Human Nature

, Volume 25, Issue 4, pp 465–475 | Cite as

Causes, Consequences, and Kin Bias of Human Group Fissions

  • Robert S. WalkerEmail author
  • Kim R. Hill


Fissions of human communities are monumental occasions with consequences for cultural and genetic variation and divergence through time by means of serial founder effects. An ethnographic review shows that most human group fissions are fueled primarily by internal political conflict and secondarily by resource scarcity. 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. 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. 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. 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. 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. 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. A fission of Piaroa (Venezuela) fits this scenario. While previous discussions of fission have emphasized similarities among human studies and even other social mammals, we highlight the full range of potential kin bias in the formation of new communities.


Group splits Lineal fissions Human kinship Genealogical relatedness Cross-cultural comparison 



This paper benefited from help and conversations with Peter Whiteley, Greg Blomquist, Rob Boyd, Craig Palmer, and Stephen Beckerman. Financial support was provided by a National Geographic Society Research and Exploration grant (#9165-12).


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© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.Department of AnthropologyUniversity of MissouriColumbiaUSA
  2. 2.School of Human Evolution and Social ChangeArizona State UniversityPhoenixUSA

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