First Principles of Experience Design · First Principles
First Principle: People Act on Mental Models, Not Interfaces
Users do what they believe the system does, not what it actually does — design for the belief, or correct it, but never ignore it.
Every user carries a working theory of how systems like yours behave, and when their theory and your system disagree, they follow the theory. Behavior that looks irrational is usually rational under the user's belief — recover the belief, then match it or visibly correct it.
Interfaces communicate a fraction of a system's actual behavior; the rest is filled in by belief, assembled from other products, past experiences, and metaphors. Teams who live inside the system design for how it works instead of how it is believed to work.
When the theory and the system disagree, the person follows the theory.
Illustrative pattern: a banking app processes transfers instantly but shows a 'pending' state for compliance review. Customers believe pending means failed — everywhere else it does — so they send the transfer again. The duplicate-payment tickets are a mental-model collision, not a UI bug.
- When behavior looks irrational, recover the belief that makes it rational.
- Ask: what must this person believe the system is doing for their action to make sense?
- Then match the belief — or visibly correct it. Never silently contradict it.
Apply this
Reading about first principle: people act on mental models, not interfaces is one thing. Seeing where it applies in your journey is the useful part.