One of the most obvious social situations in the middle Benue Valley is the expansion in all directions of the Tiv, who number some 800,000 and are the largest pagan tribe in Northern Nigeria. One of the most puzzling aspects of Tiv expansion has been that, although it is obviously connected in many areas with land shortage and accompanying economic factors, it is precisely in those areas in which land shortage is least severe that the rate of migration appears to be most rapid.
Centrifugal expansion is intimately associated with the social and political structure of the Tiv people—is, in fact, a facet of it. In order to analyse this association, and incidentally to explain the paradox of the rate of expansion, it is necessary to divide the movement into two sorts: expansion and disjunction. Expansion refers to that type of movement which leads to the enlargement in area of a lineage's territory. It may eventually lead to a change in geographical location of the lineage territory; it does not affect the over-all juxtaposition of lineage areas. Disjunction, on the other hand, characteristically brings about a separation of groups in space and affects not only the geographical position of a lineage territory, but also the juxtaposition of lineage territories.
The first section of this article will set forth briefly the relationship between movements of lineages, territorial organization, and the structure of political lineages. The second section will examine expansion; the third, disjunction. The fourth will show that the two types of migration are both present in all parts of the country, and that it is the primacy of one or the other which gives rise to an apparent difference in rate of migration.