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, betweengroup 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.warfare | social networks | collective action | pastoralists | emergence I ntergroup violence is common, worldwide, and harmful. Global annual deaths from large-scale warfare, for example, range from 0.5 to 1 million, and this does not include nonfatal physical and mental injuries (1). A diverse set of approaches has been used to study intergroup violence and warfare. Evolutionary models have credited collective violence with an important role in the development of modern human behavior (2-7), whereas cultural and ecological factors have been shown to influence small and largescale violence (8-13). More recently, there has been increased interest in understanding the dynamics of group-based violence and the social processes that can contribute to it in the setting of insurgent and terrorist groups (14, 15); for example, online records suggests small, self-organizing groups coalesce into larger groups preceding terrorist attacks (16). Warfare has also been studied as a collective action problem-because individuals must mobilize to engage in a group activity with shared gains (e.g., deterrence, territory) and individual risks (e.g., injury, death) (17, 18).Despite these advances, fundamental questions remain about how violent groups are formed, and the extent to which they may self-organize and emerge organically. Theoretical work suggests interindividual differences may be important for initiating and sustaining risky collective action, but empirical evidence in humans supporting this is sparse (19,20). Research in primate behavior provides some clues regarding the emergence of violent intergroup conflict. Wild chimpanzees engage in lethal coalitionary violence against other communities (21), and a few "impact" individuals sh...

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