Anyone can appreciate the aesthetic beauty of rounded corners, but not everyone can explain where exactly that beauty comes from. The answer to that is literally in your eye.
Some experts say that rectangles with rounded corners are easier on the eyes than a rectangle with sharp edges because they take less cognitive effort to visually process. The fovea is fastest at processing circles. Processing edges involve more “neuronal image tools” in the brain . Thus, rectangles with rounded corners are easier process because they look closer to a circle than a regular rectangle.
Scientific research done on corners by the Barrow Neurological Institute found that the “perceived salience of a corner varies linearly with the angle of the corner. Sharp angles generated stronger illusory salience than shallow angles” . In other words, the sharper the corner, the brighter it seems. And the brighter a corner appears, the more it’ll affect visual processing.
Another explanation on why we have an eye for rounded corners is because they’re more organic to how we use everyday objects in the physical world . Rounded corners are everywhere. And as children, we quickly learn that sharp corners hurt and that rounded corners are safer. That’s why when a child plays with a ball, most parents aren’t alarmed. But if a child were to play with a fork, the parents would take the fork away for the fear of the child hurting itself. This provokes what neuroscience calls an “avoidance response” with sharp edges. Thus, we tend to “avoid sharp edges because in nature, they can present a threat” .
Rounded corners are more effective for maps and diagrams because they allow our eyes to easily follow lines “as it suits better to the natural movement of the head and eyes respectively” . Sharp corners throw your eyes off the path of the line so you end up experiencing abrupt pauses when the line changes direction. But with rounded corners, the line leads your eyes around each corner to continue along the path smoothly.
Rounded corners also make effective content containers. This is because the rounded corners point inward towards the center of the rectangle. This puts the focus on the contents inside the rectangle. It also makes it easy to see which side belongs to which rectangle when two rectangles are next to each other. Sharp corners point outward putting less focus on the contents inside the rectangle. They also make it hard to tell which of the two sides belong to which rectangle when two rectangles are next to each other. This is because each rectangle side is exactly a straight line. However, the sides of a rounded rectangle are unique because the lines curve towards the rectangle it belongs to.