Laws of UX & Cognitive Heuristics

Named principles to reason about and defend interface decisions.

Law	Principle
Doherty Threshold	System response under 400ms keeps users in a productive, engaged state.
Goal-Gradient Effect	Motivation rises as users near a goal — show progress, pre-fill early steps.
Zeigarnik Effect	People remember incomplete tasks better — use progress bars and "unfinished" cues.
Peak-End Rule	Users judge an experience by its most intense moment and its ending, not the average.
Serial Position Effect	First and last items are best remembered — place key nav at the ends.
Von Restorff (Isolation) Effect	The visually different item is the one remembered — use for the primary CTA.
Postel's Law (Robustness)	Be liberal in what you accept (forgiving inputs), conservative in what you send.
Parkinson's Law	Tasks expand to fill the time allowed — impose deadlines and constraints.
Pareto Principle (80/20)	~80% of effects come from ~20% of causes — optimize the vital few flows.
Occam's Razor	Remove elements until breaking point; the simplest solution that works wins.
Flow (Csikszentmihalyi)	Match challenge to skill to keep users absorbed and low-friction.
Banner Blindness / Selective Attention	Users ignore anything that looks like an ad or is irrelevant to their goal.

dec-ux-laws