The experiments in this thesis measured light source biases in different populations to investigate why observers assume that light originates from above and to the left. All the experiments use the same greyscale stimulus, the “honeycomb”, which consists of a hexagon surrounded by six hexagons. Light and dark edges give the impression of the stimulus being lit from one side, although there is no explicit light source in the picture. Participants viewed the honeycomb presented at different orientations and stated whether they perceived the central hexagon as pushed in or out compared to the surrounding hexagons. Participants’ light biases were calculated from these judgements, the angle at which they assumed the light was originating. The effect of long term experience is explored in Chapters Two and Three, showing that the cultural experience of language modulates the left lighting bias as biases were significantly reduced in a group of first language Hebrew speakers, who read and write from right to left, compared to English speakers. However, the experience of hemispatial neglect, an attentional disorder commonly seen after stroke, does not cause a shift in the light bias. The role of hemispheric asymmetry is examined in Chapters Three, Four, and Five by measuring the effects of lesions after stroke, simulated lesions in healthy participants, and the natural decline of hemispheric asymmetry with ageing. These experiments show that the lighting bias is modulated by hemispheric asymmetry; however the specific role of the right hemisphere is not clear. Chapter Five shows the lighting bias shifts to the right with age, as the right hemisphere degenerates, yet conversely Chapters Three and Four found that a disruption of the right hemisphere causes a leftward shift in participants’ light biases, regardless of lesion location. These findings make a significant contribution to the light assumptions literature, showing that the left lighting bias is caused by an interaction between fixed factors, hemispheric asymmetry, and also environmental experience.