Interaction design laws
Aesthetic-usability effect
People see attractive designs as easier to use, and are more forgiving of small problems in a product they find beautiful.
Good looks buy patience. People rate a pleasing interface as more usable even when it isn't, and they tolerate minor issues in it.
That makes visual craft worth the effort, and it's a warning for research: a polished prototype can hide problems that a rough one would show.
In practice
Two versions of the same banking app with identical flows: testers often rate the better-looking one as easier, even when their task times are the same.
Common mistake
Testing only with polished designs and concluding there are no usability problems. Watch what people do, not just what they say.
Where it comes from
First shown by Masaaki Kurosu and Kaori Kashimura at the Hitachi Design Center in 1995, with cash machine layouts. Noam Tractinsky repeated the study in Israel in 1997.