Humans: Between-Group Conflicts
Conflict among human groups or populations, usually of different cultural backgrounds, involves violent interactions. The aggression usually leads to death of part of group members of both sides and to advantages (material or social) to surviving victors.
The central aim of evolutionary approaches to the study of intergroup violence is to examine whether between-group violent conflicts and killings are in any way adaptive, and to investigate their evolutionary history. Lethal intergroup conflict among humans during the late Pleistocene and early Holocene is still a controversial subject, with little agreement on its extent (Bowles 2009). It is important to turn to prestate societies, as these are thought to typify the kinds of aggregations that humans belonged to for the great majority of human history. Their study facilitates our understanding of the contexts from which modern behaviors are derived and of those in which human universals had their evolutionary origins. A great deal of interest revolves around illuminating the frequency, intensity, and importance of the types of lethal intergroup killings in small-scale societies such as feuds, raids, massacres, treachery, and warfare. Different hypotheses have been proposed to explain the adaptive value of intergroup conflict, some involving individual-level selection and others group-level selection. Empirical evidences for these hypotheses are reviewed here.
Despite scientific discussions, intergroup killings have been found to occur across a variety of human societies (Guilaine and Zammit 2008). Contrary to the claims of some perspectives, lethal violence between groups did not originate with nation-states. According to archeological data, intergroup killings were already committed by small-scale societies before the emergence of states. Evidence supporting the occurrence of lethal violence in the past includes Paleolithic, Mesolithic, and Neolithic art in Italy, France, and Spain, as well as osteological evidence, settlement patterns, and weapons retrieved from prehistoric sites across Africa, Eurasia, and North America.
In human subsistence societies, death rates from intergroup aggression are reported to be higher compared to rates of death from intragroup aggression (Heider 1997). The underlying motives for between-group killings in small-scale societies include (Gat 2010): competition for subsistence resources and for mates; contests for prestige and status; retaliation after suffering raids and massacres; elimination of future rivals through pre-emptive attacks; clash of cosmovisions, ideologies, or accusations of supernatural wrongdoing (e.g., witchcraft); and security dilemmas (the actions of one community are misinterpreted as warmongering by another group). Lethal violence in subsistence societies of human hunter–gatherers and farmers occurs in similar average rates (and similar variability in the rates) when compared to those reported for chimpanzee societies. Estimates are considerably high: for instance, in seven subsistence farming societies for which data were compiled, median percentage of deaths accounted for by warfare was 28.5% for men and 6.1% for women (Keeley 1996). These figures are higher than those for hunter-gatherer societies or Western-industrialized societies, especially across recent decades of modern history, although several wars among Western forces in recent decades have resulted in extreme mortality rates that surpass even the upper bounds of war-related mortality estimates for farming societies (e.g., the War of the Triple Alliance between 1864 and 1870). In spite of interpopulational variance in frequency of warfare and mortality rates due to such conflicts, evidence for warfare exists for at least 90–95% of human societies (Keeley 1996), indicating it is a human universal.
A conceptual model for human evolution that has recently gained widespread popularity is the Ecological Dominance hypothesis, which claims that modern humans have so mastered the natural environment that the major contemporary sources of selection are predominantly social. Although this model has much merit, and is related to the “social brain” hypothesis in primatology, it is possible that the case for human autonomy from natural selective pressures has been somewhat overstated. For example, climate change has recently been shown to exert severe selective pressures upon human populations over historical time (Zhang et al. 2007) – e.g., cold historical periods have been associated with elevated rates of intergroup conflict. In such cases, although the more proximal selective pressures behind intergroup conflict and killing might be social (e.g., conflicting views), the more distal selective pressures behind them might be natural (e.g., climate change and reduced resource availability). Game-theoretic models combined with updated information on archeological evidence on causes of death during the late Pleistocene and early Holocene, and with ethnographic and historical reports on hunter-gatherer populations suggest that parochial altruism (i.e., altruism geared toward those of the same group) and intergroup warfare have coevolved during these periods (Bowles 2009). Recent historical analyses in fact support this notion, for instance demonstrating that regions more frequently involved in intergroup warfare throughout Japanese history present higher levels of collectivism (as opposed to individualism or independence; Fernandes et al. 2015). It is important to stress, however, that resource competition is not the only driver of intergroup conflict, and in several human societies studied does not appear to predict warfare.
