A fission of Piaroa (Venezuela) fits this scenario. On the other extreme, a lineage assortment index near unity represents a lineal fission that maximizes the relatedness of subgroups such as the perfect split of a lineage into sublineages. Two fissions of Ache villages (Paraguay) are examples of this type of fission with a low lineage assortment index not significantly different from zero as evaluated with controlled simulations. We develop a lineage assortment index where low values represent subgroups with coefficients of relatedness near those expected if nuclear and extended households had chosen to associate into random groupings. However, random individual assortment is not an appropriate null model for evaluating lineage assortment because nuclear families and extended households are expected to remain together, which in and of itself forces higher relatedness in smaller subgroups. Fission processes that increase the average relatedness of subgroups are important because relatedness governs how strongly kin/group selection favors social behaviors such as warfare, peacekeeping, and other forms of collection action. As found for other social animals, human fissions lead to subgroups that have higher levels of relatedness as compared with the original community because of kin-biased assortment known as the lineal effect. An ethnographic review shows that most human group fissions are fueled primarily by internal political conflict and secondarily by resource scarcity. Causes, Consequences, and Kin Bias of Human Group Fissions Causes, Consequences, and Kin Bias of Human Group Fissionsįissions of human communities are monumental occasions with consequences for cultural and genetic variation and divergence through time by means of serial founder effects.
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