Where is the sun?

When we interpret a shaded picture as a three-dimensional (3D) scene, our visual system often needs to guess the position of the light source in order to resolve a convex-concave ambiguity. For more than a century, psychologists have known that the visual system assumes that light comes from above and have argued that this assumption is ecologically justified because our everyday light source (the sun) is overhead. Our experiments reveal that people's preferred lighting direction is not directly overhead, but rather shifted to the left, and this preference is reflected in art spanning two millennia. Furthermore, we find a strong correlation between people's handedness and their preferred lighting. We suggest that what counts is not so much where the sun is, but where you like the sun to be