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The law
When several similar things are presented together, the one that stands out is the one people notice and remember.
It is also called the isolation effect. Difference draws attention, and what draws attention is more likely to be remembered.
Where it comes from
Hedwig von Restorff, a German psychologist working with Wolfgang Köhler, published the study in Psychologische Forschung in 1933. Participants studied lists of items that were mostly of one kind, such as numbers, with a single item of a different kind, such as a syllable, placed among them. The isolated item was recalled better than items of the same position in lists without an isolate.
She showed the effect did not depend on the odd item being placed first or last: its difference from its surroundings was enough. The finding fits the Gestalt view that perception organizes things into groups, and an item outside every group gets its own place in memory.
What it means for an interface
- Make the primary action different. One call to action in a distinct colour or weight is found faster than one that looks like its neighbours.
- Use contrast for one thing at a time. The effect depends on isolation. If many elements are highlighted, none of them stands out.
- Do not rely on colour alone. People with colour vision deficiencies may not see a colour difference. Add shape, size, weight or a label.
- Remember it works in both directions. An accidental difference, such as an oddly styled link, also draws attention, whether you wanted it to or not.
Examples
Pricing tables. Marking one plan with a border, a label and a different background makes it the plan people look at first. It works because the other plans look alike.
Primary buttons. A form with one filled button and several plain ones tells people which action the product expects. Two filled buttons side by side cancel each other out.
Notifications. A single unread badge stands out. A navigation bar with badges on every item teaches people to ignore all of them.
Where it is misapplied
- Highlighting everything. Bold, colour and badges on every element remove the isolation the effect depends on.
- Steering people against their interest. Making the expensive option or the consent button the only prominent one is a dark pattern, not a use of memory research.
- Isolating with motion. Animation stands out, but it also distracts and can harm people with vestibular disorders. Prefer static contrast.
- Forgetting accessibility. A difference only in colour fails people who cannot see it and fails contrast checks.
Checklist
- Each screen has at most one visually isolated primary action.
- Highlights carry a second cue besides colour, such as weight, shape or a label.
- Badges and highlights are reserved for things that actually need attention.
- No highlight is used to steer people towards a choice that mainly benefits the business.
- Unintended visual differences have been checked for and removed.
Related
- Serial Position Effect: position, as well as difference, decides what people recall.
- Law of Similarity: similarity creates the groups an isolated item stands out from.
- Aesthetic-Usability Effect: visual treatment also shapes how usable a product feels.