 The four strata social system – high lineage, low lineage, caste groups and slaves – found among the Somalis has been common in the Horn of Africa region, states Donald Levine, and is also found among ethnic groups such as Afar, Amhara, Borana, Leqa, Sidamo, Kefa, Janjero and other peoples.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.