The social strata have included a free category called "geer", a castes category called "nyeenyo" or "neeno", and a servile category of slaves called "jaam". Caste status has been hereditary, and endogamy among the men and women of a particular caste status has been an enduring feature among the Wolof people, states Leonardo Villalón – a professor of Political Science and African Studies. The Wolof's caste status, states Villalón, has been and is a greater barrier to inter-marriage than is either ethnicity or religion in Senegal.