliefs may conflict with actual behaviour)
• Inconsistent (different parts of the model may contradict each other)
• Parsimonious (users prefer simple explanations even when they miss important detail)

Despite these imperfections, mental models are always influential: users act on what they believe the system will do, not on what it actually does.

Good design either (a) aligns with users' pre-existing mental models drawn from familiar concepts (file folders, shopping carts, physical mail) or (b) makes the system's behaviour transparent enough for accurate models to form quickly. When the system's behaviour contradicts the user's mental model, the result is confusion, errors, and frustration.

Don Norman distinguishes three related models: the designer's model (how the designer conceives the system), the system image (what the design actually communicates), and the user's model (what the user infers). The designer never interacts with the user directly — only through the system image — so everything depends on the system image being clear and accurate.