Default Lighting in Portraits
When the brain looks at an object, apparently it assumes that the light source is (a) above the object and (b) to the left of it. This is according to Mamassian, P., Jentzch, I., Bacon, B. A., & Schweinberger, S.R., Neural Correlates of Shape from Shading, NeuroReport, 14 (7), 971-975 (2003). And this seems consistent with artistic practice: it appears that often—perhaps most often—portaitists light their subjects from the upper left for the most natural look.
Does lighting from the upper left look more natural? See for yourself. Below is a picture I drew (from a Sargent charcoal). I digitized the drawing and flipped it horizontally. Here is one light source:
And here is another:
Which feels more natural? Or do they both feel the same?