Give things the same look and people assume they do the same job.
Things that look alike are seen as belonging together, or as doing the same thing.
The law of similarity is one of the Gestalt principles of perception, described by the German psychologist Max Wertheimer in a 1923 paper, translated into English as "Laws of Organization in Perceptual Forms". Wertheimer showed that people do not see a scene as separate dots and patches, but organise it into groups, and that things sharing a property such as colour or shape are grouped together. Later researchers found that similarity can even outweigh nearness: evenly spaced dots in alternating colours read as columns or rows of the same colour, not as one even grid.
Eight filter chips look identical, though three are switched on. Choose how an on filter should look, then squint at the row: the three become a group only when they share a look the others lack.
Give things with the same role or state the same look, and make different things look clearly different.
On most well-designed sites, every link shares one colour and every primary button shares one style, so people learn once what is clickable and then recognise it everywhere. Filters work the same way: in a good shop, every applied filter is filled and every other one is outlined, so the selected ones read as a group across size, colour and fit. Break that, for example by colouring a heading the same blue as the links, and people start clicking things that do nothing.
Similarity groups things, so do not use it where you want one thing to stand out. The recommended plan or the one urgent warning should look different from its neighbours, which is the Von Restorff Effect. And similarity is not a reason to make everything identical: things that do different jobs, such as a primary action and a destructive one, need to look clearly different even if they sit side by side.
Blur your eyes or shrink the screen until you cannot read the words. Can you still tell which items are clickable, which are selected, and which belong together?
The law of similarity is a Gestalt principle: people see things that look alike, in colour, shape or size, as belonging to the same group. In UX it means giving things with the same role or state the same look.
All links on a site sharing one colour, so people recognise them as clickable, or applied filters in a shop being filled while the rest are outlined, so the selected ones read as a group.
It is one of the Gestalt principles described by the German psychologist Max Wertheimer in a 1923 paper on how people organise what they see into groups.
Proximity groups things that are close together. Similarity groups things that look alike. When the two conflict, similarity can win, so items that look alike are grouped even if they are spread apart.
Give each role one consistent style, such as one look for links, one for primary buttons and one for selected items, and make different roles look clearly different. Add a cue beyond colour so it works for everyone.
A clothing shop's filter screen shows size, colour and fit as rows of chips. The chips that are switched on look almost exactly like the ones that are off, so shoppers can't tell what is applied and keep tapping to check.