title: Formation of raiding parties for intergroup violence is mediated by social network structure.
abstract: Intergroup violence is common among humans worldwide. To assess how within-group social dynamics contribute to risky, between-group conflict, we conducted a 3-y longitudinal study of the formation of raiding parties among the Nyangatom, a group of East African nomadic pastoralists currently engaged in small-scale warfare. We also mapped the social network structure of potential male raiders. Here, we show that the initiation of raids depends on the presence of specific leaders who tend to participate in many raids, to have more friends, and to occupy more central positions in the network. However, despite the different structural position of raid leaders, raid participants are recruited from the whole population, not just from the direct friends of leaders. An individual's decision to participate in a raid is strongly associated with the individual's social network position in relation to other participants. Moreover, nonleaders have a larger total impact on raid participation than leaders, despite leaders' greater connectivity. Thus, we find that leaders matter more for raid initiation than participant mobilization. Social networks may play a role in supporting risky collective action, amplify the emergence of raiding parties, and hence facilitate intergroup violence in small-scale societies.
authors: Department of Human Evolutionary Biology, Harvard University, Cambridge, MA 02138; Program for Evolutionary Dynamics, Harvard University, Cambridge, MA 02138; Yale Institute for Network Science, Yale University, New Haven, CT 06520; The Institute for Advanced Study in Toulouse, 31015 Toulouse, France.
authors: Luke
authors: L
authors: Glowacki
authors: Yale Institute for Network Science, Yale University, New Haven, CT 06520; Department of Physics, Harvard University, Cambridge, MA 02138.
authors: Alexander
authors: A
authors: Isakov
authors: Department of Human Evolutionary Biology, Harvard University, Cambridge, MA 02138.
authors: Richard W
authors: RW
authors: Wrangham
authors: Department of Political Science, Brown University, Providence, RI 02906.
authors: Rose
authors: R
authors: McDermott
authors: Department of Medicine, University of California, San Diego, CA 92093; Political Science Department, University of California, San Diego, La Jolla, CA 92093.
authors: James H
authors: JH
authors: Fowler
authors: Yale Institute for Network Science, Yale University, New Haven, CT 06520; Department of Sociology, Yale University, New Haven, CT 06520; Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520 nicholas.christakis@yale.edu.
authors: Nicholas A
authors: NA
authors: Christakis
mesh_terms: D000293:Adolescent; D000328:Adult; D005002:Ethiopia; D006801:Humans; D007857:Leadership; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D060756:Social Networking; D012959:Socioeconomic Factors; D014754:Violence / Q000523:psychology*; D014857:Warfare
keywords: collective action; emergence; pastoralists; social networks; warfare