Key individuals catalyse intergroup violence

Abstract

Intergroup violence is challenging to understand: why do individuals cooperate to harm members of other groups when they themselves may be killed or injured? Despite progress in understanding the evolutionary and proximate mechanisms that underlie violence, we still have little insight into the processes that lead to the emergence of coalitionary aggression. We argue that an overlooked component is the presence of individuals who have a crucial role in initiating violence. In instigating intergroup violence, these key individuals may expect to face lower costs, receive greater benefits, or garner benefits that have a greater value to them than others. Alternatively, key individuals may be motivated by individual traits such as increased boldness, propensity for aggression or exploratory behaviour. Key individuals catalyse the emergence of coalitionary violence through one of several processes including altering the costs and benefits that accrue to others, paying a greater share of the startup costs, signalling privileged knowledge, or providing coordination, among other factors. Here we integrate diverse lines of empirical research from humans and non-human animals demonstrating that inter-individual variation is an important factor in the emergence of intergroup violence. Focusing on the role of key individuals provides new insights into how and why violence emerges. This article is part of the theme issue ‘Intergroup conflict across taxa’.

Leaders of war: modelling the evolution of conflict among heterogeneous groups

“…Therefore, individuals that are most influential in contribution towards a group victory (figure 2b) are also the most likely to die when the group suffers mortalities (figure 2c). More specifically, the probability of death within a group is determined as the effort of the individual divided by the summed effort of the group (also figure 2a,c), meaning the costs are dependent on the actions of all group members (also see [30]):…”

“…Social groups are intrinsically heterogeneous [20], with members that may differ in leadership status [21], social rank [14,[20][21][22][23], sex [13,20,21,24,25], size [20], personality [20,26,27] or age [20,21,24], among other factors. This heterogeneity is likely to lead to individual differences in the risks and rewards of fighting [13,20,25,[28][29][30]. For example, in African lions (Panthera leo), there are consistent differences between individuals in their willingness to engage in territory disputes, as some 'laggard' females consistently hang back and exploit the fighting effort of others [26,30,31].…”

“…This heterogeneity is likely to lead to individual differences in the risks and rewards of fighting [13,20,25,[28][29][30]. For example, in African lions (Panthera leo), there are consistent differences between individuals in their willingness to engage in territory disputes, as some 'laggard' females consistently hang back and exploit the fighting effort of others [26,30,31]. Similarly, in many primates such as vervet monkeys (Chlorocebus pygerythrus) [23], blue monkeys (Cercopithecus mitis) [32], Japanese macaques (Macaca fuscata) [24] and Verreaux's sifakas (Propithecus verreauxi) [33], individuals contribute more towards intergroup conflict the higher their social rank and position in the dominance hierarchy.…”

“…How inter-individual differences in the risks and rewards of conflict affect evolved levels of conflict effort and conflict initiation has been little studied to date. However, see [25,30] (both from this issue) for a consideration of how sex and personality differences influence conflict behaviours.…”

“…We adopted heterogeneity of decision making from Johnstone et al [13]. Specifically, only leaders were responsible for conflict decision making, which is consistent with leadership as defined in the relevant literature [22,30,42]. In keeping with both Gavrilets & Fortunato [29] and Johnstone et al [13], we assume that leaders benefit more from the rewards of fighting.…”