According to Catherine Besteman - a professor of Anthropology, the widespread purchase of non-Somali African slaves during the medieval age helped structure the complex status hierarchy among the Somalis. However, adds Besteman, the Somali people from the upper strata have also been egalitarian in matters of clan leadership, while they have included concepts of social status, inferiority and exclusion of "Sáb" and slaves. In the northern regions where Somalis are traditionally found, states Iaon Lewis, Somali communities have traditionally distinguished between the artisanal Somali castes and their slaves, but in the south they have blurred these distinctions.