Comparative studies of hunter-gatherer societies and farmer societies indicate that, although common, material rewards possibly obtained in looting do not appear to be sufficient to explain motivations for individual engagement in group efforts toward warfare (Glowacki and Wrangham 2013). Moreover, capturing individuals who were defeated in battles (thus kept as slaves, for instance) is not a widespread in bands and tribes. On the contrary, when large raids (i.e., massacres) occur, most of the individuals in the attacked group are killed, thus not serving as a direct socioeconomic boost to victors. Hence, acquiring captives is more of a common practice of chiefdoms or other sociopolitically complex societies. Nevertheless, in those cases where it has been observed, most of the time captives are reproductively active females. Increased status, honor, and prestige appear to be the most common individual benefit for warriors. Increased fertility, be it direct or in the long-term as a result of cumulative combat efforts and recognition, has been observed to be associated with warrior status, such as among the Yanomamo foraging horticulturalists of Venezuela and the Nyangatom nomadic pastoralists of East Africa.
Archeological information permits examining how widespread and prevalent warfare-related deaths were throughout historical periods of human societies. It is worth noting, however, that despite the benefits of relying on the archeological record, its imperfection imposes a limit to the type of inferences that can be drawn. A critical issue stems from the difficulty in distinguishing between violent deaths that resulted from intergroup killings and warfare or from interpersonal attacks within a community. Related to this is the difficulty in ascertaining the number of perpetrators responsible for any given case of violent death. Though these challenges are present, the following sites represent the strongest and most significant examples of likely intergroup warfare in the archeological record (Guilaine and Zammit 2008). The Jebel Sahaba cemetery 117 in Northern Sudan, with an estimated age of 14kya–12kya, offers one of the earliest evidences of seemingly intentional and effective killing between groups, and with a lethality of ~40%. Furthermore, it has been hypothesized that either an increase in population pressure or in ecological distress led to the escalation of competition for resources along the Nile valley, leading to the violence at Jebel Sahaba. Similarly, recent findings from Nataruk, Kenya, provide evidence for intergroup aggression against a band of hunter gatherers in the late Pleistocene-early Holocene (Lahr et al. 2016). However, different for Jebel Sahaba, the remains in Nataruk were not intentionally buried, but rather covered by the sediment of the lagoon. In Europe many sites exhibit evidence of lethal conflict (Keeley 1996). Furthermore, across archeological sites around the world, the lethality of such encounters has been calculated to be close to that displayed by hunter-gatherers (~ 23%).
Another line of evidence for the widespread existence of aggressive between-group conflict in human societies comes from cross-cultural and ethnographic analyses. An examination of 11 Amazonian societies found a higher number of deaths as a consequence of between-group conflicts compared to within-group killings (Walker and Bailey 2013). For example, the Ache in Paraguay, as well as the Waorani (precontact) and the Jivaro from Ecuador, experienced considerable levels of lethality (average estimates are 56% for the Waorani, 43% for the Ache, and 42% for the Jivaro). In all 11 societies, males were more likely to be the victims in both between and within-group attacks. Comparatively, only 2% of the aggressors died during the attacks. The intense and escalated competition observed among lowland South American, coupled with (1) comparatively low levels of within-group killing, (2) high genetic differentiation among populations, and (3) small direct individual benefits to warriors, suggest that group selection may be an important force shaping warfare in these societies.
Different lines of study of between-group conflicts emphasize different evolutionary hypotheses for their origin and adaptive value, which involve individual and group selection approaches. However, neither account for the evidence completely, suggesting that complementary explanations employing the integrative multilevel selection theory, rather than relying on one mode of selection, are parsimonious. An adaptationist view of between-group conflict and killing cannot be applied to only a portion of human societies or a portion of human evolutionary history, as comparative ethnographic and archeological data point to the ubiquity of this phenomenon despite cultural and temporal fluctuations in frequency, specific strategies employed, and death rates.
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